Lecture 7
100BASE-T:
a low-cost, Ethernet-compatible 100 Mbps LAN.
All of the 100BASE-T options use the IEEE 802.3 MAC protocol and frame format
same as 10BASE-T.
100BASE-X refers to a set of options that use the physical medium specifications
originally defined for fiber distributed data interface.
All of the 100BASE-X schemes use two physical links between nodes; one for
transmission and one for reception.
100BASE-TX makes use of shielded twisted pair (STP) or high-quality (category 5)
unshielded twisted pair (UTP).
100BASE-FX uses optical fiber.
100BASE-X options require the installation of new cable so 100BASE-T4 defines a lower-cost alternative that can use category 3,
voice-grade UTP in addition to the higher-quality category 5 UTP.
To achieve the 100-Mbps data rate over lower-quality cable, 100BASE-T4 dictates
the use of four twisted-pair lines between nodes, with the data transmission
making use of three pairs in one direction at a time.
For all of the 100BASE-T options, the topology is similar to that of
10BASE-T : a star-wire topology.
100BASE-X
For all of the transmission media specified under 100BASE-X, a
unidirectional data rate of 100 Mbps is achieved transmitting over a single link
(single twisted pair, single optical fiber). For all of these media, an
efficient and effective signal encoding scheme is required.
The basis of the encoding scheme is one originally defined for FDDI and can be
referred to as 4B/5B-NRZI. To understand the significance of this choice, first
consider the simple alternative of a NRZ (nonreturn to zero) coding scheme.
With NRZ, one signal state represents binary one and one signal state represents
binary zero.
Disadvantage of NRZ : Lack of synchronization.
- Solution: Use the Manchester encoding.
Disadvantage of Manchester encoding :
-Efficiency is only 50% .
Ex: A signaling rate of 200 Mbps is needed to achieve a data
rate of 100 Mbps.
To achieve greater
efficiency, the 100BASE-X standard specifies the use of a code referred to as
4B/5B.
- Encoding is done 4 bits at a time;
- Each
4 bits of data are encoded into a symbol with 5 code bits
- A set of five code bits is known as
a code group (data code
and non-data code)
In effect, each set of 4 bits is encoded as 5 bits. The efficiency is thus
raised to 80 percent; 100 Mbps is achieved with 125 Mbps.
To ensure synchronization, each
code group of the 4B/5B stream is treated as a binary value and encoded using
nonreturn to zero inverted (NRZI)
The advantage of NRZI is that it employs differential encoding.
Differential
encoding, the signal is decoded by comparing the polarity of adjacent signal
elements rather than the absolute value of a signal element. A benefit of this
scheme is that it is generally more reliable to detect a transition in the
presence of noise and distortion than to compare a value to an absolute
threshold.
Non-data group code
• Idle. It consists of a constant flow of binary ones, which in NRZ
comes out as a continuous alternation between the two signal levels This
continuous fill pattern establishes and maintains synchronization and is used in
the CSMA/CD protocol to indicate that the share medium is idle.
• Start-of-stream delimiter.
This is used to delineate the starting boundary of a data transmission sequence;
consists of two different code groups.
• End-of-stream
delimiter. This is used to terminate normal data transmission sequences;
• Transmit
error: This code group is interpreted as a signaling error.
The 100BASE-X designation
includes two physical medium specifications, one
• for
twisted pair, known as 100BASE-TX
• for
optical fiber, known as 100-BASE-FX.
100BASE-TX makes use of two pairs of twisted-pair cables, one pair used
for transmission and one for reception ( STP or cat-5 UTP are allowed. For
transmission over twisted pair, the 4B/5B signal is subject to further encoding
to achieve desirable transmission characteristics. The following steps are
involved:
1. NRZI-to-NRZ conversion: The 4B/5B NRZI signal of the basic 100BASE-X is converted
back to NRZ.
2. Scrambling: The bit stream is scrambled
to produce a more uniform spectrum distribution for the next stage.
3. Encoder: The scrambled bit stream is encoded using a scheme
known as MLT-3.
4. Driver: The resulting encoding is transmitted.
The effect of the MLT-3
scheme is to concentrate most of the energy in the transmitted signal below 30
MHz, which reduces radiated emissions. This in turn reduces problems due to
interference. ( Note M = 2 x pi
x SQRT(L1.L2) )
100BASE-FX makes use of two
optical fibers, one for transmission and one for reception.
With 100BASE-FX, a means is
needed to convert the 4B/5B-NRZI code groups stream into optical signals.
The technique used is known
as intensity modulation.
A binary 1 is represented by
a burst or pulse of light; a binary 0 is represented by either the absence of a
light pulse or a light pulse at very low intensity.
100BASE-T4
100BASE-T4
is designed to produce a 100-Mbps data rate over lower-quality category 3 cable,
thus taking advantage of the large installed base of category 3 cable in office
buildings. (The use of category 5 cable is optional. )
100BASE-T4 does not transmit
a continuous signal between packets, which makes it useful in battery-powered
applications.
For 100BASE-T4 using
voice-grade category 3 cable, it is not reasonable to expect to achieve 100 Mbps
on a single twisted pair. Instead, 100BASE-T4 specifies that the data stream to
be transmitted is split up into three separate data streams, each with an
effective data rate of 33 Mbps.
• NRZ
encoding scheme is not used for 100BASE-T4.
• Instead,
a ternary signaling scheme is used.
• With
ternary signaling, each signal element can take on one of three
values (positive voltage, negative voltage, zero voltage).
• A
pure ternary code is not attractive for the lack of
synchronization.
However, there are schemes, referred to as block-coding methods, which approach
the efficiency of ternary and overcome this disadvantage known as 8B6T is used
for 100BASE-T4.
With 8B6T the data to be
transmitted are handled in 8-bit blocks. Each block of 8 bits is mapped into a
code group of six ternary symbols. The stream of code groups is then transmitted
in round-robin fashion across the three output channels (Figure 7.5). Thus the
ternary transmission rate on each output channel is 6/8 x 33 x 1/3 = 25 Mbps
Configuration and Operation
In its simplest form, a 100BASE-T network is configured in a star-wire
topology, with all stations connected directly to a central point referred to as
a multiport repeater.
In this configuration, the repeater has the responsibility for detecting
collisions rather than the attached stations. The repeater functions as follows:
• A valid signal appearing on any single input is repeated on all
output links.
• If two inputs occur at the same time, a jam signal is transmitted on
all links.
Thus the star-wire topology functions logically in the same manner as a bus
topology CSMA/CD network
The term collision domain is used to define a single CSMA/CD network. This means
that if two stations transmit at the same time, a collision will occur.
Stations separated by a simple multiport repeater are within the same collision
domain, whereas stations separated by a bridge are in different collision
domains.
The bridge operates in a store-and-forward fashion and therefore
participates in two CSMA/CD algorithms, one for each of the two collision
domains that it connects.
The 100BASE-T standard defines two types of repeaters.
A class I repeater can support unlike physical media segments,
such as 100BASE-T4 and 100BASE-TX.
Increased internal delay in the repeater to handle the conversion from one
signaling scheme to another.
Therefore, only a single class I repeater is used in a collision domain.
A class II repeater is limited to a single physical media type, and two class II
repeaters may be used in a single collision domain.
Full-Duplex Operation
A traditional Ethernet is
half-duplex: A station can either transmit or receive a frame, but it cannot do
both simultaneously.
With full-duplex operation, a station can transmit and receive simultaneously.
If a 100-Mbps Ethernet ran in full-duplex mode, the theoretical transfer rate
would become 200 Mbps.
Several changes are needed to operate in full-duplex mode. The attached stations
must have full-duplex rather than half-duplex adapter cards.
The central point in the star wire cannot be a simple multiport repeater but
must be some sort of switched hub, such as a bridge => each station
constitutes a separate collision domain. In fact, there are no collisions and
the CSMA/CD algorithm is no longer needed.
Full-duplex operation is currently not part of the 100-BASE-T standard but is
under consideration. However, a number of vendors offer a full-duplex Ethernet
scheme.
Auto-negotiation
is an optional capability of the 100BASE-T standard that enables two devices
connected to the same link to exchange information about their capabilities.
At a minimum it enables a device to indicate whether it operates at 100 or 10
Mbps. This capability makes it possible to implement a hub that support a
mixture of devices that conform to the various 100BASE-T and the 10BASE-T medium
options.
Auto-negotiation is performed by passing information encapsulated within a burst
of closely separated pulses known as link integrity pulses.
The pulses are defined such that a 10BASE-T receiver will recognize these as
part of a normal link maintenance procedure but not respond.
Similarly, a 100BASE-T receiver that does not implement auto-negotiation will
recognize the pulse burst as a link maintenance signal. The pulse burst is
transmitted only during idle times on the link and does not interfere with
normal traffic.
100VG-ANYLAN
• 100VG-AnyLAN
2 is the extension to the 10-Mbps Ethernet and to support IEEE 802.3 frame
types.
• It
also provides compatibility with IEEE 802.5 token ring frames.
• 100VG-AnyLAN
is designed to operate efficiently over category 3 cable as well as category 5
cable.
• 100VG-AnyLAN
uses a new MAC scheme (IEEE 802.12) known as demand priority to determine the
order in which nodes share the network. (CSMA/CD is not used in 100VG-ANYLAN)
• The
IEEE 802.12 MAC algorithm is quite effective.
• When
multiple stations offer high loads, the protocol behaves much like a token ring
protocol.
• At
low load, the protocol behaves in a fashion similar to CSMA/CD under low load: A
single requester gains medium access almost immediately.
Topology
• The
topology for a 100VG-AnyLAN network is hierarchical star.
• More
complex arrangements are possible in multiple hierarchy.
Note: The hub is responsible for handling the 802.3 frames and converting between 802.3 and 802.5 frame formats if necessary (not necessary in this example).
Medium Access Control
The MAC algorithm for 802.12
is a round-robin scheme with two priority levels.
Single-Hub Network
When a station wishes to transmit a
frame, it first issues a request to the central hub and then awaits permission
from the hub to transmit.
A station must designate each request
as normal priority or high priority.
The central hub continually scans all of its ports for a request in
round-robin fashion.
The hub maintains two pointers: a high-priority pointer and a
normal-priority pointer.
During one complete cycle, the hub grants each high priority
request in the order in which the requests are encountered. If at any time there
are no pending high-priority requests, the hub will grant any normal-priority
requests that it encounters.
Hierarchical Network.
In a hierarchical network all the end-system ports on all hubs are
treated as a single set of ports for purposes of the round robin algorithm. The
hubs are configured to cooperate in scanning the ports in the proper order. Put
another way, the set of hubs are treated logically as a single hub.
The order is generated by walking a tree representation of the network,
in which the branches under each node in the tree are arranged in increasing
order from left to right. With this convention, the port order is generated by
traversing the tree in what is referred to as preorder traversal, which is
defined recursively as follows:
1. Visit the root.
2. Traverse the subtrees from left to right.
This method of traversal is also known as a depth-first search of the
tree.
The scheme
described so far does enforce a round-robin discipline among all attached
stations, but two refinements are needed.
• First,
a preemption mechanism is needed.
• Second,
a mechanism to prevent nonroot hub to keep the the control indefinitely.
Frame
Transmission.
The
current version of IEEE 801.12 calls for the use of four-pair unshielded twisted
pair (UTP) using category 3, 4, or 5 cable. Future versions will also support
two-pair category 5 UTP, shielded twisted pair, and fiber optic cabling.
Signal Encoding.
A key objective of the
100VG-AnyLAN effort is to be able to achieve 100 Mbps over short distances using
ordinary voice-grade (category 3) cabling. The advantage of this is that in many
existing buildings, there is an abundance of voice-grade cabling and very little
else. Thus, if this cabling can be used, installation costs are minimized.
With present technology, a
data rate of 100 Mbps over one or two category 3 pairs is impractical. To meet
the objective, 100VG-AnyLAN specifies a novel encoding scheme that involves
using four pairs to transmit data in a half-duplex mode. Specifically, a MAC
frame is divided into 5-bit chunks (quintets) and each successive chunk is
transmitted over a different channel in round-robin fashion. Thus to achieve a
data rate of 100 Mbps, a data rate of only 25 Mbps is needed on each channel.
To ensure adequate
transitions on each line for synchronization, an encoding scheme known as 5B6B
is used. The 5B6B scheme is based on the same strategy as the 4B/5B scheme
described in Section 7.1. In this case, each group of 5 input bits is mapped
into a set of 6 output bits. Thus for an effective data rate of 25 Mbps, a
signaling rate of 30 Mbps is required.