Ethernet - Computer Busses - page 427

26.5.1 Media access control (MAC)
The CSMA/CD function is implemented in the MAC layer. The functions of the MAC layers
are:

• When sending frames - receive frames from LLC; control whether the data fills the LLC

data field, if not add redundant bits; make the number of bytes an integer, and calculate
the FCS; add the preamble, SFD and address fields to the frame; send the frame to the
PLS in a serial bit stream.

• When receiving frames - receive one frame at a time from the PLS in a serial bit stream;

check whether the destination address is the same as the local node; ensure the frame
contains an integer number of bytes and the FCS is correct; remove the preamble, SFD,
address fields, FCS and remove redundant bits from the LLC data field; send the data to
the LLC.

• Avoid collisions when transmitting frames and keep the right distance between frames

by not sending when another node is sending; when the medium is free, wait a specified
period before starting to transmit.

• Handle any collision that appears by sending a jam signal; generate a random number

and back off from sending during that random time.

26.5.2 Physical signalling (PLS) and physical medium attachment (PMA)
PLS defines transmission rates, types of encoding/decoding and signalling methods. In PMA
a further definition of the transmission media is accomplished, such as coaxial, fibre or
twisted-pair. PMA and MDI together form the media attachment unit (MAU), often known
as the transceiver.

26.6 Ethernet transceivers

Ethernet requires a minimal amount of hardware. The cables used to connect it are typically
either unshielded twisted-pair cable (UTP) or coaxial cables. These cables must be termi-
nated with their characteristic impedance, which is 50 Ω for coaxial cables and 100 Ω for

UTP cables.
Each node has transmission and reception hardware to control access to the cable and
also to monitor network traffic. The transmission/reception hardware is called a transceiver
(short for trans mitter/re ceiver ) and a controller builds up and strips down the frame. For
10 Mbps Ethernet, the transceiver builds the transmitted bits at a rate of 10 Mbps - thus the
time for one bit is 1/10 10 6 , which is 0.1

s (100 ns).
The Ethernet transceiver transmits onto a single ether. When there are no nodes transmit-
ting, the voltage on the line is

× µ

0.7 V. This provides a carrier sense signal for all nodes on the
network, it is also known as the heartbeat. If a node detects this voltage then it knows that the
network is active and there are no nodes currently transmitting.
Thus, when a node wishes to transmit a message it listens for a quiet period. Then, if two
or more transmitters transmit at the same time, a collision results. When they detect a colli-
sion, each node transmits a 'jam' signal. The nodes involved in the collision then wait for a
random period of time (ranging from 10 to 90 ms) before attempting to transmit again. Each
node on the network also awaits for a retransmission. Thus, collisions are inefficient in net-
works as they stop nodes from transmitting. Transceivers normally detect collisions by moni-

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