Introduction to Computer Networking
Ethernet Technologies
This lecture is divided into hyperlinked sections
Introduction
Coaxial Versions of
Legacy Ethernet
10BASE-T - Legacy
using
Twisted Pair Cabling
Commonality in Legacy Ethernet
Ethernet Frame Components
Digital
Encoding
of Digital Data
10 BASE-T goes Full-Duplex
100 Mbps Ethernet
4B/ 5B Encoding
100 BASE-TX
100 BASE-FX
Gigabit Ethernet (1000 BASE-X)
1000 BASE-T
1000 BASE-SX and 1000 BASE-LX
10 Gbps Ethernet
Conclusion
Introduction
Ethernet and the IEEE 802.3 protocols have become
the world's most widespread LAN technologies. One of the reasons for
this
is the ease of installation and the evolution of this technology as
user
needs have changed since its inception. As a consequence, the expertise
required to install and maintain Ethernet LANs is widespread and this
helps to maintain its market position.
In this lecture we shall examine the different
Ethernet technologies available.
Firstly we shall examine
older Ethernet
technologies and see the common features between
implementations
Then we will examine 100 Mbps Ethernet and see the changes necessary to
implement
this technology.
We will see the differences and similarities involved with Gigabit
Ethernet
and finally look at the implications of 10 Gbps Ethernet.
Coaxial
Versions
of Legacy Ethernet
We shall not dwell long on the older Ethernet technologies. These used
coaxial cable as the physical layer and the cabling scheme in use was
the bus topology. This meant that a length of coaxial cable would be
laid around the area in which the LAN was to be installed and the LAN
nodes would be directly connected to the cable. The signalling used in
the early installations (10BASE5 and 10BASE2)
was baseband i.e. digital meaning that only one signal could exist on
the
bus cabling at any one time. The distance that the LAN could cover
(without
repeaters to return the signals to full strength) dropped from 500m to
185m
as Ethernet migrated from 10BASE5 to 10BASE2.
10BASE-T Legacy
using
Twisted Pair Cabling
The next Ethernet technology available was 10BASE-T. This made a change
from the coaxial medium used up to this point and began to use
Unshielded Twisted Pair (UTP)
copper cabling. Unfortunately, the maximum length (without repeaters)
of a network
segment became reduced to 100m and thus the need for repeaters (two
port
devices) and multiport repeaters (hubs) was born. This allowed 10BASE-T
networks
to increase their coverage.
UTP cabling is much easier to install then coaxial
cabling due to its flexibility and ease of connection and soon many
LANs were rewired to take advantage of this new technology. Although
10BASE-T could use Category 3 cabling, many LAN owners used Cat 5
cabling as their physical layer.
It is instructive to note that this was a good policy because when
higher data rate Ethernet (100 Mbps) became available a few years
later, it was just
the NICs and hubs that would need replacing and the cabling could
remain untouched.
This made for a large cash saving whan migration occurred. In general,
it is much more
expensive to pay for the labour when a cable is laid than the cost
of the cable itself!
As LANs became more popular, users began to take advantage of the power
of a LAN and the data traffic increased on LANs. The broadcast nature
of
Ethernet became a serious limitation to its operation and a solution
was
sought. Salvation came in the form of the Layer 2 switch. This allowed
point-to-point
links between communicating devices and freed precious network
bandwidth
for the use of other user on the LAN, however Layer 2
switches remained extremely expensive until the turn of the century.
Commonality in Legacy
Ethernet
These three older technologies (10BASE5, 10BASE2, 10BASET) shared the
10 Mbps data rate and this allowed re-use of the:
Timing parameters;
Frame format;
Transmission process;
Basic Design Rules.
The table below shows some of the parameters
associated with 10 Mbps Ethernet.
Parameter
|
Value
|
Bit
time
|
100
nanoseconds
|
Slot
time
|
512
bit times = 512 x 100 x 109 = 51.2 microseconds
|
Interframe
spacing
|
96
bits = 9.6 microseconds
|
Collision
attempt limit
|
16
|
Collision
backoff limit
|
10
|
Collision
jam size
|
32
bits = 3.2 microseconds
|
Maximum
frame size
|
1518
bytes
|
Minimum
frame size
|
64
bytes
|
Ethernet
Frame
Components
The size of an Ethernet frame is fixed to lie between 64 and 1518
bytes. The frame is a digital structure that carries the data for the
user and control information for the network. This structure therefore
carries addressing information, some control information and error
checking information. Together, these three parts occupy 18 bytes of
the frame.
Legacy Ethernet is an asynchronous technology. This means
that frames may be transmitted at any time and therefore the recieving
Network Interface Card (NIC) must be given some warning of the arrival
of an incoming frame. When a frame is transmitted on a LAN,
the receiving NIC synchronises with the arriving frame by listening for
a 7 byte preamble consisting of alternating 1s and 0s followed by a 1
byte Start of Frame field. These 8 bytes are not considered part of the
frame - they only serve to allow receiving NICs to synchronise with the
arriving data.
Below is shown the structure of an Ethernet frame for 10 Mbps Ethernet.
Preamble - 7 bytes
|
Start of Frame Delimiter - 1
byte
|
Destination -6 bytes
|
Source - 6 bytes
|
Length/ Type - 2 bytes
|
Data - 46 - 1500 bytes
|
FCS - 4 bytes
|
Shown in yellow above are the sections that compromise the frame.
The first two sections, coloured blue, are required for synchronisation
when the frame is being received.
The data that makes up the frame must be converted to electrical
signals so that it can be transmitted on the network cabling. The line
encoding used is called Manchester Encoding and also manages to carry
clocking information to the recieving NIC.
Digital
Encoding
of Digital Data
The most common, and easiest, way to transmit
digital signals is to use baseband signaling to provide two different
voltage levels for
the two binary digits. Typically, a negative voltage is used to
represent binary one and a positive voltage is used to represent binary
zero. This code is known as Nonreturn-to-Zero-Level (NRZ-L) meaning the
signal never returns to zero voltage, and the value during a bit time
is kept level
being either a negative or a positive voltage.
A significant disadvantage of NRZ transmission is
that it is difficult for the receiver to determine where one bit ends
and another begins. To picture the problem, consider that with a long
string of ones or zeros for NRZ-L, the output is a constant voltage
over a long period. Under these circumstances, any drift between the
timing of transmitter and receiver results in the loss of
synchronisation between the two.
As such, this is unsuitable for an Ethernet network.
A signaling method that carries timing information is required to help
solve this. Manchester encoding is used in Ethernet networks to carry
the signals across the wiring.
Figure depicting
various
methods available for Digital Signal Encoding
Manchester encoding overcomes the problem of timing
with long strings of 1s and 0s, using a set of alternative coding
techniques, grouped
under the term biphase. Two of these techniques, Manchester, (used in
Ethernet)
and Differential Manchester, are in common use.
All the biphase techniques require at least one
transition per bit time and may have as many as two transitions. Thus,
the maximum modulation
rate is twice that for NRZ; this means that the bandwidth required is
correspondingly
greater. To compensate for this, the biphase schemes have the following
advantages:
Synchronisation: Because there is a
predictable transition during each bit time, the receiver can
synchronise on that transition. For this reason, the biphase codes are
known as self-clocking codes.
Error detection: The absence of an expected
transition can be used to detect errors. Noise on the line would have
to invert both the signal before and after the expected transition to
cause an undetected error.
Ethernet uses the Manchester encoding system. In the
Manchester code, there is a transition at the middle of each bit
period. The mid-bit transition serves as a clocking mechanism and also
as data: A high-to-low transition represents a zero, and a low-to-high
transition represents a one.
In Differential Manchester, the mid-bit transition
is used
only to provide clocking. The encoding of a zero is represented by the
presence
of a transition at the beginning of a bit period, and a one is
represented
by the absence of a transition at the beginning of a bit period.
Differential Manchester has the added advantage of
employing differential encoding. In differential encoding, the signal
is decoded by comparing the polarity one signal element with its
preceding element rather than determining the absolute value of a
signal element. One benefit of this
scheme is that it may be more reliable to detect a transition in the
presence
of noise than to compare a value to a threshold. Another benefit is
that
with a complex transmission layout, it is easy to lose the sense of the
polarity of the signal. For example, on a multipoint twisted-pair line,
if the leads from an attached device to the twisted pair are
accidentally inverted, all ones and zeros for NRZ-L will be inverted
and the sense of the message will be lost. This cannot happen with
differential encoding.
10 BASE-T goes Full-Duplex
In 10 BASE-T networks, all cables are plugged into a hub (multiport
repeater) or a switch. The hub contains the circuitry to simulate the
shared medium and thus it is as if the bus cabling from the coaxial
implementations of Ethernet
were collapsed into the hub itself.
Originally, 10 BASE-T was specified as a half-duplex network, however
it was later redesigned to allow for full-duplex operation. It is an
asynchronous network meaning that frames may arrive at any time and
therefore there may be long inter-frame periods during which no
transmissions take place. To fill
this silence on the network cabling, link pulses are released every 125
microseconds.
On the cabling are four pairs of wires. Only two pairs are used for
communication in 10 BASE-T. One pair is used for transmission and
another pair is used for
receiving signals. This means that no collisions can take place on the
network
cabling because there are two separate transmission paths. When used in
conjunction
with a hub, the collision of two frames from different stations will
not
occur in the network cabling but within the hub itself. Therefore it is
only
possible to run a network in half duplex when using a hub.
Hubs are subject to the 5-4-3-2-1 rule that states that there can only
be a maximum of 5 network segments separated by 4 hubs. This limits the
range of coverage of the network. The rule exists to prevent violation
of the timing rules that are present to prevent collisions of frames.
This limitation can be overcome by replacing all hubs with layer 2
switches. By using switches, the network can be extended and the
5-4-3-2-1 rule can be ignored. Circuitry within the switch allows the
switch to deal with timing issues and collisions.
Switches may be daisy chained and so long as the 100m cable length
is not violated, the network may be extended as much as required.
100
Mbps
Ethernet
This is often known as Fast Ethernet. There are two implementations, a
copper-based physical layer (100BASE-TX) and a fibre based physical
layer (100BASE-FX). The 100BASE-TX solution requires the use of Cat 5
(or better) cabling and the 100BASE-FX solution requires the use of
multimode fibre optic cabling. Both implementations
share the same frame format, timing parameters and transmission process.
The table below shows parameters for 100 Mbps Ethernet
Parameter
|
Value
|
Bit
time
|
10
nanoseconds
|
Slot
time
|
512
bit times = 512 x 10 x 109 = 5.12 microseconds
|
Interframe
spacing
|
96
bits = 0.96 microseconds = 960 nanoseconds
|
Collision
attempt limit
|
16
|
Collision
backoff limit
|
10
|
Collision
jam size
|
32
bits = 0.32 microseconds = 320 nanoseconds
|
Maximum
untagged frame size
|
1518
bytes
|
Minimum
frame size
|
64
bytes
|
If we compare this to the previous table,
it
can be seen that the bit time is 1/10 th of that for 10 Mbps Ethernet.
All
other parameters that depend on the timing are reduced correspondingly
by
1/10th also, however the basis for the technology remains the same.
Because the data rate is 10 times that of 10 Mbps Ethernet, the bits
arrive 10 times faster. This approaches the frequency limit of Category
5 cabling. Manchester
encoding is no longer suitable here and a different technology has to
be
used to overcome new issues raised by the increase of data rate. These
issues
are synchronisation and signal to noise ratio (SNR). At 100 Mbps, the
SNR becomes more
of an issue because a momentary spike has a 10 fold increase in
duration compared
to 10 Mbps implementations.
4B/ 5B
Encoding
This encoding scheme takes nibbles of data (4 bits) and
converts them
to 5 bit representations.
Each group of 4 bits is encoded into a 5-bit (binary) symbols
to
guarantee that the transmitted signals will contain no more than two
consecutive
0s. This prevents errors occurring due to timing drift at the receiver.
The table below gives a list of 4B/5B Codes for frame data. Other
5-bit
groups are also used to send control information across the network
(not
shown here). The other 5 bit groups are used to
convey
control information such as start of frame, end of frame or to indicate
that
the line is idle. One symbol is transmitted during idle
conditions.
This symbol allows autosensing equipment to detect whether the network
it
is connected to is 10 or 100 Mbps. These are known as Fast Link Pulses.
| 4-bit vs. 5-bit group |
4-bit vs. 5-bit group |
0000
0001
0010
0011
0100
0101
0110
0111 |
11110
01001
10100
10101
01010
01011
01110
01111 |
1000
1001
1010
1011
1100
1101
1110
1111 |
10010
10011
10110
10111
11010
11011
11100
11101 |
Every time 4 bits of data are generated by
the user, they are converted to a 5 bit symbol. This means that to
transmit
100 Mbits, 125 Mbits need to be transmitted across the physical medium.
More
careful testing of the physical layer is required to ensure that the
cabling
is suitable to carry such frequencies.
Now we have bits to transmit, it is necessary to place tham onto the
network
cabling. This differs whether copper or fibre optic cabling is used.
100
BASE-TX
In copper based Fast Ethernet, switches must be used instead of hubs if
full
duplex operation is required. Dual speed switches (10/ 100 Mbps) were
introduced
to allow for older legacy 10 Mbps equipment to be used alongside the
faster 100
Mbps equipment.
As there are two separate data paths, no collisions will occur on the
cabling
and collisions will be mitigated in the switches. The network may be
configured
for half or full duplex operation. When using switches, all connections
will
be point-to-point and collisions are administratively detected for half
duplex
operation (even though they do not occur!) and the rules of CSMA/ CD
must
be used. In full duplex operation, both parties in the point-to-point
link
may transmit simultaneously raising the data rate to 200 Mbps. Most of
the
time it is wise to set Fast Ethernet to operate in full duplex mode.
To place the signal on the cabling, an encoding method known as MLT-3
is
used. This encoding scheme uses THREE voltage levels to convey data
across
the physical layer (+5v, 0v, -5v). To represent a 1, there is a
voltage
transition; to represent a 0 there is no transition. Hence the
occurrence
of repeated 1s will provide a transition (preventing timing drift) and
a
the maximum number of 0s that can appear together (see 4b/ 5B table
above)
is two, therefore timing drift at the receiver is unlikely over 2 bit
times.
The figure below depicts a bit stream encoded using MLT-3 encoding. If
there
is a mid-bit transition, it indicates the presence of a 1, otherwise
the
voltage remains the same indicating a 0.
100
BASE-FX
The fibre option for Fast Ethernet was created to allow for vertical
cabling
and backbone applications. It is also suitable for high-noise
environments
or areas where electricity is undesirable (e.g. where explosive
materials
are present).
100 BASE-FX uses 4B/ 5B to encode the data but for line encoding uses
NRZI
line encoding. Fibre links operate by sending pulses of light at either
high
or low (almost no light) intensity. NRZI indicates a 1 with a mid-bit
transition
(either high or low) and a 0 by no transition. The figure below shows
the
pattern generated for a particular bit pattern.
When using fibre, one strand is used for transmission of data and
another is used for
reception of data.
Gigabit Ethernet (1000 BASE-X)
This version of Ethernet transmits 10 times faster then
Fast
Ethernet. The frame format, MAC addressing and CSMA/ CD are retained,
however
there are changes to the MAC sublayer, and the physical layer. Gigabit
is
designed to operate in full duplex mode. Although designed for fibre,
it
is possible to run Gigabit over UTP (1000 BASE-T).
The table below shows the parameters for Gigabit Ethernet.
Parameter
|
Value
|
Bit
time
|
1
nanosecond
|
Slot
time
|
4096
bit times = 4096 x 1 x 109 = 4.096
microseconds
|
Interframe
spacing
|
96
bits = 96 nanoseconds
|
Collision
attempt limit
|
16
|
Collision
backoff limit
|
10
|
Collision
jam size
|
32
bits = 32 nanoseconds
|
Maximum
frame size
|
1518
bytes
|
Minimum
frame size
|
64
bytes
|
Burst
limit
|
65
536 bits = 8192 bytes
|
As the data rate is even higher now, encoding is needed to
overcome
noise and timing issues. On 1000 BASE-T networks, the encoding used is
called
8Bit-1 Quinary Quarter (8B1Q4) and the line encoding (for copper) is
known
as 4-Dimensional 5 Level Pulse Amplitude Modulation (4D-PAM5). For UTP
operation,
all 4 pairs of cable are used in parallel. In this technology, the
cables
are used in full duplex mode which requires complex decoding of
arriving
signals because both transmitted and received signals will be present
on
the wires.
1000
BASE-T
This runs Gigabit over UTP and needs to use all 4
pairs
simultaneously to manage such a high data rate. Signals to be
transmitted
are split into 4 separate paths and transmitted over the 4 pairs within
the
cabling. This means that the data rate is 250 Mbps per pair of cables.
The
4 signal paths are recombined on arrival at the destination NIC.
Full duplex operation means that there are constant collisions on the
wire
pairs, however for Gigabit, this is perfectly permissible. Complex
circuitry
in the NICs use various techniques to separate the transmitted and
received
signals. One technique known as
echo
cancellation assists in reception of signals. Another technique known
as
Forward Error Correction (FEC) helps achieve Gigabit data rates.
This complex system relies on the adherence to the cabling, termination
and
noise guidelines.
1000 BASE-SX and 1000 BASE-LX
These technologies are both fibre based. 1000
BASE-SX
uses short wavelength light (850 nm) and operates over multimode fibre.
1000
BASE-LX uses long wavelenegth light and operates over single mode
fibre.
An encoding scheme known as 8B/ 10B is used and the line encoding is
NRZ.
Here, a low level of light indicates a 0 and a high level of light
indicates
a 1. Two fibres are used, one for transmission and the other for
reception.
10
Gbps
Ethernet
This was introduced during 2002 as IEEE 802.3ae-2002.
It offers suitability for
LANs and also MANs and WANs. The frame format is still the same as
earlier
versions. The physical layer standards have changed to allow a maximum
distance
of 40 km over single mode fibre. 10 Gbps Ethernet (10 GE) is also
compatible with
SONET/ SDH networks and ATM.
All implementations of 10 Gbps Ethernet run over fibre cable, different
implementations
suiting different applications. This is full duplex only. CSMA/ CD has
finally been removed from this Ethernet specification. When 10 Gbps
Ethernet
is run over a MAN or a WAN, it is likely that the amount of data that
is
carried in a frame will be increased to take advantage of the low error
rates
associated with fibre. This will finally break the original frame
format
that can only carry 1500 bytes of user data.
Conclusion
There are many common parameters between all implementations of
10 Mbps Ethernet and the frame format used.
The frame format does not change across all Ethernet implementations up
to
10 Gbps Ethernet.
Manchester line encoding is used at 10 Mbps to
carry timing information for the benefit of the receiver.
At 100 Mbps, a technique known as 4B/ 5B is used to
provide
extra symbols for signalling across the network.
Different techniques for
line encoding are used depending whether copper or fibre is used as the
physical
layer.
At Gigabit speeds, all 4 pairs are used in full duplex mode to achieve
the data
rate.
Complex techniques are used to recover the data from the cabling.
The
line encoding varies once again depending on the physical layer used.
10 Gbps Ethernet allows Ethernet to be used across MANs and WANs.
(c) MM Clements 2010