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Overview of Line Protocols

1-3

Data Flow

When a node on an Ethernet LAN (

endstation)

transmits data, every endstation on

the LAN receives the data. Each endstation checks each data unit to see whether

the destination address matches its own address. If the addresses match, the

endstation accepts and processes the packet. If they do not match, it disregards the

packet.

Medium Access Control

Endstations use Carrier Sense Multiple Access with Collision Detection

(CSMA/CD) to monitor the medium and wait until it is idle before transmitting

data.

Carrier Sense Multiple Access

Before attempting to transmit a message, an endstation determines whether or not

another endstation is transmitting a message on the medium. If the medium is

available, the endstation transmits the message; if not, the endstation delays its

transmission until the other endstation has ﬁnished sending.

Collision Detection

If two endstations transmit data simultaneously, a collision occurs and the result is

a composite, garbled message. All endstations on the network, including the

transmitting endstations, detect the collision and ignore the message. Each

endstation that wants to transmit waits a random amount of time and then attempts

to transmit again. The random transmission delays reduce the probability that the

endstations will transmit simultaneously again.

Ethernet Frame Formats

There are two MAC-layer frame format speciﬁcations used in Ethernet LANs.

The ﬁrst speciﬁcation is called Ethernet. The second, standardized by the IEEE, is

called 802.3. One way that they differ is in message format (F

igure 1-3). Instead

of a Length ﬁeld in the MAC-layer header, Ethernet messages include a Type

ﬁeld, indicating which higher-layer protocol is used in the Data ﬁeld.

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Avaya Configuring Line Services Manual do Utilizador Página 27