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being superseded as 10 Gigabit Ethernet is available and being widely used.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.
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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.
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