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Packets, Frames and Error Detection

Based on Chapter 7 in Computer Networks and Internets (Comer,

Third Edition)

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Modem error-checking protocols

• Data is chunked into "blocks" of a certain byte size and sent to the destination modem.

• The destination modem checks each block for errors and returns – ACK if no errors are found – NAK if errors are found, which leads to a

retransmission.

• The kind of checking (checksum or cyclic redundancy checking) varies from protocol to protocol.

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ASCII

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Error Detection• If certain timing restrictions of the protocol are not

met, the receiver may not be able to interpret the signal that arrived as binary data – Recall that in the RS232 protocol two 1’s is distinguished

from one 1 by the length of time the signal is at –15 V

• But other problems can yield a signal which can be interpreted as binary data — but not the binary data sent.

• For this reason, protocols include error detection schemes (like parity, checksums or cyclic redundancy checks) to validate the data received.

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Error Detection (Cont.)

• Error detection schemes involve transmitting additional information which the receiver can use to validate the data.

• Error detection can inform the receiver of the presence of an error but cannot fix the error, the package must be resent.

• Error correction schemes (which are distinct) attempt to pinpoint the error.

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Parity

• Parity is a simple error detection scheme.

• The transmitter and receiver agree ahead of time on the type of parity they will use (even or odd). Let us assume even.

• The transmitter takes a string of binary data and calculates whether the number of 1’s in the string is even or odd.

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Parity (Cont.)

• The transmitter includes an extra bit: – 0 if the number of data-bit 1’s is even; – 1 if the number of data-bits 1’s is odd.

• The string consisting of the data plus parity bit is now guaranteed to have an even number of 1’s.

• The receiver checks that the data plus parity bit have even parity (an even number of 1’s).

• If the data received has odd parity, some transmission error must have occurred.

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Parity (Cont.)

D D D D D D D D P

0 0 0 1 1 1 0 0 1

0 0 1 0 1 0 0 0 0

0 1 0 1 1 1 0 0 ?

1 1 1 1 0 0 0 1 ?

D – data P – parity

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Parity (Cont.)

• (You may read that the number of 1’s is counted, this is not true and is unnecessary complicated.)

• A multi-input XOR (excluded OR) gate will determine the parity bit, this circuitry is much simpler than what is required for counting the number of 1’s.

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2-input and 3-input XOR Truth Tables

A B Output

0 0 0

0 1 1

1 0 1

1 1 0

A B C Output

0 0 0 0

0 0 1 1

0 1 0 1

0 1 1 0

1 0 0 1

1 0 1 0

1 1 0 0

1 1 1 1

Output is true when number of inputs is odd.

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Parity (Cont.)

• Parity is used when data is moved around within computers as well as when data is sent from computer to computer.

• Modems use parity. Users may also select no parity, meaning that the modems will not include the parity bit. Sending fewer bits speeds up transmission.

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Checksum• The transmission string of data bits is broken up

into units, for instance of consisting of 16 bits each. • These 16-bit numbers are then added (any carries, if

the sum is larger than a 16-bit number, are added to the sum).

• The sum is sent along as part of the frame’s trailer field.

• The receiver repeats the calculation. If the sums match, it's assumed that no transmission error occurred.

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Checksum (Cont.)

• Actually instead of including the sum in the trailer, one can transmit instead its negative.

• The receiver then repeats the sum over the data and includes in the summation the negative of the sum sent in the trailer.

• The result should be zero. It is easier to test for a result of all zeros.

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Catching errors

• Since the 16-bit string might end up in any of 216 (65536) possible states, you might assume that the checksum would catch errors 65535/65536 99.9985% of the time.

• But this would assume that all possible errors are equally likely, which is not true.

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Burst errors

• For multi-bit errors, it is more likely that a group of consecutive bits are affected as opposed to randomly selected bits, such an error is known as a burst error.

• It is also common for periodic errors to occur (e.g. the first two bits in every byte).

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CRC

• Cyclic Redundancy Check is better at catching burst errors than checksums.

• The idea is the same, perform some mathematical operation on the data, send the result, have the receiver do the same calculation and check that the same answer is obtained.

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CRC (Cont.)

• In CRC the operation is somewhat like division – If one thinks of the data as a large number, one

can divide it by another number N giving a whole number answer and a remainder.

– The remainder could be any number between 0 and N-1.

– The problem here is that division is a fairly difficult computation (more circuitry and a slow process).

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CRC (Cont.)

• CRC uses a variation on division that while mathematically abstract is very simple to build a circuit for.

• The circuitry needs only a shift register and some (2-input) XOR gates.

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Shift Register

• A register is a small piece of memory that holds values.

• In addition to holding values, a shift register performs a simple operation on the values; it moves them to the left to to the right.

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Shift Register

0 1 0 1 0 1 1 1

1 0 1 0 1 1 1 0

0 1 0 1 1 1 0 1

1 0 1 1 1 0 1 0

InputOutput Shift register

time

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How shift registers are used?

• Multiplication

• Adding floats

• Converting Parallel Data (the form inside the computer) to Serial (the form sent over transmission lines)

• Cyclic Redundancy Check (CRC)

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CRC: Shift register + XORs

Excluded Or gates

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CRC (Cont.)

• Again the 16-bit string might end up in any of 216 (65536) possible states, so you might expect that CRC would catch errors 65535/65536 99.9985% of the time, same as a 16-bit checksum.

• But CRC is better at detecting burst errors which are more likely than purely random errors.

• The positions of the XORs are important in determining what kinds of burst errors are detected.

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CRC: Transmission Only

• The data must be serialized for a Cyclic Redundancy check.

• This is fine for transmission error checking, since the data was serialized for transmission.

• However, serializing the data would waste a lot of time if the data were in a parallel form (as it is inside the computer).

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Topology

• The word “topology” is used here to convey that we are not interested in the spatial arrangement of the computers (where they are located) but rather in how they are connected. – E.g. Is Computer A directly connected to

Computer B?

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Topologically Equivalent

A

B

C

B

A

CD

D

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Point-to-Point

• One way to organize a network would be to have a connection between every pair of computers.

• Known as – Point-to-point network– Mesh network– Fully connected network

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Point-to-Point

• This is fine for small networks, in fact it can provide: – Freedom in hardware choice – Freedom in protocol choice

• Each pair can choose a protocol

– Security (communication channels are not shared)

– Robustness (one broken cable does not bring down the network)

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Point-to-Point (Cont.)

• The problem with this approach is that it does not scale.

• Scaling refers to changing the size of something.

• Something is said “to scale” if the change in size requires a minimal amount of modifications.

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Point-to-Point

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Number of Connections in a Point-to-Point Network

Number of Computers Number of connections

2 1

3 3

4 6

5 10

6 15

7 21

8 28

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Adding another

• In the point-to-point approach, adding a 9th

computer to a network of eight requires eight new connections, the new computer must be connected to the eight old computers.

• The computers will soon run out of ports (slots/cards) for these connections and there will be a massive tangle of wire.

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Star Topology

• In the star topology, the computers are connected to a hub.

• Generally, a “hub” is the central part of a wheel, where the spokes come together (the part covered by a hubcap).

• A hub is a device that computers are connected to so they can communicate.

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Star Topology (Fig. 8.3)

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Computers are not necessarily arranged in a circle (topology)

Hub

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Star Topology (Cont.)

• The star topology is scalable, physically adding a computer to a network requires only connecting it to the hub.

• It is also robust, losing one connection does not affect the entire network.

• (Losing power to the hub is another story.)

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Ring Topology

• Each computer in the network is connected to two others.

• The connections form the perimeter of a circle.

• If one followed the connections around, one would visit each computer once and only once before completing the round trip.

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Ring Topology (Fig. 8.4)

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Additions to the ring

• To add a computer to a pre-existing ring, one of the previous connections must be broken, and two new connections added.

• The ring topology is not very robust, if one connection is disrupted, then the ring is broken.

• The connection to the ring is designed so that turning off a computer does not break the ring.

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Bus topology

• The individual computers connect to a common transmission path called the bus (a.k.a. the backbone).

• Ideally, the information is placed on the bus and is only read by the intended receiver.

• Much of the information moving around inside a computer travels on buses.

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Bus Topology (Fig 8.5)

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Bus Topology (Cont.)

• To add a computer to such a network, the computer must be connected to the bus.

• If a connection from a computer to bus is broken, it does not necessarily affect the rest of the network.

• If the bus connection is broken, the network is divided into two parts. – Actually it is worse than that, buses carry signals,

which are waves. And waves tend to reflect off of surfaces (ends). The ends of the bus must be treated to prevent the undesired reflection.

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Ethernet

• A fairly standard way to connect computers is Ethernet. – It originally used the bus topology.

• Strictly speaking Ethernet is the predecessor to the IEEE’s 802.3 CSMA/CD local area network standard.

• There are a few important differences; however, the latter has also come to be known as Ethernet.

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“Ether” (Origin of the term)

• Water and sound waves require a medium, some material through which the disturbance moves

• Originally it was thought that electromagnetic waves (light included) must also have a medium, it was called “the ether.”

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Ether (Origin of the term)• Searches for the ether

(esp. by Michelson and Morley) could not find it.– The first two computers on

an ethernet were two Altos called Michelson and Morley.

• The absence of the ether was important in Einstein’s development of relativity theory.

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ALOHA net

• Predecessor to Ethernet

• A radio-based communications system connecting the Hawaiian islands, invented by Norm Abramson.

• The first wireless packet-switching network.

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ALOHA net (Cont.)• Occasionally two packets would be transmitted

simultaneously, they would interfere with each other and the information would be corrupted.

• ALOHAnet had a protocol, in which the receiver would perform an error check on each packet and transmit an acknowledgment if the packet arrived error free.

• If the transmitting computer did not receive such an acknowledgment, it would retransmit after waiting a random amount of time.

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Improving the protocol

• Bob Metcalfe improved upon this protocol. – Metcalfe (MIT, Xerox) eventually started 3Com:

Computer Communication Compatibility.

• Before sending a packet, the transmitter “listens” for any traffic on the line and only transmits if there is none.

• Of course, two distance transmitters might “sense” a quiet or idle carrier and decide to transmit packets simultaneously or nearly simultaneously

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Collisions

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Collision: If a body meet a body• If two packets are sent on an Ethernet simultaneously, it

is known as a “collision.”• When a collision is detected, the packets involved are

discarded. • Each transmitter waits a random amount of time chosen

from a certain interval and retransmits.• If a second collision occurs, the process is repeated but

the interval is doubled in size. • This process, known as binary exponential backoff,

makes it very unlikely the packets will continue to collide.

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CSMA/CD

• This protocol goes by the name Carrier Sense Multiple Access/Collision Detect (CSMA/CD).

• Carrier Sense: waits for line to be idle.• Multiple Access: various nodes share a

common transmission line.• Collision Detect: simultaneously

transmitted packets are detected, discarded and retransmitted.

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Ethernet (Cont.)• What has come to be called Ethernet is the

technology most often used for LANs. • It uses a bus topology.

– More about logical topology versus physical topology another time

• Ethernet was originally developed by Xerox and then extended by DEC, Intel and Xerox (DIX Ethernet).

• It was then specified in a standard, IEEE 802.3.

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IEEE

• Institute of Electrical and Electronics Engineers

• Self-described as “the world's largest technical professional society — promoting the development and application of electrotechnology and allied sciences for the benefit of humanity, the advancement of the profession, and the well-being of our members."

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IEEE (Cont.)

• It is a professional organization for electrical and electronic engineers.

• They hold meetings, publish journals and so on. • Their importance here is that they also develop

and promote standards, both national and international.

• The importance of standards cannot be over-emphasized.

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Manchester encoding• One aspect of the standard is the way in which bits are

represented. • Recall in the RS232 standard, the bit value is

represented by the voltage’s level (high for 0, low for 1).

• But Ethernet uses Manchester encoding in which a rising voltage (in the middle of a time interval) corresponds to a 1 and a falling voltage corresponds to a 0.

• The hardware is said to be “edge triggered” — it detects changes more easily than levels.

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Timing• Pro: Because the signal is guaranteed to vary at

least once each interval, it helps in maintaining the timing between transmitter and receiver.

• Pro: Because the important part of the signal is in the middle of the interval, other parts of the signal can effectively be ignored and noise during those times is irrelevant.

• Con: The signal has to be able to change at a rate twice as fast as the information rate (so one’s not using the full capacity of the channel).

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Manchester encodingOne interval

Rising in middle of interval

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Wiring an Ethernet

• A LAN using Ethernet generally uses coaxial cable or special grades of twisted pair wires.

• The most commonly installed Ethernet systems today are called 10BASE-T and provide transmission speeds up to 10 Mbps.– M stands for mega (1,000,000)

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Ethernet (Cont.)

• Fast Ethernet or 100BASE-T provides transmission speeds up to 100 megabits per second and is typically used for 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 or 1 billion bits per second).

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10BASE-T

• One of several physical media standardized for use in Ethernet LANs.

• Is somewhat like ordinary telephone twisted pair, but has more twists because the signals are more sensitive to interference.

• Supports 10 Mbps transmission speed.

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10BASE-T

• The "10" in the media type designation refers to the transmission speed of 10 Mbps.

• The "BASE" refers to baseband signaling, which means that only Ethernet signals are carried on the medium.

• The "T" represents twisted-pair.

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Token Ring Network

• A LAN organized in the ring (or possibly star) topology.

• The protocol used is a “token-passing” scheme.

• The token gives a transmitter permission to transmit, thereby preventing packet collisions.

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Token ring protocol

• A special frame called the token is passed around the ring, a given node is allowed to transmit when it “has the token.”

• If a node has no data to transmit, it simply passes the token on to the next computer.

• If it has data to transmit, it inserts that data into the frame along with address information and other protocol information and then alters the frame so that the next computer does not have permission to transmit.

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Token ring protocol (Cont.)• If a node receives a frame with data, it looks at the

address information. If the destination does not match the computer’s address, it simply passes the frame to the adjacent node. If the destination matches the computer’s address, it copies the data, but also passes it on (perhaps adding some acknowledgement).

• The frame eventually reaches its point of origin, the original transmitter removes the packet and puts a token frame onto the ring, giving permission to the next node to transmit.

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Token ring (Cont.)

• Tokens can also be used on bus-organized LANs.

• The standard token ring protocol IEEE 802.5.

• The Fiber Distributed-Data Interface (FDDI) also uses a token ring protocol.

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Other references

• http://www.whatis.com

• http://www.webopedia.com

• Understanding Data Communications and Networks, William Shay

• Computer Dictionary, Mitchell Shnier

• http://www.pbs.org/opb/nerds2.0.1/