Page 124 - DCAP406_DCAP_207_Computer Networks

Unit 8: Data Link Protocols

protocol packets. The IP-specific NCP protocol is the IP Control Protocol (IPCP). Aside from Notes
dealing with the calling peer’s IP address, this protocol can also negotiate whether or not to use
header compression, providing a significant speed improvement for low-speed links. If the
calling peer has an IP address, it tells the called peer what it is; if the calling peer doesn’t have an
IP address, the called peer can assign the caller one from a pool of addresses.

8.6 Multiple Access Protocols

Traditional networks were point-to-point channels based on the dedicated channels for a pair of
users. These channels due to their simplicity were not only economical but also used to provide
transmission between a pair of nodes has no effect on the transmission between another pair of
nodes even if they have a common node. The disadvantages of such channels were that they used
to require fixed topology and enormous number of dedicated connections between a pair of
channels thus imposing a challenge to design maintenance and cost effectiveness. Instead,
broadcast channels began to use in which more than a single receiver can receive every transmitted
message. The broadcast channels were good when a message is destined to a large number of
destinations than a single or a very small number of destinations because it incurred wasteful
processing results in all switches in which the message is not intended. The transmissions over
a broadcast channel were also prone to interfere with another transmission. Thus, the transmission
between a pair of nodes was no longer independent of other transmissions. To avoid such
interference, a transmission control mechanism is required. Such transmission control mechanism
known as multiple access protocol determines the access to shared channels in which allocation
of shared resources are critical for desirable performance characteristics and proper operation of
the network. These multiple access protocols are channel allocation schemes and they reside
mostly in a special layer called the Medium Access Control (MAC) layer within the data link
layer of the OSI model.
8.6.1 Multiple Access Protocols Classification

There are numerous multiple access protocols. One of them is non-centralized multiple access
protocols in which all hosts perform based on the same rules and no single host is allowed to
coordinate the activities of the others. This also does not include polling type access protocols.
Broadly, they are classified as conflict free and contention protocols.
Conflict Free Protocols: They ensure successful transmission each time without interfering with
another transmission. This further divided into static or dynamic conflict free protocols in
which hosts communicate with channel allocation statically or dynamically.

Static Channel Allocation: The channel resources in static conflict free schemes depend on time,
frequency or mixed time-frequency. The channels are divided based on the frequency range
(bandwidth) to a single host for a fraction of the time as in time division multiple access (TDMA)
or giving a fraction of the frequency range to every host all of the time as in frequency division
multiple access (FDMA) or providing every host a portion of the bandwidth for a fraction of the
time as in code division multiple access (CDMA).
Dynamic Channel Allocation: The dynamic channel allocation considers channel allocations
based on demand so that optimum uses of channel may be ensured. The hosts who demand only
little use of channel but keep the channel idle for most of the time within their allocated share in
static allocation may leave the channel resources for more active hosts. It may further be classified
into reservation and token passing scheme.

Reservation Scheme: The hosts first announce their intent to transmit in the dynamic channel
allocation by various reservation schemes and reserve their right to transmit before new hosts
obtain a chance to announce their intent to transmit.

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