Lesson #4: Exploring Access Methods
Access methods, sometimes called channel access methods,
are theoretically independent of the topologies
you just learned about. In reality, however, only a few combinations of
physical and logical topologies work well together.
When several entities share the same communications medium
(channel), some mechanism must be in place to
control access fairly. It is unproductive to have everyone in a meeting speak
at once, so rules of order were defined long ago for managing meetings.
Similar rules, or access methods, are applied to networks.
In this lesson, you will learn about the three most common channel access
methods and the topologies with which they are associated. These access
methods are as follows:
The channel access methods discussed in the following sections include
general rules that govern the devices as they access and transmit across the
channel. Access methods use a certain amount of the channel's
band width for access control.
The usable portion of the channel's band width is limited by the access
method being used. Each method has a different effect on network traffic.
Polling resembles a well-ordered meeting in which the chairman
must recognize an attendee before that person is allowed to speak. The
chairman's responsibility is to maintain order in the meeting and ensure that
each person who wants to speak has an opportunity to do so. Polling is most
closely associated with mainframe (point-to-point) computer networks. By
using polling, one device such as a mainframe
front-end processor, is designated as the primary device.
Primaries also are known as the channel access administrators,
controllers, or masters. All access to the network is
controlled by the primary.
The primary queries (polls) each of the secondary devices, also known as
slaves. As each secondary is polled, the primary inquires if the
secondary has information to be transmitted. Only when it is polled does the
secondary have access to the communication channel. Each system has rules
pertaining to how long each secondary can transmit data. The process of
polling is much like a committee chairman who asks each member in turn to vote
on an issue. Polling can be utilized in virtually any network topology.
Advantages of Polling
The advantages of polling are as follows:
Disadvantages of Polling
Many characteristics of polling can be determined centrally, including
the polling order and node priorities.
Polling ensures that channel access is
predictable and fixed. Because the time delays between the
primary and secondary devices can be calculated, this access method is
called deterministic. Deterministic access methods are suitable for
controlling some automated equipment because each piece of equipment is
guaranteed access to the network at predetermined intervals.
Polled channels cannot be over saturated
with traffic. As demand increases, traffic increases up to a maximum
level. The polling mechanism ensures that maximum traffic level cannot be
exceeded. Nor can excess traffic reduce the performance of the network.
Polling has the following disadvantages:
Some applications cannot function with the time delays required for
polling other devices.
The process of polling involves large numbers of messages that take up
available band width. Traffic is required to poll each node, even nodes
that are idle.
Some polled networks use half-duplex transmission lines. This means
that the primary and secondary devices must "turn around" the
line, requiring some band width.
Polling requires a sophisticated central control mechanism that
requires extensive configuration.
Contention on a network resembles conversation in a meeting.
Every attendee can attempt to speak at any time. When two speakers interfere,
however, the conversation is garbled, and the
speakers must begin again. Any speaker can speak at any time, and the
speakers must contend for openings in the conversation in which to state
Similarly, on a contention network, any device can transmit whenever it
needs to send information. To avoid data collisions, specific
contention protocols were developed requiring the device to listen
to the cable before transmitting information.
Contention is also known as random access because, unlike
polling and token passing, there is no fixed order in which the nodes can
STUDY NOTE: The act of
"listening" to the channel to see if any traffic exists
is called carrier sensing, and
contention-based networks are called Carrier Sense
Multiple Access (CSMA) networks.
Even though each station listens for network traffic before it attempts
to transmit, it remains possible for two transmissions to overlap on the
network. This overlap is called a collision.
As the diagram shows, collisions occur because it takes time for signals to
propagate through the network. Both stations A and D have found the network
clear and transmit a message. A few micro-seconds are required for the signal
from A to reach D. During that period, D is free to transmit, and a collision
As a result of collisions, access to a CSMA network is somewhat
unpredictable, and CSMA networks can be referred to as random or
statistical access networks.
Collisions are part of the normal operation of a CSMA network. Two
specialized methods of collision management have been developed to improve
performance: Collision Detection (CD) and
Collision Avoidance (CA).
The collision detection approach listens to the network
traffic as the card is transmitting. By analyzing network traffic, it is
possible to detect collisions and initiate retransmissions.
Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
is the access method utilized in Ethernet and IEEE 802.3.
Collision avoidance uses time slices
to make network access smarter and avoid collisions. Carrier Sense Multiple
Access with collision avoidance is the access mechanism used in Apple's
LocalTalk network and 802.11 Wi-Fi networks.
Benefits of Contention
Contention offers the following benefits:
Disadvantages of Contention
Contention is a very simple access method that has low administrative
overhead requirements. No network traffic is necessary to manage the
Actual user data throughput is rather high at low traffic levels in
comparison to the total amount of utilized network band width.
The disadvantages of contention are as follows:
Examples of Contention
At high traffic levels, data collisions and the resulting
retransmission diminish performance dramatically. It is theoretically
possible that collisions can be so frequent at higher traffic levels
that no station has a clear chance to transmit.
Channel access is probabilistic
rather than deterministic. Because of retransmissions and the
time it takes to sense collisions, automated equipment that cannot
tolerate delays cannot use this type of access. Contention offers no
means of establishing the frequency of a station's opportunities to
Examples of networks that use contention are as follows:
Token passing resembles a children's story-telling game in
which the players pass a ball around a circle. When a player receives the
ball, he or she is expected to tell part of a story. Players can talk only
when the ball is in their possession.
Token passing uses a special authorizing packet
of information to inform devices that they can transmit data. These
packets are called tokens and are passed around
the network in an orderly fashion from one device to the next. Devices can
transmit only if they have control of the token. This method distributes the
access control among all the devices.
Two approaches to token passing are available.
Token Ring uses a ring topology. Each station passes the token to the
next station in the ring. ARCnet also uses token
passing; however, with ARCnet, each station passes the token to the station
with the next higher node address, regardless of its physical location on
the network (token passing bus).
The diagram below shows examples of token passing in
Token Ring and ARCnet
Advantages of Token Passing
Token passing provides the following advantages:
Disadvantages of Token Passing
Token passing offers the highest data throughput possible under high
traffic conditions. Only one transmission can occur at a time, and
collisions cannot occur (non-contention).
Therefore, token passing experiences less performance degradation at
higher traffic levels than contention.
Token passing is deterministic.
Each station is guaranteed an opportunity to transmit each time the token
travels around the ring.
Some token passing systems enable you to set priorities for devices
that need controlled access to the token.
As the traffic increases, data throughput also increases to a certain
level, and then stabilizes.
The disadvantages of token passing are as follows:
Examples of Token Passing
Token passing involves complicated protocols for managing the network
and recovering from errors. The traffic associated with these protocols
has higher band width overhead then is required for CSMA.
All devices require complicated software that needs to be modified
whenever a station is added or removed.
Some systems require an additional central controller that adds to the
overhead and reduces throughput. Cabling and network hardware can be more
expensive for token passing networks than for CSMA networks.
Examples of token passing networks include the following:
IEEE 802.4, also known as token bus. Token bus uses token
passing access control and a bus topology. (similar to ARCnet)
IEEE 802.5, also known as Token Ring
. Token Ring uses token passing access control and a
ARCnet uses token passing
based on node addresses, using a star-wired network with a logical bus
TokenTalk is Apple's standard for networking Macintosh
computers on Token Ring networks.
Token passing and CSMA, the most common access methods used in LANs, have
different performance characteristics (see the graph). The
"Load" x-axis represents the
demand being placed on the network. The "
Throughput" y-axis represents the data actually being
Notice that the throughput of a CSMA network rises smoothly
with increased traffic levels up to a point. At that point, collisions begin
to occur with greater frequency, resulting in a gradual reduction in network
throughput. At some point, network throughput reaches unacceptably low
Token passing exhibits reduced performance at lower traffic
levels than CSMA. This is a result of the many administrative mechanisms
required for token access. Throughput rises smoothly until the network is
fully utilized. At that point, throughput stabilizes. Throughput does not
degrade because no collisions can occur. However, beyond the plateau, all
workstations are sharing a strictly limited band width. Although total
throughput remains stable, the bandwidth available to a given station
diminishes as demand increases.
The user's perception is that the
network's performance is diminishing as the load demand increases. This graph
above illustrates how throughput decreases as a percentage of demand.
Basically, as demand increases, a smaller percentage of the demand can be
satisfied. With contention-based networks, the fall-off after a certain point
is fairly rapid until the number of collisions interferes with virtually all
traffic on the network and few, if any, packets are actually delivered.
Perceived performance of a token passing network also declines, but never
reaches zero. Each user is guaranteed a fair share of the network's band
width (deterministic), although this share may, at some point, be
considered inadequate for the user's needs.
Combining Architectures and Access Methods
The following table summarizes common types of networks in terms of their
topologies and access methods.
Notice that CSMA technologies are only applicable in networks that are
logical buses. This is the case, because each
station must be able to sense all network traffic to determine if the network
is busy. On a ring network, a node can detect only the data transmissions
that happen to pass it, and cannot determine whether traffic exists elsewhere
on the network. CSMA is, therefore, not applicable to rings.
On the other hand, the only access method that works with
logical rings is token passing. This is the case because
each station must be able to receive each packet it transmits after that
packet travels around the logical ring.
Summary of Networks, Topologies, and Access Controls|
Token Ring/Token Talk