most data link protocols perform the following functions:
■ Arbitration—Determines when it is appropriate to use the physical medium
■ Addressing—Ensures that the correct recipient(s) receives and processes the data that is
sent : Each data-link protocol defines its own unique addressing structure. For instance, Ethernet
uses Media Access Control (MAC) addresses, which are 6 bytes long and are represented as
a 12-digit hexadecimal number. Frame Relay typically uses a 10-bit-long address called a
data-link connection identifier (DLCI)—notice that the name even includes the phrase data
link.
■ Error detection—Determines whether the data made the trip across the physical medium successfully
■ Identification of the encapsulated data—Determines the type of header that follows the data link header
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Ethernet is best understood by first considering the early DIX Ethernet specifications, called 10BASE5 and 10BASE2.
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The CSMA/CD algorithm works like this:
1. A device with a frame to send listens until the Ethernet is not busy.
2. When the Ethernet is not busy, the sender begins sending the frame.
3. The sender listens to make sure that no collision occurred.
4. Once the senders hear the collision, they each send a jamming signal, to ensure that all
stations recognize the collision.
5. After the jamming is complete, each sender randomizes a timer and waits that long.
6. When each timer expires, the process starts over with Step 1.
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To ensure a
unique MAC address, the Ethernet card manufacturers encode the MAC address onto the
card, usually in a ROM chip. The first half of the address identifies the manufacturer of the
card. This code, which is assigned to each manufacturer by the IEEE, is called the
organizationally unique identifier (OUI). Each manufacturer assigns a MAC address with its
own OUI as the first half of the address, with the second half of the address being assigned
a number that this manufacturer has never used on another card.
An Ethernet cable between two hubs or switches often is called a trunk.
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When IP multicasts over an Ethernet, the multicast MAC addresses used
by IP follow this format: 0100.5exx.xxxx, where any value can be used in the last half
of the addresses.
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LAN Header Format
802.2 SAP and SNAP Type Fields
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The IEEE 802.3 specification limits the data portion of the 802.3 frame to a maximum of 1500 bytes. The Data field was designed to hold Layer 3 packets; the term maximum transmission unit (MTU) defines the maximum Layer 3 packet that can be sent over amedium. Because the Layer 3 packet rests inside the data portion of an Ethernet frame, 1500 bytes is the largest IP packet allowed over an Ethernet.
When the IEEE first created the 802.2 standard, anyone with a little cash could register favorite protocols is th the IEEE and receive a reserved value with which to identify those favorite protocols in he DSAP field. For instance, Novell registered IPX and was assigned hex E0 by the IEEE.
However, the IEEE did not plan for a large number of protocols—and it was wrong. As it
turns out, the 1-byte-long DSAP field is not big enough to number all the protocols.
To accommodate more protocols, the IEEE allowed the use of an extra header, called a
Subnetwork Access Protocol (SNAP) header. In the second frame of Figure 3-13, the DSAP field is AA, which implies that a SNAP header follows the 802.2 header, and the SNAP header includes a 2-byte protocol type field. The SNAP protocol type field is used for the same purpose as the DSAP field, but because it is 2 bytes long, all the possible protocols can be identified .
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The IEEE defines Gigabit Ethernet in standards 802.3z for optical cabling and 802.3ab for
electrical cabling.
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PP و HDLC
