Gigabit Ethernet Explained

Gigabit Ethernet is a transmission technology that can theoretically provide a data rate of one Gigabit (one billion bits per second). This is equivalents to 128 Megabytes per second. This technology is based on Ethernet protocols which are widely used in local area networks (LANs).

Ethernet works on link layer of the TCP/IP model. In the OSI model, it touches both the first (physical) and second layer (data link layer). There are two units of transmission, packet and frame.


A frame consists of a small portion of data being transmitted along with identification information of the source/destination, error-correction information, VLAN tagging, etc. Each frame is further wrapped in a packet which adds information required to establish connections and marking start/end of a frame.

Hardware/IEEE Specifications

Gigabit Ethernet was first introduced by Intel, Digital and Xerox in the early 1970s. The first Gigabit Ethernet standard was labelled 802.3z, certified by the IEEE 802.3 Committee, in 1998.

It was very quickly adopted and became widely used LAN technology systems for data sharing worldwide. As of now, it is the backbone of many large organisations.

Gigabit Ethernet, primarily, uses optical fibres for communication. In an optical fibre, data is transmitted by sending signals down very this (hair-like) strands of glass or plastic fibre.

The signal is guided across the core of the thread. Data can be transmitted using a single mode or multimode optical fibre. Single mode fibres are deployed for long-range communication while multimode fibres are deployed in places such as offices universities, etc. IEEE 802.3z incorporated 1000 BASE-SX and 1000 BASE-LX as standards for multimode and single mode fibre communication.

For very short distances, the speed of a Gigabit can be achieved using copper cable and twisted pair cable. 1000BASE-CX is the standard for copper cables included in IEEE 802.3z.

For transmission of twisted pair cable, a different encoding sequence is required defined by 1000BASE-T.

Alternate technology

Gigabit Ethernet competes with an alternative technology called asynchronous transfer mode (ATM). ATM works by organising digital data into 53-byte cell units. This ensures that all the data packets are of the same size.

These cells units are transmitted over a physical medium using a predetermined fixed route. Each unit is processed asynchronously relative to other units which make the implementation easier on the hardware.

The idea behind this technology is to reduce queueing delays, thus speeding up transmission data. Data transmission with ATM can also go up to one Gbps.

How fast is it in practice?

In a practical scenario, several factors play a role in the speed of data transmission such as network protocol overhead, re-transmissions due to collisions, etc. These factors slow down the data transmission speed. Under normal conditions, you can reach effective data transfer rate of 900 Mbps for brief periods.

Other factors that slow down network speed include compatibility of your computers. For instance, spinning rate traditional hard drives range between 5400 and 9600 rpm (rotations per minute). Thus, they can handle a data transfer rate between 25 and 100 MBps.

Efficient data transfer rate also depends on users connected to the network. For instance, if five users are connected to a gigabit ethernet network, theoretical speed per user would be approximately 200 Mbps.


Connecting Slow Devices to Gigabit Ethernet

Gigabit Ethernet provides backward compatibility to older 100 Mbps and 10 Mbps network equipment. So, if you plug a device that supports 100 Mbps Ethernet into a gigabit-capable port, you will be able to work on the slower speed.

Similarly, if you connect a gigabit enabled device into a slower network, you will experience slower transmission rates. Nowadays, all new broadband routers come with support for Gigabit Ethernet.


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