Gigabit Ethernet including 1000Base-T

- an overview of the Gigabit Ethernet standard, IEEE802.3 including the wire based 1000Base-T variant and details of Gigabit Ethernet Cables and Gigabit Ethernet Switches.

Gigabit Ethernet is the next development of the Ethernet standard beyond the popular 100Base-T version. As the name suggests, Gigabit Ethernet allows the transfer of data at speeds of 1000 Mbps or 1Gbps. It is particularly easy to install because the 1000Base-T variant is designed to run over Cat 5 UTP (unshielded twisted pair) that is widely and cheaply available.

Initially Gigabit Ethernet was only used for applications such as backbone links within large networks, but as the technology has become more affordable it is being used more widely, and the 1000Base-T variant is often incorporated within PCs themselves. However even 1 Gigabit Ethernet is being superseded as 10 Gigabit Ethernet is available and being widely used. Despite this, the 1 Gigabit version will still be designed into new product for many years to come.

Gigabit Ethernet development

The success of the Ethernet standard has been its ability to evolve and move forward in such a way that it can keep up with or even ahead of the networking requirements for local area networks. The original development of Ethernet took place in 1970s at the Xerox Corporation. Since was launched on to the market it has steadily evolved, seeing versions including 10Base-T and later 100Base-T become networking standards.

With its success the Ethernet standard was taken over by the IEEE under their standard IEEE 802.3. Accordingly IEEE 802.3ab, which defines Gigabit Ethernet was ratified in 1999 and it became known as 1000Base-T.

Gigabit Ethernet basics

Although the 1000Base-T version of Gigabit Ethernet is probably the most widely used, the 802.3ab specification also details versions that can operate over other media:

• 1000Base-CX   This was intended for connections over short distances up to 25 metres per segment and using a balanced shielded twisted pair copper cable. However it was succeeded by 1000Base-T.



• 1000Base-LX   This is a fiber optic version that uses a long wavelength



• 1000Base-SX    This is a fiber optic version of the standard that operates over multi-mode fiber using a 850 nanometer, near infrared (NIR) light wavelength



• 1000Base-T    Also known as IEEE 802.3ab, this is a standard for Gigabit Ethernet over copper wiring, but requires Category 5 (Cat 5) cable as a minimum.

The specification for Gigabit Ethernet provides for a number of requirements to be met. These can be summarised as the points below:

• Provide for half and full duplex operation at speeds of 1000 Mbps.



• Use the 802.3 Ethernet frame formats.



• Use the CSMA/CD access method with support for one repeater per collision domain.



• Provide backward compatibility with 10BASE-T and 100BASE-T technologies.

Note on CSMA/CD: The CSMA/CD protocol used for Ethernet and a variety of other applications falls into three categories. The first is Carrier Sense. Here each station listens on the network for traffic and it can detect when the network is quiet. The second is the Multiple Access aspect where the stations are able to determine for themselves whether they should transmit. The final element is the Collision Detect element. Even though stations may find the network free, it is still possible that two stations will start to transmit at virtually the same time. If this happens then the two sets of data being transmitted will collide. If this occurs then the stations can detect this and they will stop transmitting. They then back off a random amount of time before attempting a retransmission. The random delay is important as it prevents the two stations starting to transmit together a second time.

Like 10Base-T and 100Base-T, the predecessors of Gigabit Ethernet, the system is a physical (PHY) and media access control (MAC) layer technology, specifying the Layer 2 data link layer of the OSI protocol model. It complements upper-layer protocols TCP and IP, which specify the Layer 4 transport and Layer 3 network portions and enable communications between applications.

Gigabit transport mechanism for 1000Base-T

In order to enable Gigabit Ethernet, 1000Base-T to operate over standard Cat 5 or Cat 5e cable, the transmission techniques employed operate in a slightly different way to that employed by either 10Base-T or 100Base-T. While it accomplishes this it still retains backward compatibility with the older systems.

Cat 5 cables have four sets of twisted pair wires of which only two are used for 10Base-T or 100Base-T. 1000BaseT Ethernet makes full use of the additional wires.

To see how this operates it is necessary to look at the wiring and how it is used. For 10Base-T and 100BaseT one pair of wires is used for the transmitted data and another for the received data as shown below:

Pin	Wire colour	Function
1	White + Green	+TD
2	Green	-TD
3	White + Orange	+RD
4	Blue	Not used
5	White + Blue	Not used
6	Orange	-RD
7	White + Brown	Not used
8	Brown	Not used

Wiring for Cat 5 cable used for 10 and 100 Base-T

The data is transmitted along the twisted pair wires. One wire is used for the positive and one for the negative side of the waveform, i.e. send and return. As the two signals are the inverse of each other any radiation is cancelled out. From the table the lines are labelled RD for received data and TD for transmitted data.

The Cat 5 cable used for transmitting 100BaseT Ethernet actually has a maximum clock rate of 125 Mbps. The reason for this is that the signal is coded so that 8 bits are coded into a 10 bit signal in a scheme known as 8B/10B. Thus to transmit at 100 Mbps the maximum clock rate is 125 MHz. This factor can also be used to advantage by 1000BaseT, Gigabit Ethernet.

To achieve the rate of 1000 Mbps, Gigabit Ethernet, 1000Base-T uses a variety of techniques to retain the maximum clock rate of 125 MHz while increasing the data transfer rate of a Gigabit. In this way the standard Cat 5 cable can be used as Gigabit Ethernet cable.

The first technique is that rather than using two wires to enable it to carry a signal representing a "0" or "1", it uses two sets of twisted pair and in this way four different data combinations can be transmitted: "00", "01", "10", and "11". This gives a four-fold increase in transmission speed. To give a further increase in speed by a factor of two, each twisted pair is used for transmission and reception of data, i.e. each twisted pair is bi-directional.

This method of transmission is known as 4D-PAM5, and the maximum data rate is 125 Mbps x 4 x 2 = 1000Mbps.

A further voltage is used for error correction.

Although the same cables are sued for Gigabit Ethernet, the designations for the individual lines in the Gigabit Ethernet cable is changed to map the way in which the data is carried. The letters "BI" indicate the data is bi-directional and the letters DA, DB, … etc indicate Data A, Data B, … etc.

Pin	Wire colour	Function
1	White + Green	+BI-DA
2	Green	--BI-DA
3	White + Orange	+BI-DB
4	Blue	+BI-DC
5	White + Blue	-BI-DC
6	Orange	-BI-DB
7	White + Brown	+BI-DD
8	Brown	-BI-DD

Line designations for Cat 5 Gigabit Ethernet cable

Gigabit Ethernet is rapidly becoming an accepted standard not just for use for high speed links in networks, but also for standard links between PCs and the relevant servers. Many PCs have Gigabit Ethernet fitted as standard and this also means that networks require to use Gigabit Ethernet switches, and routers, etc. However the fact that standard Cat 5 cable can be used for the 1000Base-T variant means that Gigabit Ethernet will rapidly take over from the previous variants of Ethernet, allowing speeds to be steadily increased.

Practical aspects

Gigabit Ethernet has been developed with the idea of using ordinary Cat 5 cables. However several companies recommend the use of higher spec Cat 5e cables when Gigabit Ethernet applications are envisaged. Although slightly more expensive, these Cat 5e cables offer improved crosstalk and return loss performance. This means that they are less susceptible to noise. When data is being passed at very high rates, there is always the possibility that electrical noise can cause problems. The use of Cat 5e cables may improve performance, particularly when used in a less quiet electrical environment, or over longer runs.