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Unit I

DATA NETWORK FUNDAMENTALS

Network hierarchy

The diagram above organizes the content of theweb pages according to theOSI reference model. Each box in thediagram may be clicked to go to a page which introduces the appropriate section of the course.

The OSI Reference Model

The OSI reference model specifies standards for describing "Open Systems Interconnection" with the term 'open' cto emphasize the fact that by using these international standards, a system may be defined which is open to all systems obeying the same standards throughout the world. The definition of a common technical language has bmajor catalyst to the standardization of communications protocolsand the functions of a protocol layer.
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The seven layers of the OSI reference model showing a connection between two end systemscommunicating using oneintermediate system.

The structure of the OSI architecture is given in the figure above, which indicates the protocolsused to exchange data between two users A and B. The figure shows bidirectional(duplex) information flow; information in either directio passes through all seven layers at the end points. When the communication is via a networkof intermediate systems, onlythe lower three layersof the OSI protocolsare used in the intermediate systems.

Services provided by each Protocol Layer

The OSI layers may be summarized by:

1. Physical layer: Provides electrical, functional, and procedural characteristics to activate, maintain, deactivate physical links that transparently send the bit stream; only recognizes individual bits, not charactemulti character frames.

2. Data link layer : Provides functional and procedural means to transfer data between network entities (possibly) correct transmission errors; provides for activation, maintenance, and deactivation of data liconnections, grouping of bits into characters and message frames, character and frame synchronization, control, media access control, and flow control (examples include HDLCand Ethernet)

3. Network layer : Provides independence from data transfer technology and relaying and routing consideratimasks peculiarities of data transfer medium from higher layers and provides switching androuting functions toestablish, maintain, and terminate network layer connections and transfer data between users.

4. Transport layer : Provides transparent transfer of data between systems, relieving upper layers from conwith providing reliableand cost effective data transfer; provides end-to-end control and information interchawith quality of service needed by the application program; first true end-to-end layer.

5. Session layer : Provides mechanisms for organizing and structuring dialogues between application procesmechanisms allow for two-way simultaneous or two-way alternate operation, establishment of major and m

synchronization points, and techniques for structuring data exchanges.6. Presentation layer : Provides independence to application processes from differences in data representat

which is, in syntax; syntax selection and conversion provided by allowing the user to select a "presentacontext" with conversion between alternative contexts.

7. Application layer : Concerned with the requirements of application. All application processes use the servelements provided by the application layer. The elements include library routines which perform inter procommunication, provide common procedures for constructing application protocols and for accessingservices provided by servers which reside on the network.

The communications engineer is concerned mainly with the protocolsoperating at the bottom four layers (physical, datalink, network, and transport) in the OSI reference model. These layers provide the basic communications servicelayers above are primarily the concern of computer scientists who wish to build distributed applications programs the services provided by the network.

"Hop-by-Hop" "Network-wide" and "End-to-End" Communication

The two lowest layers operate between adjacent systems connected via the physical link and are said to work "hop byhop ". The protocol control information is removed after each "hop" across a link (i.e. by each System) and a suinew header added each time the information is sent on a subsequent hop.
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The network layer (layer 3) operates"network-wide" and is present in all systems and responsible for overall cordination of all systems along the communications path.

The layers above layer 3 operate "end to end" and are only used in the End Systems (ES) which are communicating. TLayer 4 - 7 protocol control information is therefore unchanged by the IS in the network and is delivered tcorresponding ES in its original form. Layers 4-7 (if present) in Intermediate Systems (IS) play no part in the end-tcommunication.

Medium Access Control (MAC)

The Medium Access Control (MAC) protocol is used to provide the data link layer of theEthernetLAN system. TheMAC protocolencapsulatesa SDU (payload data) by adding a 14 byte header (Protocol Control Information (P before the data and appending anintegrity checksum, The checksum is a 4-byte (32-bit)Cyclic Redundancy Check(CRC)after the data. The entire frame is preceded by a small idle period (the minimum inter-frame gap, 9.6 micros(S)) and a 8 byte preamble (including the start of frame delimiter).

Preamble

The purpose of the idle time before transmission starts is to allow a small time interval for the receiver electronics inof the nodes to settle after completion of the previous frame. A node starts transmission by sending an 8 byte (64 preamble sequence. This consists of 62 alternating 1's and 0's followed by the pattern 11. Strictly speaking the lastwhich finished with the '11' is known as the "Start of Frame Delimiter". When encoded using Manchester encoding,Mbps, the 62 alternating bits produce a 10 MHz square wave (one complete cycle each bit period).

The purpose of the preamble is to allow time for the receiver in each node to achieve lock of the receiver Digital Phase

Lock Loopwhich is used to synchronise the receive data clock to the transmit data clock. At the point when the firsof the preamble is received, each receiver may be in an arbitrary state (i.e. have an arbitrary phase for its local cDuring the course of the preamble it learns the correct phase, but in so doing it may miss (or gain) a number of bspecial pattern (11), is therefore used to mark the last two bits of the preamble. When this is received, the Ethreceive interface starts collecting the bits into bytes for processing by the MAC layer. It also confirms the polarity otransition representing a '1' bit to the receiver (as a check in case this has been inverted).
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Header

MAC encapsulation of a packet of data

The header consists of three parts:

A 6-byte destination address, which specifies either a single recipient node(unicast mode), a group of recipiennodes (multicast mode), or the set of all recipient nodes(broadcast mode).

A 6-byte source address, which is set to thesender's globally unique node address. This may be used by thenetwork layer protocol to identify the sender, but usually other mechanisms are used (e.g. arp). Its main functionis to allow address learning which may be used to configure the filter tables in a bridge.

A 2-byte type field, which provides a Service Access Point (SAP) to identify the type of protocol being ca(e.g. the values 0x0800 is used to identify theIP network protocol, other values are used to indicateothernetwork layer protocols). In the case of IEEE 802.3 LLC, this may also be used to indicate the length of the da

part. Th type field is also be used to indicate when a Tag field is added to a frame.

CRC

The final field in an Ethernet MAC frame is called a Cyclic Redundancy Check (sometimes also known as a FCheck Sequence). A32-bit CRC provides error detection in the case where line errors (or transmission collisionEthernet) result in corruption of the MAC frame. Any frame with an invalid CRC is discarded by the MAC recwithout further processing. The MAC protocol does not provide any indication that a frame has been discarded dueinvalid CRC.

The link layer CRC therefore protects the frame from corruption while being transmitted over the physical med(cable). A new CRC is added if the packet is forwarded by the router on another Ethernet link. While the packet is processed by the router the packet data is not protected by the CRC. Router processing errors must be detectenetwork or transport-layer checksums.

Inter Frame Gap

After transmission of each frame, a transmitter must wait for a period of 9.6 microseconds (at 10 Mbps) to allowsignal to propagate through the receiver electronics at the destination. This period of time is known as the Inter-FGap (IFG). While every transmitter must wait for this time between sending frames, receivers do not necessarily"silent" period of 9.6 microseconds. The way in which repeaters operate is such that they may reduce the IFG be

the frames which they regenerate.

Byte Order

It is important to realise that nearly all serial communications systems transmit the least significant bitof each byte firstat the physical layer. Ethernet supports broadcast, unicast, and multicast addresses. The appearance of a multicast adon the cable (in this case anIP multicast address, with group set to the bit pattern 0xxx xxxx xxxx xxxx xxxx xxxxtherefore as shown below (bits transmitted from left to right):
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0 23 IP Multicast Address Group 47| | |1000 0000 0000 0000 0111 1010 xxxx xxx0 xxxx xxxx xxxx xxxx| |Multicast Bit 0 = Internet Multicast

1 = Assigned for other uses

However, when the same frame is stored in the memory of a computer, the bits are ordered such that the least signi bit of each byte is stored in the right most position (the bits are transmitted right-to-left within bytes, bytes transmleft-to-right):

0 23 47| | |0000 0001 0000 0000 0101 1110 0xxx xxxx xxxx xxxx xxxx xxxx

| Multicast Bit IP Multicast Address Group

CSMA /CD

The Carrier Sense Multiple Access (CSMA) with Collision Detection (CD) protocol is used to control access to thshared Ethernet medium. A switched network (e.g. Fast Ethernet) may use a full duplex mode giving access to the fulink speed when used between directly connected NICs, Switch to NIC cables, or Switch to Switch cables.

Receiver Processing Algorithm

Runt Frame

Any frame which is received and which is less than 64 bytes is illegal, and is called a "runt". In most cases, such frarise from a collision, and while they indicate an illegal reception, they may be observed on correctly functionetworks. A receiver must discard all runt frames.
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Giant Frame

Any frame which is received and which is greater than the maximum frame size, is called a "giant". In theory, the jabbercontrol circuit in the transceiver should prevent any node from generating such a frame, but certain failures in physical layer may also give rise to over-sized Ethernet frames. Like runts, giants are discarded by an Ethernet recei

Jumbo Frame

Some modern Gigabit Ethernet NICs support frames that are larger than the traditional 1500 bytes specified by the This new mode requires support by both ends of the link to support Jumbo Frames. Path MTU Discoveryis required fora router to utilise this feature, since there is no other way for a router to determine that all systems on the end-to-enwill support these larger sized frames.

A Misaligned Frame

Any frame which does not contain an integral number of received bytes (bytes) is also illegal. A receiver has no wknowing which bits are legal, and how to compute the CRC-32 of the frame. Such frames are therefore also discardthe Ethernet receiver.

Other Issues

The Ethernetstandard dictates a minimum size of frame, which requires at least 46 bytes of data to be present in eMAC frame. If the network layer wishes to send less than 46 bytes of data the MAC protocol adds sufficient numbzero bytes (0x00, is also known as null padding characters) to satisfy this requirement. The maximum size of data wmay be carried in a MAC frame using Ethernet is 1500 bytes (this is known as the MTUin IP).

A protocol known as the"Address Resolution Protocol" (arp)is used to identify the MAC source address of remo

computers when IP is used over an EthernetLAN.

Exception to the Rule

An extension to Ethernet, known as IEEE 802.1p allows for frames to carry a tag. The tag value adds an extra lePCI to the Ethernet frame header. This increases the size of the total MAC frame when the tag is used. A side effethis is that NICs and network devices designed to support this extension require a modification to the jabber detectioncircuit.

Token Passing
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In the access method known as token passing, a special type of packet, called a token, circulates around a cable ringcomputer to computer. When any computer on the ring needs to send data across the network, it must wait for atoken. When a free token is detected, the computer will take control of it if the computer has data to send.

The computer can now transmit data. Data is transmitted in frames, and additional information, such as addressiattached to the frame in the form of headers and trailers. Let's talk more about these later. For now, only the comthat has the token can transmit on the network.

While the token is in use by this one computer other computers cannot send data. Because only one computer at acan use the token, no contention and no collision take place, and no time is spent waiting for computers to resend tdue to network traffic on the cable

T he token passing access method is a non-contention method that works very differently from the contention me previously discussed. Token passing is a more orderly way for a network to conduct its business. A signal called a tokengoes from one computer to the next. In a Token Ring network, the token goes around the ring; in a token bus netwogoes down the line of the bus. If a computer has data to transmit, it must wait until the token reaches it; thencomputer can capture the token and transmit data.

High Level Link Control (HDLC) Protocol

The HDLC protocol is a general purpose protocolwhich operates at the data link layer of the OSI reference model. The protocol uses the services of a physical layer, and provides either a best effortor reliablecommunications path betweethe transmitter and receiver (i.e. with acknowledgeddata transfer). The type of service provided depends upon the HDLCmode which is used.

Each piece of data is encapsulatedin an HDLC frame by adding a trailer and a header. The header contains an HDLCaddressand an HDLC control field. The trailer is found at the end of the frame, and contains a Cyclic Redundancy Check

(CRC) which detects any errors which may occur during transmission. The frames are separated byHDLC flagsequences which are transmitted between each frame and whenever there is no data to be transmitted.

HDLC Frame Structure showing flags, header (address and control), data and trailer (CRC-16).

HDLC (High Level Data Link Protocol) has been defined by the International Standards Organization for use on bmultipoint and point-to-point links. HDLC is a bit-oriented protocol. It is a predecessor to the local area network da protocols. The two most common modes of operation for HDLC are:

Unbalanced normal response mode (NRM) . This is used with only one primary (or master) station initiating alltransactions.
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Asynchronous balanced mode (ABM) . In this mode each node has equal status and can act as either a primary orsecondary node.

FrameFormat The standard format is indicated Figure. The three different classes of frames used are as follows:Unnumbered frames: Used for setting up the link or connection and to define whether NRM or ABM is to be used.They are called unnumbered frames because no sequence numbers are included.Information frames: Used to convey the actual data from one node to another.Supervisory frames: Used for flow control and error control purposes. They indicate whether the secondary station available to receive the information frames; they are also used to acknowledge the frames. There are two forms of econtrol used: a selective re-transmission procedure because of an error, or a request totransmit a number of previous frames.

Frame Content: The frame contents are as follows: The flag character is a byte wi th the value 01111110. To ensure that the receiver always knows that the character itreceives is unique (rather than merely some other character in the sequence); a procedure called zeroinsertion is adopted. This requires the transmitter to insert a 0 af ter a sequence of five 1s in the text, so that the flagcharacter can never appear in the message text. The receiver removes the inserted zeros. The frame check sequence (FCS) uses the CRC -CCITT methodology, with sixteen 1s to the tail of the message beforthe CRC calculation proceeds, and the remainder is inverted. The address field can contain one of three types of address for the request or response messages to or from thesecondary node:

Standard secondary address Group addresses for groups of nodes on the network Broadcast addresses for all nodes on the network (here the address contains all 1s)

Where there are a large number of secondaries on the network, the address field can be extended beyond eight bits byencoding the least significant bit as a 1. This then indicates that there is another byte to follow in the address field. The control field is indicated in Figure

Protocol Operation:


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A typical sequence of operations for a multidrop link is given below:1 The primary node sends a Normal Response Mode frame, with the P/F bit set to 1, together with the address of thesecondary node.2 The secondary node responds with an unnumbered acknowledgment with the P/F bit set to 1. If the receiving nodeunable to accept the setup command, a disconnected mode frame is returned instead.3 Data is transferred with the information frames.4 The primary node sends an unnumbered frame containing a disconnect in the control field.5 The secondary node responds with an unnumbered acknowledgment. A similar approach is followed for a point-to point link using asynchronous balanced mode, except that both nodes can initiate the setting up of the link and thetransfer of information frames, and the clearing of the point-to-point link. The following differences also apply: When the secondary node transfers the data, it transmits the data as a sequence of information frames with the P/F bitset to 1 in the final frame of the sequence. In NRM mode, if the secondary node has no further data to t ransfer, it responds with a Receiver Not Ready frame witthe P/F bit set to 1.

Error & Flow Control: For a half duplex exchange of information frames, error control is by means of sequence

numbers. Each end maintains a transmit sequence number and a receive sequence number. When a node successfullreceives a frame, it responds with a supervisory frame containing a receiver ready (RR) indication and a receive seqnumber. The number is that of the next frame expected, thus acknowledging all previous frames.

If the receiving node responds with a negative acknowledgment (REJ) frame, the transmitter must transmit all frfrom the receive sequence number in the REJ frame. This happens when the receiver detects an out-of-sequence frais also possible for selective retransmission to be used. In this case the receiver would return a selection rejection fcontaining only the sequence number of the missing frame.

A slightly more complex approach is required for a point-to-point link using asynchronous balanced mode withduplex operation, where information frames are transmitted in two directions at the same time. The same philosopfollowed as for half duplex operation except that checks for correct sequences of frame numbers must be maintain both ends of the link.

Flow control operates on the principle that the maximum number of information frames awaiting acknowledgment atime is seven. If seven acknowledgments are outstanding, the transmitting node will suspend transmission untacknowledgment is received. This can be either in the form of a receiver ready supervisory frame, or piggybackedinformation frame being returned from the receiver.

If the sequence numbers at both ends of the link become so out of sequence that the number of frames awacknowledgment exceeds seven, the secondary node transmits a frame reject or a command reject frame to the pri

node. The primary node then sets up the link again, and on an acknowledgment from the secondary node, both reset all the sequence numbers and commence the transfer of information frames.

It is possible for the receiver to run out of buffer space to store messages. When this happens it will transmit a recnot ready (RNR) supervisory frame to the primary node to instruct it to stop sending any more information frames.

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)


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The Ethernetnetwork may be used to provide shared access by a group of attached nodes to the physical medium wconnects the nodes. These nodes are said to form a Collision Domain. All frames sent on the medium are physireceived by all receivers, however the Medium Access Control (MAC)header contains a MAC destination address whicensure only the specified destination actually forwards the received frame (the other computers all discard the frwhich are not addressed to them).

Consider a LANwith four computers each with a Network Interface Card (NIC)connected by a common Ethernet cable

One computer (Blue) uses a NIC to send a frame to the shared medium, which has a destination address correspondthe source address of the NIC in the red computer.

The cable propagates the signal in both directions, so that the signal (eventually) reaches the NICs in all four ocomputers. Termination resistors at the ends of the cable absorb the frame energy, preventing reflection of the s back along the cable.

All the NICs receive the frame and each examines it to check its length and checksum. The header destination Maddress is next examined, to see if the frame should be accepted, and forwarded to the network-layer software icomputer.

Only the NIC in the red computer recognises the frame destination address as valid, and therefore this NIC forwards the contents of the frame to the network layer. The NICs in the other computers discard the unwanted fram
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The shared cable allows any NIC to send whenever it wishes, but if two NICs happen to transmit at the same timcollision will occur, resulting in the data being corrupted.

ALOHA & Collisions

To control which NICs are allowed to transmit at any given time, a protocol is required. The simplest protocol is k

as ALOHA (this is actually an Hawaiian word, meaning "hello"). ALOHA allows any NIC to transmit at any timestates that each NIC must add a checksum/CRCat the end of its transmission to allow the receiver(s) to identify whetthe frame was correctly received.

ALOHA is therefore a best effortservice, and does not guarantee that the frame of data will actually reach the remrecipient without corruption. It therefore relies on ARQ protocols to retransmit any data which is corrupted. An ALnetwork only works well when the medium has a low utilisation, since this leads to a low probability of the transmcolliding with that of another computer, and hence a reasonable chance that the data is not corrupted.

Carrier Sense Multiple Access (CSMA)

Ethernet uses a refinement of ALOHA, known as Carrier Sense Multiple Access (CSMA), which improves performwhen there is a higher medium utilisation. When a NIC has data to transmit, the NICfirst listens to the cable (using atransceiver) to see if a carrier (signal) is being transmitted by another node. This may be achieved by monitoring wha current is flowing in the cable (each bit corresponds to 18-20 milliAmps (mA)). The individual bits are seencoding them with a 10 (or 100 MHz for Fast Ethernet) clock using Manchester encoding. Data is only sent when nocarrier is observed (i.e. no current present) and the physical medium is therefore idle. Any NIC which does not netransmit, listens to see if other NICs have started to transmit information to it.

However, this alone is unable to prevent two NICs transmitting at the same time. If two NICs simultaneouslytransmit, then both could see an idle physical medium (i.e. neither will see the other's carrier signal), and both

conclude that no other NIC is currently using the medium. In this case, both will then decide to transmit and a colwill occur. The collision will result in the corruption of the frame being sent, which will subsequently be discarded breceiver since a corrupted Ethernet frame will (with a very high probability) not have a valid 32-bit MAC CRCat theend.

Collision Detection (CD)

A second element to the Ethernet access protocol is used to detect when a collision occurs. When there is data wait be sent, each transmitting NIC also monitors its own transmission. If it observes a collision (excess current above wis generating, i.e. > 24 mA for coaxial Ethernet), it stops transmission immediately and instead transmits a 32-bisequence. The purpose of this sequence is to ensure that any other node which may currently be receiving this framreceive the jam signal in place of the correct 32-bit MAC CRC, this causes the other receivers to discard the frame da CRC error.

To ensure that all NICs start to receive a frame before the transmitting NIC has finished sending it, Ethernet defiminimum frame size (i.e. no frame may have less than 46 bytes of payload). The minimum frame size is related tdistance which the network spans, the type of media being used and the number of repeaters which the signal mayto pass through to reach the furthest part of the LAN. Together these define a value known as the Ethernet Slot Tcorresponding to 512 bit times at 10 Mbps.
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When two or more transmitting NICs each detect a corruption of their own data (i.e. a collision), each responds same way by transmitting the jam sequence. The following sequence depicts a collision:

At time t=0, a frame is sent on the idle medium by NIC A.

A short time later, NIC B also transmits. (In this case, the medium, as observed by the NIC at B happens to be idle to

After a period, equal to the propagation delay of the network, the NIC at B detects the other transmission from A, aaware of a collision, but NIC A has not yet observed that NIC B was also transmitting. B continues to transmit, senthe Ethernet Jam sequence (32 bits).

After one complete round trip propagation time (twice the one way propagation delay), both NICs are aware ocollision. B will shortly cease transmission of the Jam Sequence, however A will continue to transmit a completeSequence. Finally the cable becomes idle.

Retransmission Back-Off

An overview of the transmit procedure is shown below. The transmitter initialises the number of transmissions ocurrent frame (n) to zero, and starts listening to the cable (using the carrier sense logic (CS) - e.g., by observing th Rxsignal at transceiverto see if any bits are being sent). If the cable is not idle, it waits (defers) until the cable is idle. Itwaits for a small Inter-Frame Gap (IFG) (e.g., 9.6 microseconds) to allow to time for all receiving nodes to retu prepare themselves for the next transmission.

Transmission then starts with the preamble, followed by theframe data and finally theCRC-32. After this time, thetransceiver Tx logicis turned off and the transceiver returns to passively monitoring the cable for other transmissions
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During this process, a transmitter must also continuoulsy monitor the collision detection logic (CD) in the transceivertodetect if a collision ocurs. If it does, the transmitter aborts the transmission (stops sending bits) within a few bit peand starts the collision procedure, by sending a Jam Signal to the transceiver Tx logic. It then calculates a retransmissiotime.

If all NICs attempted to retransmit immediately following a collision, then this would certainly result in anocollision. Therefore a procedure is required to ensure that there is only a low probability of simultaneous retransmiThe scheme adopted by Ethernet uses a random back-off period, where each node selects a random number, multthis by the slot time (minimum frame period, 51.2 S) and waits for this random period before attempting retransmiThe small Inter-Frame Gap (IFG) (e.g., 9.6 microseconds) is also added.

On a busy network, a retransmission may still collide with another retransmission (or possibly new frames being sethe first time by another NIC). The protocol therefore counts the number of retransmission attempts (using a variain the above figure) and attempts to retransmit the same frame up to 15 times.

For each retransmission, the transmitter constructs a set of numbers:

{0, 1, 2, 3, 4, 5, ... L} where L is ([2 to the power (K)]-1) and where K=N; K


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As the level of utilisation of the network increases, particularly if there are many NICs competing to share the bandwan overload condition may occur. In this case, the throughput of Ethernet LANs reduces very considerably, and muthe capacity is wasted by the CSMA/CD algorithm, and very little is available for sending useful data. This is the rwhy a shared Ethernet LAN should not connect more than 1024 computers. Many engineers use a threshold ofUtilisation to determine if a LAN is overloaded. A LAN with a higher utilisation will observe a high collision ratelikely a very variable transmission time (due to back off). Separating the LAN in to two or more collision domains bridges or switcheswould likely provide a significant benefit (assuming appropriate positioning of the bridges orswitches).

Shared networks may also be constructed using Fast Ethernet, operating at 100 Mbps. Since Fast Ethernet alwaysfibre or twisted pair, a hub or switchis always required.

Ethernet Capture

A drawback of sharing a medium using CSMA/CD, is that the sharing is not necessarily fair. When each compconnected to the LAN has little data to send, the network exhibits almost equal access time for each NIC. Howevone NIC starts sending an excessive number of frames, it may dominate the LAN. Such conditions may occuinstance, when one NIC in a LAN acts as a source of high quality packetised video. The effect is known as "EthCapture".

Ethernet Capture by Node A.

The figure above illustrates Ethernet Capture. Computer A dominates computer B. Originally both computers haveto transmit. A transmits first. A and B then both simultaneously try to transmit. B picks a larger retransmission intthan A (shown in red) and defers. A sends, then sends again. There is a short pause, and then both A and B attemresume transmission. A and B both back-off, however, since B was already in back-off (it failed to retransmit), it ch

from a larger range of back-off times (using the exponential back-off algorithm). A is therefore more likely to sucwhich it does in the example. The next pause in transmission, A and B both attempt to send, however, since this fathis case, B further increases its back-off and is now unable to fairly compete with A.

Ethernet Capture may also arise when many sources compete with one source which has much more data to send. Uthese situations some nodes may be "locked out" of using the medium for a period of time. The use of higher stransmission (e.g. 100 Mbps) significantly reduces the probability of Capture, and the use full duplex cabling elimithe effect.
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The Internetwork Protocol (IP)

The IP (Internet Protocol) is a protocol that uses datagramsto communicate over a packet-switched network. The IP protocol operates at the network layer protocol of the OSI reference modeland is a part of a suite of protocolsknown asTCP/IP. Today, with over 1.5 billion users worldwide, the current Internet is a great success in terms of conne people and communities. Even though the current Internet continues to work and is capable of fulfilling its cumissions, it also suffers from a relative ossification , a condition where technological innovation meets naturesistance, as exemplified by the current lack of wide deployment of technologies such as multicast or Internet Protocolversion 6 (IPv6).

The Internetwork Protocol (IP)[RFC791] provides a best effort network layer service for connecting computers to forcomputer network. Each computer is identified by one or more gloablly uniqueIP addresses. The network layer PDUsare known as either "packets" or"datagrams". Each packet carries the IP addressof the sending computer and also thaddress of the intended recipient or recipients of the packet. Other management information is also carried.

The IP network service transmits datagrams between intermediate nodes using IProuters. The routers themselves arsimple, since no information is stored concerning the datagrams which are forwarded on a link. The most complex pan IP router is concerned with determining the optimum link to use to reach each destination in a network. This pris known as"routing". Although this process is computationally intensive, it is only performed at periodic intervals.

An IP network normally uses a dynamic routing protocol to find alternate routes whenever a link becomes unavaiThis provides considerable robustness from the failure of either links or routers, but does not guarentee reliabledelivery.Some applications are happy with this basic service and use a simple transport protocol known as theUser DatagramProtocol (UDP)to access this best effort service.

Most Internet users need additional functions such as end-to-end error and sequence control to give a reliable se(equivalent to that provided by virtual circuits). This reliability is provided by the Transmission Control Protocol (TCP

which is used end-to-end across the Internet.

In a LAN environment, the protocol is normally carried byEthernet, but for long distance links, other link protocousing fibre optic links are usually used. Other protocols associated with the IP network layer are theInternet ControlMessage Protocol (ICMP)and the Address Resolution Protocol (arp).

IP the Next Generation, IPv6

The IPv4 protocol although widely used, is slowly being superceded byIPv6 [RFC2460], a next-generation networklayer protocol. IPv6 is now widely implemented, and deployed in many networks.

The gradual transition from IPv4 towards majority IPv6 deployment will take many years and IPv4 may never its phased out completely. In the meantime the two protocols can co-exist and be used together in various ways. IPv6ultimately succeed the current version, IPv4, to become the dominant version of IP used in the Internet. IPv6 chmany things, one of the most obvious from the Ethernet perspective is that it uses a different Ether-Types and uses t Neighbor-Discovery(ND) protocol in place of ARP.
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Unit II

INTER NETWORKING

Network Classification

Types of Networks


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Network Hardware Componenets

Ethernet Bridges & Switches

A bridge is a LAN interconnection device which operates at the data link layer (layer 2) of the OSI reference model. Itmay be used to join two LAN segments (A,B), constructing a larger LAN. A bridge is able to filter traffic pa between the two LANs and may enforce a security policy separating different work groups located on each of the LBridges were first specified in IEEE 802.1D(1990) and later by ISO (in 1993).

The format of PDUs at this layer in an Ethernet LAN is defined by theEthernet frame format (also known as MAC -Medium Access Control). It consists of two 6 byte addresses and a one byte protocol ID / length field. The address fallows a frame to be sent to single and groups of stations. The MAC protocol is responsible for access to the mediumfor the diagnosis of failure in either the medium or the transceiverwhich attaches to the medium.

Operation of a Bridge

The simplest type of bridge, and that most frequently used is the Transparent Bridge (meaning that the nodes us bridge are unaware of its presence). The bridge therefore has to forward (receive and subsequently transmit) framesone LAN (e.g. LAN A below) to another (e.g. LAN B). Obviously, the bridge could forward all frames, but then it behave rather like arepeater; it would be much smarter if the bridge only forwarded frames whichneed to travel fromone LAN to another. To do this, the bridge need tolearn which computers are connected to which LANs. More formalit need to learn whether to forward to each address.
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multicast frames to parts of the network for which there are no interested receivers. Some bridges implementextra processingto control the flooding of multicast frames.

Managing the Interface Tables

A bridge may implement an interface table using a software data structure or use a Contents Addressable Me

(CAM) chip. In either case, the size of the table is finite, and usually constrained to 1000's - 10 000's of entries. In aLAN this may be a limit. To help keep the table small, most bridges maintain a check of how recently each addresused. Addresses which have not been used for a long period of time (e.g. minutes) are deleted. This has the efferemoving unused entries, but if the address is again used, before a frame is received from the same source, it will rthe frame to be flooded to all ports.

A useful side effect of deleting old addresses is that the bridge interface table records only working MAC addresses NIC stops sending, its address will be deleted from the table. If the NIC is subsequently reconnected, the entry wrestored, but if the connection is made to another port (the cable is changed) a different (updated) entry will be inscorresponding to the actual port associated with the address. (The bridge always updates the interface table for source address in a received MAC frame, therefore even if a computer changes the point at which it is connected wfirst having the interface table entry removed, the bridge will still update the table entry).

Filter Tables

In some managed bridges, a system administrator may override the normal forwarding by inserting entries in a filter tto inhibit forwarding between different work groups (for example to provide security for a particular set of Maddresses). The filter table contains a list of source or destination addresses. Frames which match entries in the table will only be forwarded to specific configured ports. This can be used to implement security polcies and alconstrcut Virtual LANs.

Multiple Port Bridges (Switches)

A bridge with more than two interfaces (ports) is also known as a switch. There are important differences betswitches and hubs. In particular, the way in which they forward frames.
http://www.erg.abdn.ac.uk/~gorry/course/intro-pages/uni-b-mcast.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/switch.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/switch.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/nic.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/vlan.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/vlan.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/hub.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/hub.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/vlan.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/vlan.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/nic.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/switch.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/switch.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/intro-pages/uni-b-mcast.html


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A Hub sending a packet form F to C.

A hub (or repeater) forwards a received frame out of all the interfaces (ports), resulting in the frame reachinconnected equipment, even though the frame may be only destined for a system connected to one particular interfacin the above diagram).

A switch, on the other hand, forwards the frame to only the required interface. The switch learns the association bethe systems MAC addresses and the interface ports in the same way as a bridge (see above). By sending the packet where it need to go, the switch reduces the number of packets on the other LAN segments (and hence the load onsegments), increasing the overall performance of the connected LANs. The switch also improves security, since fronly travel where they are intended (and can not in this case, for instance, be observed by an unauthorised comattached to segment A).

A Switch sending a packet from F to C

Switches (like bridges) normally forward allmulticast and broadcast packets to all receivers (some switches have extra processingto help improve performance of multicast forwarding). More details about this, and the operation of swimay be found in a related page (see below).
http://www.erg.abdn.ac.uk/~gorry/course/lan-pages/10bt.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/mac.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/intro-pages/uni-b-mcast.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/switch.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/switch.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/switch.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/switch.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/intro-pages/uni-b-mcast.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/mac.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/10bt.html


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is forwarded, the processor checks the Maximum Transfer Unit (MTU) of the specified interface. Packets larger thainterface's MTU must be fragmented by the router into two or more smaller packets. If a packet is received which hDon't Fragment (DF) bit set in the packet header, the packet is not fragmented, but instead discarded. In this case,ICMP error messageis returned to the sender (i.e. to the original packet's IP source address) informing it of interface's MTU size. This forms the basis for Path MTU discovery (PMTU).

The routing and filter tables resemble similar tables in link layer bridges and switches. Except, that instead of speclink hardware addresses(MAC addresses), the router table sepcify network(IP addresses). The routing table lists knowIP destination addresses with the appropraite network interface to be used to reach that destiantion. A default entry be specified to be used for all addresses not explicitly defined in the table (this is very common in routers close tedge of the networ, where the default routes packets towards the Internet backbone).

A filter table may also be used to ensure that unwanted packets are discarded. The filter may be used to deny acce particular protocols or to prevent unauthorised access from remote computers by discarding packets to a spedestination address. Routers at the edge of ISP networks also often perform filtering of the IP source address, as a w prevent "spoofing" of addresses belonging to other networks.

A router forwards packets from one IP network to another IP network. Like other systems, it routes based on the lo prefxi match of the IP addresss in the routing table. One exception to this rule is when a router receives an IP packenetwork broadcast address. In this case, the router will process the packet internally (to see if it needs to respondthen discards the packet. Forwarding broadcast packet can lead to severe storms of packets, and if uncontrolled clead to network overload.

A router introduces delay (latency) as it processes the packets it receives. The total delay observed is the sum of components including:

Time taken to process the frame by the data link protocol

Time taken to select the correct output link (i.e. filtering and routing) Queuing delay at the output link (when the link is busy) Other activities which consume processor resources (computing routing tables, network management, gener

of logging information)

The router queue of packets waiting to be sent also introduces a potential cause of packet loss. Since the router finite amount of buffer memory to hold the queue, a router which receives packets at too high a rate may experiefull queue. In this case, the router ahs no other option than to simply discard excess packets. If required, these may be retransmitted by a transport protocol.
http://www.erg.abdn.ac.uk/~gorry/course/inet-pages/ip-packet.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/inet-pages/icmp.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/inet-pages/mtu.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/mac.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/inet-pages/ip-address.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/inet-pages/ip-address.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/mac.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/inet-pages/mtu.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/inet-pages/icmp.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/inet-pages/ip-packet.html


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Architecture of a router

Routers are often used to connect together networks which use different types of links (for instance an HDLC or PPPlinkconnecting a WAN to a localEthernet LAN). The optimum (and maximum) packet lengths (i.e. themaximumtransmission unit (MTU)) is different for different types of network. A router may therefore uses IP to provsegmentation of packets into a suitable size for transmission on a network.

Associated protocols perform network error reporting(ICMP), communication between routers (to determine appropriroutes to each destination) and remote monitoring of the router operation (network management).

The operation of a simple modern router is described on a separate page. If you want to know how the router actuworks click HERE.

Repeaters

Repeaters operate within the physical layerof the OSI reference modeland regeneratethe signal . Repeaters are used iLANs MAN and WANs. They may be used to provide more flexibility in design of a network or to extend the distover which a signal may travel down a cable. One example of a repeater is an Ethernet Hub.

The 5-4-3 rule

The 5-4-3 rule is important when considering using repeaters (or 10BT hubs) to build a larger LAN. This rule statea single Ethernet LAN should not have more than:5 No path between any two end systems (network interface carother equipment) may cross more than FIVE Ethernets segments. 4 No path between any two end systems may more than FOUR Ethernet hubs or repeaters.3 No more than THREE of the five segments on the longest path mactive segments (i.e. segments with more than two nodes and/or repeater ports). The remaining two segments mu point-to-point links.

Hub / Switch / Bridge / Router

In data communications, ahub is a place of convergence where data arrives from one or more directions and forwarded out in one or more other directions. A hub usually includes a switch of some kind.
http://www.erg.abdn.ac.uk/~gorry/course/dl-pages/hdlc.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/enet.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/intro-pages/lan.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/inet-pages/mtu.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/inet-pages/mtu.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/inet-pages/icmp.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/inet-pages/router-opn.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/inet-pages/router-opn.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/phy-pages/phy.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/intro-pages/osi.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/intro-pages/ber.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/intro-pages/lan.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/intro-pages/man.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/intro-pages/wan.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/10bt.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/10bt.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/intro-pages/wan.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/intro-pages/man.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/intro-pages/lan.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/intro-pages/ber.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/intro-pages/osi.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/phy-pages/phy.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/inet-pages/router-opn.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/inet-pages/router-opn.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/inet-pages/icmp.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/inet-pages/mtu.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/inet-pages/mtu.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/intro-pages/lan.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/lan-pages/enet.htmlhttp://www.erg.abdn.ac.uk/~gorry/course/dl-pages/hdlc.html


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In telecommunications, aswitch is a network device that selects a path or circuit for sending a unit of data to inext destination.

A switch may also include the function of therouter , a device or program that can determine the route aspecifically what adjacent network point the data should be sent to.

In general, a switch is a simpler and faster mechanism than a router, which requires knowledge about network and how to determine the route

In telecommunication networks, abridge is a product that connects a local area network (LAN) to another loarea network that uses the same protocol (for example, Ethernet or token ring).

On the Internet, arouter is a device or, in some cases, software in a computer, that determines the next netw point to which a concept of packet should be forwarded toward its destinat

The router is connected to at least two networks and decides which way to send each information packet bon its current understanding of the state of the networks it is connected to.

A router is located at any gateway (where one network meets another), including each Internet poin presence. A router is often included as part of a network switch.

Ethernet

developed by Xerox in 1973 1975

standardized as IEEE 802.3

has replaced token ring, FDDI and ARCNET

usually uses twisted pair cable ( RJ-45)

IEEE 802.3: Ethernet is the most widely-installed local area network (LAN) protocol. Specified in a standIEEE 802.3, Ethernet was originally developed by Xerox and then developed further by Xerox, DEC, and An Ethernet LAN typically uses coaxial cable or special grades of twisted pair wires.

10BASE-T: The most commonly installed Ethernet systems are called 10BASE-T and provide transmis

speeds up to 10 Mbit/s. Devices are connected to the cable and compete for access using a Carrier SMultiple Access with Collision Detection (CSMA/CD) protocol.

100BASE-T or Fast Ethernet provides transmission speeds up to 100 megabits per second and is typically ufor LAN backbone systems, supporting workstations with 10BASE-T cards.

Gigabit Ethernet provides an even higher level of backbone support at 1000 megabits per second (1 gigabit billion bits per second).


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ARCnet (Attached Resource Computer network)

ARCnet is a widely-installed local area network (LAN) technology that uses a token-bus scheme for manaline sharing among the workstations and other devices connected on the LAN.

The LAN server continuously circulates empty message frames on a bus (a line in which every message through every device on the line and a device uses only those with its address).

GENERIC ARCNET BOARD

GENERIC ARCNET BOARD


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GENERIC ARCNET BOARD FEATURES

80c31 CPU 16 MHZ CLOCK 64K RAM 64K FLASH ROM

8 bit I/O port 256 bytes (available for memory mapped I/O) 1 serial port OR 2 bit bi-directional I/O port 2 bit I/O port (dedicated) Automatic reset on power-up 16 user definable chip selects Two standard 16 bit counters. ARCNET software interface built-in Firm ware upload feature up to 39K Token removal (turn off ARCNET stop noise) 2.5Mbits/s data rate (ARCNET) works in high magnetic fields (external power supply needed) optoisolation between each node.

What is ARCNET?

Attached Resource Computer NETwork

Token-Passing Local Area Network (LAN)

Originally 2.5 Mbps data rate

255 Nodes or Stations

Variable Packet Length

Bus or Distributed Star Wiring

Unicast or Broadcast Messages

One to one or one to all

Coaxial, Fiber Optic, Twisted-pair Cabling

Over 20 Million Installed Nodes

Originally developed by Datapoint Corporation as an office network

Chip sets available from SMSC

ATA 878.1-1999 Local Area Network: Token Bus

Ideally suited for an industrial network


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What are ARCNETs Benefits?

Broad Acceptance

Large Installed Base

Deterministic Performance

Simple to Install

Low Cost per Node

Robust Design

Multiple Cable Media Support

Multi-master Communication

Where is ARCNET Used?

HVAC

Motor Drives

Power Generation

Data Acquisition and Control

Manufacturing Information Systems

Office Automation

Shipboard Automation Printing Press Controls

Telecommunications

Gaming Machines

Vehicular Navigation

Security Systems

ARCNET Protocol Only Five Simple Commands

ITT - Invitation to transmit

FBE - Free buffer enquiry

PAC - Packet

ACK - Acknowledgement


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NAK - Negative acknowledgement

ARCNET Protocol Features

Deterministic Token Passing

Packet Flow Control

Error Detection

Auto Reconfiguration

Variable Packet Size

Supports Various Transceivers & Media

Supports Various Software Drivers

Up to 255 Nodes Per Network

Token Passing - Transmitting on the network is only permitted when a node has the token

Every node can transmit once during each token rotation

Benefits:

Every node has a guaranteed response time to transmit

Deterministic behavior

Auto-Reconfiguration - Network is automatically reconfigured when a node joins or leaves the network

Token pass is automatically reconfigured Typical time 20 - 30 ms

Supports live node insertion and deletion

Variable Packet Size

From 1 to 507 bytes per packet

Packet Flow Control - Transmitter checks receiver to make sure it is ready to receive a packet

Reduced software overhead Increased bandwidth

No lost packets due to input buffer overruns

Error Detection - 16 bit CRC checks each packet

Corrupted packets automatically rejected

Transmitter is aware of the error


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Reduced software overhead

Better CPU utilization


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Unit III

HART AND FIELDBUS

HART Overview

The majority of smart field devices installed worldwide today are HART-enabled. But some new in the automationmay need a refresher on this powerful technology.

Simply put, the HART (Highway Addressable Remote Transducer) Protocol is the global standard for sendingreceiving digital information across analog wires between smart devices and control or monitoring system.

More specifically, HART is a bi-directional communication protocol that provides data access between intelligent instruments and host systems. A host can be any software application from technician's hand-held device or laptop plant's process control, asset management, safety or other system using any control platform.

A DIGITAL UPGRADE FOR EXISTING PLANTS

HART technology offers a reliable, long-term solution for plant operators who seek the benefits of intelligent dewith digital communication that is included in the majority of the devices being installed. In many cases howemost applications cannot retrofit their existing automation systems with a system that can accept the digital data wh provided by the HART Protocol.

Because most automation networks in operation today are based on traditional 4-20mA analog wiring, HART technoserves a critical role because the digital information is simultaneously communicated with the 4-20mA signal. Withthere would be no digital communication.

A CRITICAL, DIGITAL ROLE

HART technology is easy to use and very reliable when used for commissioning and calibration of smart devices as as for continuous online diagnostics.

There are several reasons to have a host communicate with smart devices. These include:

Device Configuration or re-configuration Device Diagnostics Device Troubleshooting Reading the additional measurement values provided by the device Device Health and Status Much more: There are many benefits of using HART technology, and more users are reporting benefits in

projects on a continual basis. For more information please visit Success Stories

Years of success using these benefits explain why HART technology is the largest of all communication protoinstalled in more than 30 million devices worldwide.

If you've ever used a land-line telephone and noticed the Caller ID display to take note of who is calling, you alknow half of what the HART Protocol does it tells "who" is calling. In an industrial automation network "who" imicroprocessor-based smart field device. In addition to letting such smart field devices "phone home," HCommunication lets a host system send data to the smart instrument.
http://en.hartcomm.org/hcp/tech/applications/applications_success.htmlhttp://en.hartcomm.org/hcp/tech/applications/applications_success.html


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HART emerged in the late1980s based on the same technology that brought Caller ID to analog telephony. Iundergone continued development, up to and including automation products now shipping with built-in WirelessHCommunication.

How HART Works

HART is an acronym for Highway Addressable Remote Transducer. The HART Protocol makes use of the Bell 202Frequency Shift Keying (FSK) standard to superimpose digital communication signals at a low level on top of t20mA.

Frequency Shift Keying (FSK)

This enables two-way field communication to take place and makes it possible for additional information beyond junormal process variable to be communicated to/from a smart field instrument. The HART Protocol communica1200 bps without interrupting the 4-20mA signal and allows a host application (master) to get two or more dupdates per second from a smart field device. As the digital FSK signal is phase continuous, there is no interferencethe 4-20mA signal.

HART technology is a master/slave protocol, which means that a smart field (slave) device only speaks when spok by a master. The HART Protocol can be used in various modes such as point-to-point or multidrop for communicinformation to/from smart field instruments and central control or monitoring syste

HART Communication occurs between two HART-enabled devices, typically a smart field device and a contromonitoring system. Communication occurs using standard instrumentation grade wire and using standard wiringtermination practices

The HART Protocol provides two simultaneous communication channels: the 4-20mA analog signal and a digital sThe 4-20mA signal communicates the primary measured value (in the case of a field instrument) using the 4-2current loop - the fastest and most reliable industry standard. Additional device information is communicated usdigital signal that is superimposed on the analog sign


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The digital signal contains information from the device including device status, diagnostics, additional measurecalculated values, etc. Together, the two communication channels provide a low-cost and very robust completecommunication solution that is easy to use and configure.

Two Communication Channels

The HART Protocol provides for up to two masters (primary and secondary). This allows secondary masters suhandheld communicators to be used without interfering with communications to/from the primary mastercontrol/monitoring system.

Primary and Secondary Masters


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The HART Protocol permits all digital communication with field devices in either point-to-point or multidrop netconfigurations:

Multidrop Configuration

There is also an optional "burst" communication mode where a single slave device can continuously broadcast a staHART reply message. Higher update rates are possible with this optional burst communication mode and use is normrestricted to point-to-point configuration.

HART Protocol Specifications

The HART Protocol was developed in the late 1980's and transferred to the HART Foundation in the early 1990's.

then it has been updated several times. When the protocol is updated, it is updated in a way that ensures backcompatibility with previous versions. The current version of the HART Protocol is revision 7.3. The "7" denotemajor revision level and the "3" denotes the minor revision level.

The HART Protocol implements layers 1,2, 3, 4 and 7 of the Open System Interconnection (OSI) 7-layer protocol m

The HART Physical Layer is based on the Bell 202 standard, using frequency shift keying (FSK) to communicat1200 bps. The signal frequencies representing bit values of 0 and 1 are 2200 and 1200Hz respectively. This signsuperimposed at a low level on the 4-to-20mA analog measurement signal without causing any interference witanalog signal.


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The HART Data Link Layer defines a master-slave protocol - in normal use, a field device only replies when spoken to. There can be two masters, for example, a control system as a primary master and a handheld Hcommunicator as a secodary master. Timing rules define when each master may initiate a communication transactioto 15 or more slave devices can be connected to a single multidrop cable pair.

The Network Layer provides routing, end-to-end security, and transport services. It manages "sessions" for end-tocommunication with correspondent devices.

The Transport Layer: The Data-Link Layer ensures communications are successfully propagated from one devicanother. The Transport Layer can be used to ensure end-end communication is successful.

The Application Layer defines the commands, responses, data types and status reporting supported by the Protocothe Application Layer, the public commands of the protocol are divided into four major groups:

1. Universal Commands - provide functions which must be implemented in all field devices2. Common Practice Commands - provide functions common to many, but not all field devices3. Device Specific Commands - provide functions that are unique to a particular field device and are specifie

the device manufacturer

4.

Device Family Commands - provide a set of standardized functions for instruments with particular measuretypes, allowing full generic access without using device-specific commands.

HART Commands

The HART Protocol is a master-slave communication protocol which means that during normal operation, each slafield device) communication is initiated by a request (or command) from the master (host) communication devicemaster or host is generally a distributed control, PLC, or PC-based asset management system for example. The device is typically a field measurement device such as pressure, level, temperature, flow or other transmitters.

In order to make certain any HART-enabled device from any supplier can communicate properly and respondcommand with the correct information, the set and types of commands are defined in the HART Specifications

implemented in all HART registered devices.

Users need not worry about these commands because they are included in the functions of the host. The specapabilities of a device (device specific commands) are available to the host when the host is given the instrucincluded in the Device Description (DD) of a specific device.

An important point is that defined device status indications are included with each communication response to theThe host then interprets these status indicators and may provide basic device diagnostic information.

The HART Command Set provides uniform and consistent communication for all field devices. Host applications implement any of the necessary commands for a particular application. The command set includes three classes:

UniversalAll devices using the HART Protocol must recognize and support the universal commands. Universal commands praccess to information useful in normal operations (e.g., read primary variable and units).

Common Practice commands provide functions implemented by many, but not necessarily all, HART Communicdevices.

Device Specific commands represent functions that are unique to each field device. These commands access setupcalibration information, as well as information about the construction of the device. Information on Device Specommands is available from device manufacturers.


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A Partial List of HART Commands:

Universal Commands Common Practice Commands Device Specific Commands

Read manufacturer and devicetype

Read primary variable (PV) andunits

Read current output and percentof range

Read up to four pre-defineddynamic variables

Read or write eight-charactertag, 16-character descriptor, date

Read or write 32-charactermessage

Read device range values, units,and damping time constant

Read or write final assemblynumber

Write polling address

Read selection of up to fourdynamic variables

Write damping timeconstant

Write device range values Calibrate (set zero, set span) Set fixed output current Perform self-test Perform master reset Trim PV zero Write PV unit Trim DAC zero and gain Write transfer function

(square root/linear) Write sensor serial number Read or write dynamic

variable assignments

Read or write low-flow cutoff

Start, stop, or clear totalizer Read or write density

calibration factor Choose PV (mass, flow, o

density) Read or write materials o

construction information Trim sensor calibration PID enable Write PID set point Valve characterization Valve set point Travel limits User units Local display information

Highway Addressable Remote Transducer Protocol HART

Protocol Information

Type ofNetwork

Device Bus (Process Automation)

PhysicalMedia

Legacy 4-20 mA analoginstrumentation wiring or 2.4 GHzWireless

NetworkTopology

One-on-One, Multidrop, WirelessMesh

MaximumDevices

15 in multidrop

MaximumSpeed

Depends on Physical Layer employed

DeviceAddressing

Hardware/Software

GoverningBody

HART Communication Foundation
http://en.wikipedia.org/wiki/4-20_mAhttp://en.wikipedia.org/wiki/4-20_mA


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The HART Communications Protocol (Highway Addressable Remote Transducer Protocol) is an early implementof Fieldbus, a digital industrial automation protocol. Its most notable advantage is that it can communicate over legacy 4-20 mAanalog instrumentation wiring, sharing the pair of wires used by the older system. According to Emerson,[1] dueto the huge installed base of 4-20 mA systems throughout the world, the HART Protocol is one of the most poindustrial protocols today. HART protocol has made a good transition protocol for users who were comfortable usinlegacy 4-20 mA signals, but wanted to implement a "smart" protocol. Industries seem to be using Profibus DP/PAFoundation fieldbus (also by Rosemount) more as users become familiar with later technology and look to advantage of the enhanced diagnostics they can provide.

The protocol was developed by Rosemount Inc., built off the Bell 202early communications standard, in the mid-198as proprietary digital communication protocol for their smart field instruments. Soon it evolved into HART. In 19was made an open protocol. Since then, the capabilities of the protocol have been enhanced by successive revisions tospecification.

ModesThere are two main operational modes of HART instruments: analog/digital mode, and multidrop mode.

In point-to-point mode (analog/digital) the digital signals are overlaid on the4-20 mA loop current. Both the 4-20 mAcurrent and the digital signal are valid output values from the instrument. The polling address of the instrument is set t"0". Only one instrument can be put on each instrument cable signal pair. One signal, generally specified by the usspecified to be the 4-20 mA signal. Other signals are sent digitally on top of the 4-20 mA signal. For example, precan be sent as 4-20 mA, representing a range of pressures, and temperature can be sent digitally over the same wir point-to-point mode, the digital part of the HART pr

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