Compatibility Between Gigabit & Fast Ethernet


Compatibility issues between Gigabit and Fast Ethernet segments of a network can cause data loss and transmission errors. A Gigabit interface communicating at full speed transmits data at too great a rate for Fast Ethernet interfaces to handle, forcing the slower interface to discard packets in an attempt to keep up. However, Gigabit interfaces are usually able to reduce their transfer rates to allow communication with devices using the Fast Ethernet standard.

Two Types of Ethernet

  • The Gigabit and Fast Ethernet standards are fundamentally very similar, with both able to run on identical copper cables and both using identical plugs to connect devices together. As such, it may not always be visually obvious which parts of a network use which standards. The principal difference between the two is one of data transfer rate. Fast Ethernet allows devices to transfer data at up to 100 Mbps, while Gigabit Ethernet allows for transfer at up to 1000 Mbps.

Data Transfer

  • The Gigabit Ethernet standard is backwardly compatible with Fast Ethernet, allowing devices using different standards to communicate on the same network. The Gigabit interface reduces its transmit and receive speeds to 100 Mbps to do this, allowing it to effectively act as a Fast Ethernet interface. Similarly, both Gigabit and Fast Ethernet interfaces are able to reduce their speed to 10 Mbps in order to communicate with devices using the original Ethernet standard; as such, the two standards are sometimes referred to as “10/100/1000 Ethernet” and “10/100 Ethernet," respectively.

Negotiating a Speed

  • The speed of a Gigabit interface can be set either manually or automatically. Managed switches and routers allow the users to define the speeds at which each interface transmits and receives data. To enable communication between Gigabit and Fast Ethernet interfaces you would select the Gigabit interface and manually instruct it to transmit at a slower rate. Alternatively, auto-negotiation sees interfaces and sends their respective speeds to each other before data transfer begins so that both interfaces know what speed to use without human intervention.

Design Issues

  • Networks with interconnecting Gigabit and Fast Ethernet sections should be designed with care, as the reduction in speed caused by Fast Ethernet can result in data bottlenecks. The rate of data transfer between two points on a network can only be as fast as the slowest component in between those points, so using Fast Ethernet on busy or centralized parts of the network could slow all traffic down to 100 Mbps even if most of the network is built on Gigabit technology.

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