22-09-2016, 02:19 PM
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Introduction
A network of computers based on multi-access medium requires a protocol for effective
sharing of the media. As only one node can send or transmit signal at a time using the
broadcast mode, the main problem here is how different nodes get control of the medium
to send data, that is “who goes next?”. The protocols used for this purpose are known as
Medium Access Control (MAC) techniques. The key issues involved here are - Where and
How the control is exercised.
‘Where’ refers to whether the control is exercised in a centralised or distributed
manner. In a centralised system a master node grants access of the medium to other
nodes. A centralized scheme has a number of advantages as mentioned below:
o Greater control to provide features like priority, overrides, and guaranteed
bandwidth.
o Simpler logic at each node.
o Easy coordination.
Although this approach is easier to implement, it is vulnerable to the failure of the
master node and reduces efficiency. On the other hand, in a distributed approach all the
nodes collectively perform a medium access control function and dynamically decide
which node to be granted access. This approach is more reliable than the former one.
‘How’ refers to in what manner the control is exercised. It is constrained by the
topology and trade off between cost-performance and complexity. Various approaches
for medium access control are shown in Fig. 5.2.1. The MAC techniques can be broadly
divided into four categories; Contention-based, Round-Robin, Reservation-based and.
Channelization-based. Under these four broad categories there are specific techniques, as
shown in Fig. 5.2.1. In this lesson we shall concentrate of the MACs of the first two
categories, which have been used in the legacy LANs of the IEEE standard. The
CSMA/CA, a collision-free protocol used in wireless LAN, will be presented in Lesson
5.5. Channalization-based MACs, which are used in cellular telephone networks, will be covered in Lesson 5.6. And the reservation-based MACs, which are used in satellite
networks, will be discussed in Lesson 5.7.
Goals of MACs
Medium Access Control techniques are designed with the following goals in mind.
• Initialisation: The technique enables network stations, upon power-up, to enter
the state required for operation.
• Fairness: The technique should treat each station fairly in terms of the time it is
made to wait until it gains entry to the network, access time and the time it is
allowed to spend for transmission.
• Priority: In managing access and communications time, the technique should be
able to give priority to some stations over other stations to facilitate different type
of services needed.
• Limitations to one station: The techniques should allow transmission by one
station at a time.
• Receipt: The technique should ensure that message packets are actually received
(no lost packets) and delivered only once (no duplicate packets), and are received
in the proper order.
• Error Limitation: The method should be capable of encompassing an
appropriate error detection scheme.
• Recovery: If two packets collide (are present on the network at the same time), or
if notice of a collision appears, the method should be able to recover, i.e. be able
to halt all the transmissions and select one station to retransmit.
Reconfigurability: The technique should enable a network to accommodate the
addition or deletion of a station with no more than a noise transient from which
the network station can recover.
• Compatibility: The technique should accommodate equipment from all vendors
who build to its specification.
• Reliability: The technique should enable a network to confine operating inspite of
a failure of one or several stations.
5.2.3 Round Robin Techniques
In Round Robin techniques, each and every node is given the chance to send or transmit
by rotation. When a node gets its turn to send, it may either decline to send, if it has no
data or may send if it has got data to send. After getting the opportunity to send, it must
relinquish its turn after some maximum period of time. The right to send then passes to
the next node based on a predetermined logical sequence. The right to send may be
controlled in a centralised or distributed manner. Polling is an example of centralised
control and token passing is an example of distributed control as discussed below.
5.2.3.1 Polling
The mechanism of polling is similar to the roll-call performed in a classroom. Just like
the teacher, a controller sends a message to each node in turn. The message contains the
address of the node being selected for granting access. Although all nodes receive the
message, only the addressed node responds and then it sends data, if any. If there is no
data, usually a “poll reject” message is sent back. In this way, each node is interrogated
in a round-robin fashion, one after the other, for granting access to the medium. The first
node is again polled when the controller finishes with the remaining codes.
The polling scheme has the flexibility of either giving equal access to all the
nodes, or some nodes may be given higher priority than others. In other words, priority of
access can be easily implemented.
Introduction
A network of computers based on multi-access medium requires a protocol for effective
sharing of the media. As only one node can send or transmit signal at a time using the
broadcast mode, the main problem here is how different nodes get control of the medium
to send data, that is “who goes next?”. The protocols used for this purpose are known as
Medium Access Control (MAC) techniques. The key issues involved here are - Where and
How the control is exercised.
‘Where’ refers to whether the control is exercised in a centralised or distributed
manner. In a centralised system a master node grants access of the medium to other
nodes. A centralized scheme has a number of advantages as mentioned below:
o Greater control to provide features like priority, overrides, and guaranteed
bandwidth.
o Simpler logic at each node.
o Easy coordination.
Although this approach is easier to implement, it is vulnerable to the failure of the
master node and reduces efficiency. On the other hand, in a distributed approach all the
nodes collectively perform a medium access control function and dynamically decide
which node to be granted access. This approach is more reliable than the former one.
‘How’ refers to in what manner the control is exercised. It is constrained by the
topology and trade off between cost-performance and complexity. Various approaches
for medium access control are shown in Fig. 5.2.1. The MAC techniques can be broadly
divided into four categories; Contention-based, Round-Robin, Reservation-based and.
Channelization-based. Under these four broad categories there are specific techniques, as
shown in Fig. 5.2.1. In this lesson we shall concentrate of the MACs of the first two
categories, which have been used in the legacy LANs of the IEEE standard. The
CSMA/CA, a collision-free protocol used in wireless LAN, will be presented in Lesson
5.5. Channalization-based MACs, which are used in cellular telephone networks, will be covered in Lesson 5.6. And the reservation-based MACs, which are used in satellite
networks, will be discussed in Lesson 5.7.
Goals of MACs
Medium Access Control techniques are designed with the following goals in mind.
• Initialisation: The technique enables network stations, upon power-up, to enter
the state required for operation.
• Fairness: The technique should treat each station fairly in terms of the time it is
made to wait until it gains entry to the network, access time and the time it is
allowed to spend for transmission.
• Priority: In managing access and communications time, the technique should be
able to give priority to some stations over other stations to facilitate different type
of services needed.
• Limitations to one station: The techniques should allow transmission by one
station at a time.
• Receipt: The technique should ensure that message packets are actually received
(no lost packets) and delivered only once (no duplicate packets), and are received
in the proper order.
• Error Limitation: The method should be capable of encompassing an
appropriate error detection scheme.
• Recovery: If two packets collide (are present on the network at the same time), or
if notice of a collision appears, the method should be able to recover, i.e. be able
to halt all the transmissions and select one station to retransmit.
Reconfigurability: The technique should enable a network to accommodate the
addition or deletion of a station with no more than a noise transient from which
the network station can recover.
• Compatibility: The technique should accommodate equipment from all vendors
who build to its specification.
• Reliability: The technique should enable a network to confine operating inspite of
a failure of one or several stations.
5.2.3 Round Robin Techniques
In Round Robin techniques, each and every node is given the chance to send or transmit
by rotation. When a node gets its turn to send, it may either decline to send, if it has no
data or may send if it has got data to send. After getting the opportunity to send, it must
relinquish its turn after some maximum period of time. The right to send then passes to
the next node based on a predetermined logical sequence. The right to send may be
controlled in a centralised or distributed manner. Polling is an example of centralised
control and token passing is an example of distributed control as discussed below.
5.2.3.1 Polling
The mechanism of polling is similar to the roll-call performed in a classroom. Just like
the teacher, a controller sends a message to each node in turn. The message contains the
address of the node being selected for granting access. Although all nodes receive the
message, only the addressed node responds and then it sends data, if any. If there is no
data, usually a “poll reject” message is sent back. In this way, each node is interrogated
in a round-robin fashion, one after the other, for granting access to the medium. The first
node is again polled when the controller finishes with the remaining codes.
The polling scheme has the flexibility of either giving equal access to all the
nodes, or some nodes may be given higher priority than others. In other words, priority of
access can be easily implemented.