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OSI Layer 2: Data-Link Layer Protocols and Applications

The following field in both formats is the Data field. The only difference

is that Ethernet uses a variable byte size between 46 and 1500 for data. This
data is what will be handed to the upper-layer protocols. IEEE 802.3 uses a
variable byte size between 46 and 1500 as well, but the information here
contains the 802.2 header and the encapsulated data that will eventually be
passed to an upper-layer protocol that is defined within the Data field.

Finally, you have the last field, which is the Frame Check Sequence (FCS)

field. It is a four-byte long field that is used to store information that will be
used for calculating the CRC after the data has been sent or received.

Token Ring/IEEE 802.5

Token Ring and IEEE 802.5 have the same relationship as Ethernet and IEEE
802.3. However, in this case, the IEEE 802.5 specification follows IBM's
Token Ring much more closely. Both implementations specify baseband sig-
naling, token passing, and data rates. Token Ring is IBM's token-passing
LAN technology. It has bandwidth capabilities of either 4Mbps or 16Mbps
in a ring topology.

Token Ring/IEEE 802.5 differ greatly from Ethernet/IEEE 802.3. Ether-

net/IEEE 802.3 are CSMA/CD LANs, whereas Token Ring/IEEE 802.5 are
apportioned networks. Apportioned means that equal time is allotted to
every station on a ring. This is achieved by passing a token around the ring.
The next section explains in a little more detail.

Tokens

The physical design of Token Ring/IEEE 802.5 is just as the name indi-
cates--a ring. Multiple stations connect to the same ring, just as Ethernet/
IEEE 802.3 stations connect to the same segment.

The main idea behind Token Ring/IEEE 802.5 was that a station may not

transmit data onto the ring without first possessing the token. The token is
just a small frame containing control information. Use Figure 2.25 as a visual
reference for the following example. The frame or token is sent around the
ring. Each station on the ring waits its turn to receive the token. If a station
receives the token but doesn't have anything to transmit, it simply passes the
token on to the next station in line. However, if the station does have infor-
mation to send, it alters the frame, changing it into a start-of-frame identi-
fier, and then appends the data to the frame. While the token frame has been

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