Chapter: Computer Networks : Data Link Layer
Gigabit Ethernet
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).
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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