Chapter 1 - Local Area Network Technologies

How IP Datagrams are Encapsulated

• IP datagrams are found at the OSI Network layer

• IP datagrams are sent to the DataLink Layer prior to being sent out the physical medium

• See Diagram on handout

Items Included in Data Link Layer Header and Trailer

• Delimitation – used to distinguish frames in the Data Link Layer from each other. Both a start and end of frame is used.

• Protocol Identification – identifies what particular protocol is being used (TCP/IP, IPX, etc.)

• Addressing – source node and destination node• Bit-level integrity check – to detect bit-level errors in

the frame. A checksum is used. The checksum is computed by source node and included in frame header or trailer. The destination recalculates checksum and compares result to included checksum.

LAN Encapsulation

• The way a network encapsulates data to be transmitted is called the “frame format”

• The frame format corresponds to information placed on the frame by the Data Link Layer

• The frame format consists of a header and trailer• All nodes on the same network segment bounded

by routers must use the same frame format • Each network type (Ethernet, Token Ring, FDDI)

uses a different frame format

Ethernet

• Began as a radio transmission system developed at the University of Hawaii called ALOHA

• All transmissions share a common channel• Stations contend for access to the channel in order to

transmit• Uses CSMA/CD- transmitter listens to channel to

determine when to send• 1979 – DEC, Intel, and Xerox created industry standard

10Mbps Ethernet (known as Ethernet II)• 1981 – IEEE formed the 802.3 standard to make 10

Mbps an international standard

Ethernet – Continued

• 1995 – IEEE approved a 100 Mbps version of Ethernet – referred to as Fast Ethernet – uses CAT 5 twisted pair cable

• Ethernet existed before the 802.3 standard was developed. This has resulted in multiple Ethernet standards.

• There are thus multiple ways of encapsulating data to be transmitted on an Ethernet network.

• This situation can result in two hosts on an Ethernet network segment unable to communicate with each other

Ethernet – Continued

• 10/100 Ethernet – dual-speed Ethernet – especially helpful for transitions from 10 Mbps to 100 Mbps technology

• Gigabit Ethernet – operates at 1000 Mbps

Ethernet Frame Formats

• IP datagrams use either Ethernet II (10 Mbps) or IEEE 802.3 SubNetwork Access Protocol (SNAP) encapsulation

Ethernet II

• Created by DEC, Intel, and Xerox before 802.3 specification

• Also known as Digital Intel Xerox (DIX) frame format• Frame format contains:

– Preamble– Destination Address– Source Address– EtherType– Payload– Frame Check sequence

Ethernet II Preamble

• 8 bytes long

• 7 bytes of alternating 1s and 0s (10101010) to synchronize a receiving station

• 1 byte – 10101011 to indicate the start of the frame

• The Preamble field is not visible with Network Monitor

Ethernet II Destination and Source Address

• Destination address is 6 bytes long• Destination can be unicast, multicast, or broadcast• The unicast address is the physical MAC (Media

Access Control) address• The broadcast address is all 1s (FF-FF-FF-FF-FF-

FF)• Source address is 6 bytes long and indicates

senders unicast address

Ethernet II EtherType

• 2 bytes long

• Indicates the upper layer protocol contained within the Ethernet frame.

• Acts as an identifier for the Ethernet II frame format.

• For IP datagram, this field is set to 0x0800

Ethernet II Payload

• Consists of a protocol data unit (PDU) of an upper layer protocol

• Maximum size of 1500 bytes

• Minimum size of 46 bytes – if the PDU is smaller than this, it is padded so that it is at least this size

Ethernet II Frame Check Sequence

• 4 bytes long• Provides bit-level error detection• Also called CRC – Cyclic Redundancy Check• Source node calculates Frame Check Sequence bits and

places result in the appropriate field• Destination re-calculates FCS bits and checks these with

the bits sent• If the two values match, the destination node processes the

frame• If the two values don’t match, the frame is silently

discarded

Frame Check Sequence – Cont.

• FCS is obtained by dividing by a 33-bit prime number• This prime number is divided into the number

consisting of the bits in the frame (except for the Preamble and the FCS fields)

• The remainder of this division is placed in the FCS field (32 bits)

• This process can detect 100 percent of all single-bit errors

• It is highly improbable that random noise would damage the frame and not be detected by this process

Frame Check Sequence – Cont.

• FCS does not provide security• An intermediate node could have intercepted the

frame, altered the contents, and calculated a new FCS • Use IP Security to detect the above (Chapter 20)• This process does not indicate where the error is

located or how to correct it• Other types of CRC calculations provide additional

information• The Checksum field in the ATM header provides

error detection and limited error recovery

See P. 7 for Example of Ethernet II Frame Format from Network

Monitor

Ethernet Interframe Gap

• The end of the Ethernet frame is not explicitily indicated

• An implied postamble is used that consists of a gap between each frame

• This gap is called the Ethernet interframe gap• This gap is a specified measure of time – that

needed to send 96 bits of data• It the wire goes silent for 96 bit times, the last bit

in the received frame occurred 96 bit times ago.

Ethernet Minimum Frame Size

• All frames must be at least 46 bytes in length• This is because of how the collision detection scheme

works• To detect a collision, the transmission must be long

enough for the signal indicating the collision to be propagated back to the sending node

• See Figure 1-2 of a 10Base5 Ethernet network that obeys the 5-4-3 rule:– Maximum of 5 physical segments– Maximum of four repeaters between any two nodes– Maximum of at most three segments populated

Ethernet Minimum Frame Size

• Propagation time – time for signal to propagate from one end of network to the other

• Propagation delay for this maximum extent Ethernet is 28.8 microsec.

• Slot time – time for signal to make a round trip = 28.8 * 2 = 57.6 microsec.

• In 57.6 microsec, at a transmission rate of 10Mbps, you will transmit 57.6 * 10 = 576 bits

• Thus the entire frame, including the Preamble filed must be a minimum of 576 bits (72 bytes)

• Subtract 8+6+6+2+4 = 26 yields 46 bytes

IEEE 802.3 Ethernet Frame Format

• Result of the IEEE 802.2 and 802.3 specifications

• Contains an 802.3 header and trailer as well as an 802.2 header

• See Figure 1-3 for specific fields

IEEE 802.3 Header

• Preamble – 7 bytes long consisting of alternating 1s and 0s. Used to synchronize receiving station.

• Start Delimiter – 10101011 – Indicates start of frame

• Destination & Source Address – same • Length – 2 bytes that indicate the number of bytes

from the LLC header’s first byte to the payload’s last byte – minimum of 46 and maximum of 1500

IEEE 802.2 LLC Header

• Destination Service Access Point (DSAP)- indicated the destination upper layer protocol for the frame

• Source Service Access Point (SSAP)- indicates the source upper layer protocol

Similarities between Ethernet II and IEEE 802.3 Frame Formats

• II Preamble is identical to IEEE Preamble and Start Delimiter fields

• Source Address and Destination Address are very similar

• FCS is identical

How are the Two Frame Formats Differentiated?

• To differentiate the two, examine the first 2 bytes past the Source Address field.

• If this field is > 1500, then this is an Ethernet II frame format

• If this field is <= 1500, then it is a length field and an 802.3 frame

IEEE 802.3 SNAP

• Was created as an extension of IEEE 802.3 to allow protocols that were designed for Ethernet II to be used in an IEEE 802.3 compliant environment

What Frame Format does Windows 2000 Use?

• The default frame format for Windows 2000 is the Ethernet II format

• Windows 2000 will receive both types of frame format

• You may instruct windows 2000 to send IEEE 802.3 SNAP-encapsulated frames

Details of the MAC Address

• Individual/Group Bit – indicates whether the address is individual (0) or group (1)

• Universal/Locally Administered Bit – indicates whether address is allocated by IEEE (0) or locally administered (1)

Homework

• Obtain a listing of the defined EtherType fields allowed from the web site listed in our text

• Complete reading Chapter 1 of text