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Chapter: Computer Networks : Data Link Layer

Gigabit Ethernet

The need for an even higher data rate resulted in the design of the Gigabit Ethernet protocol (1000 Mbps).

Gigabit Ethernet


The need for an even higher data rate resulted in the design of the Gigabit Ethernet protocol (1000 Mbps). The IEEE committee calls the Standard 802.3z. The goals of the Gigabit Ethernet design can be summarized as follows:

1.        Upgrade the data rate to 1 Gbps.

2.        Make it compatible with Standard or Fast Ethernet.

3.        Use the same 48-bit address.

4.        Use the same frame format.

5.        Keep the same minimum and maximum frame lengths.

6.        To support autonegotiation as defined in Fast Ethernet.


1. MAC Sublayer


A main consideration in the evolution of Ethernet was to keep the MAC sublayer untouched. However, to achieve a data rate 1 Gbps, this was no longer possible. Gigabit Ethernet has two distinctive approaches for medium access: half-duplex and full-duplex. Almost all implementations of Gigabit Ethernet follow the full-duplex approach.


Full-Duplex Mode


In full-duplex mode, there is a central switch connected to all computers or other switches. In this mode, each switch has buffers for each input port in which data are stored until they are transmitted. There is no collision in this mode.


Half-Duplex Mode


Gigabit Ethernet can also be used in half-duplex mode, although it is rare. In this case, a switch can be replaced by a hub, which acts as the common cable in which a collision might occur. Three methods have been defined: traditional, carrier extension, and frame bursting.


A.  Traditional : In the traditional approach, we keep the minimum length of the frame as in traditional Ethernet (512 bits). The reduced slot time means that collision is detected 100 times earlier. This means that the maximum length of the network is 25 m. This length may be suitable if all the stations are in one room, but it may not even be long enough to connect the computers in one single office.


B. Carrier Extension: To allow for a longer network, we increase the minimum framelength. The carrier extension approach defines the minimum length of a frame as 512 bytes (4096 bits). This means that the minimum length is 8 times longer. This method forces a station to add extension bits (padding) to any frame that is less than 4096 bits.


C. Frame Bursting: Carrier extension is very inefficient if we have a series of short framesto send; each frame carries redundant data. To improve efficiency, frame bursting was proposed. Instead of adding an extension to each frame, multiple frames are sent.


2. Implementation


Gigabit Ethernet can be categorized as either a two-wire or a four-wire implementation. The two-wire implementations use fiber-optic cable (1000Base-SX, short-wave, or l000Base-LX, long-wave), or STP (1000Base-CX). The four-wire version uses category 5 twisted-pair cable (l000Base-T).

 3. Encoding


Gigabit Ethernet cannot use the Manchester encoding scheme because it involves a very high bandwidth (2 GBaud).

The two-wire implementations use an NRZ scheme, but NRZ does not self-synchronize properly. To synchronize bits, particularly at this high data rate,



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