Page 72 - M. Abrahim Thesis

Chapter (3) Networking Media and Data Link Layers Protocols

not particularly complicated, although some of the faster physical layer implementations are
becoming so.

Despite the basic simplicity, when a problem occurs in Ethernet, it is often quite difficult
to isolate the source of the problem. Because of the common bus architecture of Ethernet
(which can be described as a distributed single point of failure), the scope of the problem is
usually all stations within the collision domain that are attached to the segment. When repeaters
are used, this can include stations up to four segments away.

According to the rules, any station on an Ethernet network that wants to transmit a
message first listens to ensure that no other station currently is transmitting. If the cable is quiet,
the station begins transmitting immediately.
But because the electrical signal takes a small amount of time to travel down the cable
(called propagation delay), and each subsequent repeater encountered introduces a small
amount of latency in forwarding the frame from one port to the next, it is possible for more than
one station to begin transmitting at or near the same time. A collision then results.
If the attached station is operating in full duplex, the station can send and receive
simultaneously, and collisions should not be present. Full-duplex operation also changes the
timing considerations and eliminates the concept of slot time. Full-duplex operation allows for
larger network architecture designs because the timing restriction for collision detection is
removed. In half-duplex operation, assuming that a collision does not occur, the sending station
transmits 64 bits of timing synchronization information that often is known collectively as the
preamble.

The contents are as follows:

• Destination and source MAC addressing information

• Certain another header information

• The actual data payload

• A checksum (FCS) used to ensure that the message was not corrupted along the way

Stations receiving the frame recalculate the FCS to determine whether the incoming
message is valid, and hand good messages to the next higher layer in the protocol stack.

For 10-Mbps Ethernet and slower versions, which are asynchronous, each receiving
station uses the eight octets of timing information to synchronize its receive circuit to the
incoming data but then discard it. The 100 Mbps higher-speed implementations of Ethernet are
synchronous, so the timing information is not actually required at all. However, for compatibility
reasons, the preamble and SFD are present. All information following the SFD at the end of the
timing information is passed to the next higher layer.

A new checksum is calculated and compared with the checksum found at the end of the
received frame. If the frame is intact, it then must be interpreted according to the rules for
whichever protocol is indicated by the Length/Type field or the LLC-layer protocol indicated by

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