Gigabit Ethernet

In this tutorial, we will be covering the concept of Gigabit ethernet in detail in Computer Networks.

Ethernet is basically a physical and data link layer technology for Local Area Network(LAN). Gigabit Ethernet was developed in June 1998.

  • Gigabit Ethernet provides the data rate of 1000 Mbps or we can say 1Gbps.

  • The Gigabit Ethernet(IEEE 802.3z) addresses the two layers of the OSI reference Model:

    • Layer 2(Data Link Layer) mainly describes how data can be organized in the form of frames and then sent over the network.

    • Layer 1(Physical Layer) mainly describes the medium of the network and signaling specifications.

Gigabit Ethernet Design

The design of the Gigabit Ethernet can be summarized with the help of the following given points:

1.It is compatible with standard or fast ethernet.

2.It provides the data rate of 1 Gbps.

3.It makes use of an 8-bit address.

4.Provides the support of auto-negotiation.

5.It keeps the same minimum and maximum lengths of frames.

MAC(Medium Access control )Sublayer

With the evolution of the Ethernet, the main concern was to keep the MAC layer untouched. But in order to achieve the data rate of 1Gbps, it was not possible.

There are two different approaches in Gigabit Ethernet in order to access the Medium:

  • Half-Duplex

  • Full-Duplex

All the implementations of the Gigabit Ethernet follow the Full-Duplex Approach.

Full-Duplex Approach

In this approach, there is a central switch that is connected to all the computers or other with other switches. Each switch has buffers for each input port where the data gets stored until the transmission.

In this CSMA/CD is not used as there occurs no collision. As there is a lack of collision thus it implies that the maximum length of the cable is determined with the signal attenuation in the cable.

Half-Duplex Approach

It is another approach used by gigabit ethernet. In this approach the switch is usually replaced by the hub, which mainly acts as the common cable; thus there are chances of the collision to occur. This approach makes use of CSMA/CD.

The maximum length in this approach mainly depends upon the minimum frame size.

Three methods defined are:

  • Traditional

  • Carrier Extension

  • Frame Bursting


In this approach, the minimum length of the frame is the same as in traditional Ethernet (i.e. 512 bits). The slot time for the gigabit ethernet is 512 bits x 1/1000µs which is equal to 0.512µs.

The reduction in the time slot means that collision is detected 100 times earlier.

Thus the maximum length of the network is 25 m. It may be suitable in the case if all the stations are in a single room, but it may not even be long enough so as to connect the computers in one single office.

Carrier Extension

In order to allow a longer network, the minimum frame length is increased. This approach mainly defines the minimum length of a frame equals 512 bytes (that is equal to 4096 bits); which means that the minimum length is 8 times longer.

This approach basically forces a station to add extension bits (i.e padding) to any frame whose size is less than 4096 bits.

Frame Bursting

The above approach i.e, Carrier Extension was not efficient in the case if we have if there is a series of short frames to send. Each frame series also carries redundant data. In order to improve the efficiency, this approach was proposed.

Instead of adding an extension to each frame, with the help of this approach, multiple frames are sent.

Physical Layer in Gigabit Ethernet

In Gigabit Ethernet the physical layer is more complicated.

The Gigabit Ethernet is mainly designed in order to connect two or more stations. In case if there are only two stations then they can be connected point-to-point.In the case of three or more stations,we can use start topology by making the use of either hub or switch at the center.

There is another possible configuration where several star topologies can be connected.


The categorization of Gigabit Ethernet can be done by using either a two-wire or four-wire implementation.

  • The two-wire implementation makes use of Fiber-optic cable(1000Base-SX, short-wave or 1000Base-LX), or STP(1000Base-CX).

  • The Four-wire version makes the use of cat-5 twisted pair cable(1000Base-T).

Name Cable Max. Segment
1000Base-CX 2 pairs of STP 25m
1000Base-SX Fibre Optics Cable 550m
1000Base-LX Fibre Optics Cable 5000m
1000Base-T 4 pairs of UTP 100m

Advantages of Gigabit Ethernet

Given below are some of the benefits of using Gigabit Ethernet:

  • There is an increase in the bandwidth and it leads to higher performance.

  • There is a low cost of acquisition and ownership.

  • It provides full compatibility.

  • By using the full-duplex approach the effective bandwidth gets virtually doubled.

  • It can transfer a large amount of data across the network quickly.

Disadvantages of Gigabit Ethernet

  • 1000Base-T requires 4 pairs of wiring in order to transfer the data.

  • 1000Base-T does not support the full-duplex mode.