Gigabit Ethernet, & 1000BASE-T

Gigabit Ethernet, especially in the form of 1000BASE-T wired connectivity allows data to be transferred at rates of 1 Gb per second enabling high speed transfer of data.


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Gigabit Ethernet, 1GE, is the next development of the Ethernet standard beyond the popular 100 Mbps Ethernet that was often used in the form of100BASE-T version.

As the name suggests, Gigabit Ethernet, 1GE, 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) Ethernet cables that are widely and cheaply available.

Whilst the copper wire version of 1Gb Ethernet is the most commonly known, fibre versions are also available and widely used, particularly in local area networks for home and business applications.

Gigabit Ethernet is now the main standard for new equipment like computers as well as small business and home Internet routers and Ethernet switches offering speed improvements over previous versions that make it compatible with the requirements of the faster data transfers needed today.

1Gb Ethernet, Gigabit Ethernet, 1000BASE-T used in local area networks and wide area networks, i.e. WAN networks

Gigabit Ethernet, 1GE 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 requirements for local area networks, wide area networks and the like.

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. The Gigabit Ethernet standard had three different types as the need for it increased:

  • IEEE 802.3z:   This was the initial standard for Gigabit Ethernet and it was released by the IEEE in June 1998. It is often called 1000BASE-X - the -X stands for a number of versions including: CX, -SX, -LX, or even -ZX, this last one being non-standard.
  • IEEE 802.3ab:   This next gigabit standard was released in 1999 and defined Gigabit Ethernet transmission over unshielded twisted pair (UTP) category 5, 5e or 6 cabling. It became known as 1000BASE-T, and it opened up the use of 1Gb Ethernet for common usage with desktop computers, printers, routers and Ethernet switches. It lead to 1Gb Ethernet becoming the standard for copper based Ethernet connectivity.
  • IEEE 802.3ah   This later standard was released in 2004 and it added two more fibre based versions of Gigabit Ethernet: 1000BASE-BX10, and also 1000BASE-LX10 which was already widely implemented as vendor specific extension.

Initially Gigabit Ethernet was deployed to provide high capacity backbone links for high performance local area networks as well as wide area, WAN networks. However, just after the turn of the century, computers started to incorporate the 1000BASE-T technology and this triggered its widespread usage. Ethernet switches, routers, printers and other peripherals also became more widely available and costs fell.

Gigabit Ethernet versions

The Gigabit Ethernet standard accommodates data being sent over a variety of media. Each one is denoted by the version indicator: 1000BASE-nn, where nn consists of one or two letters determining the media. 1000BASE-T is possibly the most widely viewed.

1 Gb Ethernet Versions and Details
 
1 Gb Ethernet Version Details
 
1 Gb Ethernet over copper wire
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 and today it is rarely used.
1000BASE-T 1 Gb Ethernet over twinax cables as defined in IEEE 802.3ab. Maximum distance is normally 300 metres. The standard Cat 5 STP or UTP Ethernet cables are able to carry this - Cat 5e is better. Cat 6, Cat 6a and Cat 7 etc Ethernet cables can also be used and offer better performance.
1000BASE-T1 This is a form of 1000BASE-T that uses single balanced twisted pair cables and can operate over distances of up to around 15 metres.
1000BASE-TX This is a form of 1000BASE-T that uses twisted pair Cat 6 or Cat 7.
 
1 Gb Ethernet over Fibre
1000BASE-BX Fibre based version of 1G Ethernet that uses a transmission optical wavelength of 1260 - 1360 nm and a receive wavelength of 1480 - 1500 nm.
1000BASE-EX A proprietary version, i.e. not supported by the IEEE standard, of 1GE that uses fibre media with a wavelength of 1310 nm.
1000BASE-KX A version of 1Gb Ethernet used for copper based PCB backplanes,
1000BASE-LX This is a fibre optic version that uses a long wavelength
1000BASE-SX This is a fibre optic version of the standard that operates over multi-mode fibre using a 850 nanometer, near infrared (NIR) light wavelength

Gigabit Ethernet basics

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:   Traditionally Ethernet has had the capability to run both full and half duplex operation. With half duplex, the system can transmit and receive in both directions, but not at the same time. With full duplex, different lines are used for communication in different directions - for example when using copper systems, different wire pairs are used for transmitting in different directions.
  • Use the 802.3 Ethernet frame formats:   The standard frame formats for Ethernet are well established and provide a straightforward yet effective way of formatting the data to be sent. Keeping the same Ethernet frame format makes backwards compatibility much easier as well as many integrations with other systems that have relied on the Ethernet frame structure.
  • Use the CSMA/CD access method with support for one repeater per collision domain   The CSMA/CD anti-collision scheme was well established and has worked well.
  • Provide backward compatibility with 10BASE-T and 100BASE-T technologies:   The 10 and 100 Mbps Ethernet systems were in widespread use when 1 Gb Ethernet was introduced. Maintaining backwards compatibility was key to the adoption of 1GE..

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.

1000BASE-T operation

One of the most popular forms of gigabit Ethernet is the one that runs over copper wire - Cat5, Cat6 or Cat 7 Ethernet cables. As a result there is a very good selection of IT items that use it: computers, peripherals, Ethernet switches, Ethernet routers and many more items.

In order to enable Gigabit Ethernet, 1000BASE-T to operate over standard Cat 5 or similar cables, the transmission techniques employed operate in a slightly different way to that employed by either 10BASE-T or 100BASE-T. The way that was devised was ingenious and not only enabled backwards compatibility, but also allowed the continued use of the existing cable types and Ethernet cables that may already have been installed in various local area networks and other installations.

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

The data is transmitted along the twisted pair wires within the Ethernet cable. 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. Similarly the scheme provides noise immunity on reception as well. From the table below the lines are labelled RD for received data and TD for transmitted data.

Technique used for sending data over 1000BASE-T 1Gb Ethernet
Cancellation technique used for sending data over 1000BASE-T 1 Gb Ethernet

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 1000BASE-T, 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 signal lines x 2 bits per signal = 1000Mbps. Rather than increasing each signal line data rate, more data is transmitted by carrying it in more ways.

A further voltage is used for error correction. Although the twisted pairs and differential signalling minimise data errors, these are still possible, especially when the lengths are longer

Although the same cables are used for Gigabit Ethernet, the designations for the individual lines in the Gigabit Ethernet cable are 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.


1 Gb Ethernet 1000BASE-T Pin / Line Functions
 
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

Gigabit Ethernet is rapidly becoming an accepted standard not just for use for high speed links in local area networks and wide area networks, but also for standard links between PCs, routers, network switches, and other peripherals. Many PCs have Gigabit Ethernet fitted as standard and this also means that networks require to use Gigabit network 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

In order to obtain the optimum performance and set up the system to ensure it works correctly there are a few practical points that are often asked.

  • Cables:   Gigabit Ethernet, 1000BASE T has been developed with the idea of using ordinary Cat 5 Ethernet cables. However several companies recommend the use of higher spec Cat 5e or Cat 6 and 7 Ethernet cables now that they are available. Although they are slightly more expensive, the higher specification Ethernet 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 the screened and geenrally higher performance cables may improve performance, particularly when used in a noisy electrical environment, or over longer runs. The cable type is printed on the cable, so it is possible to determine what type it is.
  • Connecting slower devices:   Fortunately 1 Gb Ethernet, like other Ethernet standards is backwards compatible. Normally new computers, routers and Ethernet switches will be fitted with 1 Gb Ethernet, but many older ones may not. When there is a mix of Gigabit Ethernet and older versions like 100 or even 10 Mbps Ethernet, the system will send data at the rate that the slowest device can accommodate. There won't be any data loss, but things may appear a bit slow when talking to older devices over 10 Mbps or 100 Mbps links.
  • How can you tell is a device supports 1Gb Ethernet:   Often there will be a marking by the Ethernet port saying something like 1GE, Gigabit Ethernet, Gb Ethernet or the like. As can be seen from the image below, the marking says "Gig Ethernet." Where there is no marking, it is necessary to resort to the specification to see, or check the details via the network list on the computer..
    Rear of typical small router for use in home local area networks showing connections and marking for 1 Gb Ethernet
    Rear of typical home router showing connections and marking for 1 Gb Ethernet
    It is normally possible to check the speed rating of an active Ethernet network connection. To do this, find and open up the connection settings on the client device. From here it is normally possible to check the speed by right clicking on the device in question.

Gigabit Ethernet is now accepted as the main standard for Ethernet connections on devices like computers, printers, home and office broadband routers, network switches and the like. Using Gigabit Ethernet provides the speed needed for most applications, although as the need for increased data consumption and faster speeds are needed, more will want to use 10 Gb Ethernet for their local area networks. However for the moment, 1 Gb Ethernet, 1000BASE-T is by far the most widely used. Added to this, there are many products available ranging from Ethernet switches, Ethernet routers, computers and peripherals and many more items .

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