100Base-T Fast Ethernet / 802.3u
- an overview of the 100Base-T Ethernet standard, IEEE802.3u also known as
"Fast Ethernet" used widely for local area network, LAN applications.
100Base-T Ethernet was originally known as "Fast Ethernet"
when it was introduced, it was the fastest version of Ethernet that was
available offering a speed of 100 Mbps (12.5 MByte/s excluding 4B/5B overhead).
Now 100Base-T has been overtaken by other standards such as 1GB and more
recently 10 GB Ethernet offering speeds of 10 and 100 times that of the
100Base-T versions. Nevertheless 100Base-T is widely used for most networking
applications as it offers a performance that is more than acceptable for many
applications. Officially, the 100BASE-T standard is IEEE 802.3u.
100Base-T overview
100BaseT Ethernet, also known as Fast Ethernet is defined
under the 802.3 family of standards under 802.3u. Like other flavours of
Ethernet, 100Base-T, Fast Ethernet is a shared media LAN. All the nodes within
the network share the 100 Mbps bandwidth. Additionally it conforms to the same
basic operational techniques as used by other flavours of Ethernet. In
particular it uses the CSMA/CD access method, but there are some minor
differences in the way the overall system operates.
The designation for 100Base-T is derived from a standard
format for Ethernet connections. The first figure is the designation for the
speed in Mbps. The base indicates the system operates at baseband and the
following letters indicate the cable or transfer medium.
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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. |
There are a number of cabling versions available:
- 100Base-TX uses two pairs of Category 5 UTP *
- 100Base-T4 uses four pairs of Category 3 (now obsolete) *
- 100Base-T2 uses two pairs of Category 3 (now obsolete) *
- 100Base-FX It uses two strands of multi-mode optical fibre for receive
and transmit. Maximum length is 400 metres for half-duplex connections (to
ensure collisions are detected) or 2 kilometres for full-duplex and is
primarily intended for backbone use
- 100Base-SX It uses two strands of multi-mode optical fibre for receive
and transmit. It is a lower cost alternative to using 100Base-FX, because it
uses short wavelength optics which are significantly less expensive than the
long wavelength optics used in 100Base-FX. 100Base-SX can operate at
distances up to 300 metres
- 100Base-BX is a version of Fast Ethernet over a single strand of optical
fibre (unlike 100Base-FX, which uses a pair of fibres). Single-mode fibre is
used, along with a special multiplexer which splits the signal into transmit
and receive wavelengths.
* The segment length for a 100Base-T cable is limited to 100
metres.
Fast Ethernet data frame format
Although the frame format for sending data over an Ethernet
link does not vary considerably, there are some changes that are needed to
accommodate the different physical requirements of the various flavours. The
format adopted for Fast Ethernet, 802.3u is given below:
| PRE |
SOF |
DA |
SA |
Length / Type |
Data payload |
FCS |
| 7 |
1 |
6 |
6 |
2 |
46 - 1500 |
4 |
Fast Ethernet (802.3u) Data Frame Format
It can be seen from the diagram above that the data can be
split into several elements:
PRE This is the Preamble and it is seven
bytes long and it consists of a series of alternating ones and zeros. This warns
the receivers that a data frame is coming and it allows them to synchronise to
it.
SOF This is the Start Of Frame delimiter.
This is only one byte long and comprises a pattern of alternating ones and zeros
ending with two bits set to logical "one". This indicates that the next bit in
the frame will be the destination address.
DA This is the Destination Address and it
is six bytes in length. This identifies the receiver that should receive the
data. The left-most bit in the left-most byte of the destination address
immediately follows the SOF.
SA This is the Source Address and again it
is six bytes in length. As the name implies it identifies the source address.
Length / Type This two byte field indicates
the payload data length. It may also provide the frame ID if the frame is
assembled using an alternative format.
Data This section has a variable length
according to the amount of data in the payload. It may be anywhere between 46
and 1500 bytes. If the length of data is below 46 bytes, then dummy data is
transmitted to pad it out to reach the minimum length.
FCS This is the Frame Check Sequence which
is four bytes long. This contains a 32 bit cyclic redundancy check (CRC) that is
used for error checking.
Data transmission speed
Although the theoretical maximum data bit rate of the system
is 100 Mbps. The rate at which the payload is transferred on real networks is
far less than the theoretical maximum. This is because additional data in the
form of the header and trailer (addressing and error-detection bits) on every
packet, along with the occasional corrupted packet which needs to be re-sent
slows the data transmission. In addition to this time is lost time waiting after
each sent packet for other devices on the network to finish transmitting.
Fast Ethernet using Cat 5 cable
Fast Ethernet can be transmitted over a variety of media, but
100Base-t is the most common form and it is carried over Cat 5 cable. These
cables have four sets of twisted pair wires of which only two are used for
10Base-T or 100Base-T. For 10Base-T and 100Base-T one pair of wires is used for
the transmitted data (TD) and another for the received data (RD) as shown below.
The data is carried differentially over the wires and in this way the "+" and
"-" wires carry equal and opposite signals. As a result any radiation is
cancelled out.
| 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 100 Base-T Ethernet
Fast Ethernet Applications
Fast Ethernet in the form of 100Base-T has become one of the
most widely used forms of Ethernet. It became almost universally used for LAN
applications in view of the ease of its use and the fact that systems could
sense whether 10Base-T or 100Base-T speeds should be used. In this way 100Base-T
systems could be incorporated steadily and mixed with existing 10Base-T
equipment. The higher specification standard would be used once the two
communicating elements were both 100Base-T. In addition to this the fibre based
version is also used, but in view of the fact that Cat5 cable is so cheap and
easy to use, the wired version is more common. However the fibre version has the
advantage of being able to communicate over greater distances.
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