Page 125 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes Token Passing Scheme: A single token either in logical or physical form is circulated among the
hosts for allowing the host to transmit who possesses the token so that interferences between
transmissions of other hosts could b avoided.
Contention Schemes: Unlike the conflict free schemes, a transmitting host is not guaranteed to
be successful in contention schemes. The protocol must be embedded with some resolution
processes to resolve conflicts when they happen so all hosts could transmit successfully. The
different resolution protocols build up the contention schemes. In contention schemes, idle
hosts do not transmit and thus do not consume the channel resources.
Static Resolution: It refers to the right of the first host to transmit when a conflict happens. It is
also based on probability in which the transmission schedule for the interfering hosts is chosen
from a fixed distribution that is independent of the actual number of interfering hosts. Examples
are Aloha type protocols and the various versions of Carrier Sense Multiple Access (CSMA)
protocols.
Dynamic Resolution: It determines the highest priority or lowest priority to a packet based on
the time of arrival in the system. The resolution can also be probabilistic. Some of the protocols
based on this scheme are the multiplicity of the interfering packets and the exponential back-off
scheme of the Ethernet.

8.6.2 Aloha and Slotted Aloha

Development of Aloha protocols in itself was a pioneer effort towards computer networking. It
is also known as pure Aloha. It was developed by University of Hawaii in 1970 under the able
expertise of Norman Abramson and Franklin Kuo for project sponsored by DARPA. It laid the
foundation for the basis for the evolution of the Ethernet. Aloha network as developed with the
aim of enabling people in different locations to access the main computer systems. Unlike
ARPANET, Aloha network had used packet radio. Aloha network also initiated the concept of a
shared medium for transmission. Aloha used same frequency for each node and therefore needs
contention management. Aloha used to send data via a teletype in which the data rate usually
did not go beyond 80 characters per second. In fact, Aloha network was a true network in which
all of the computers were connected to Alohanet and they could send data at any time without
operator intervention. It did not raise any limitation on number of computers to have been
involved. It was possible as the medium used was a radio, which did not entail any fixed costs.

Aloha Protocols
The Aloha protocol works on OSI layer 2 for establishing LAN networks with broadcast domain.
The first version of the protocol was basic:
Whenever a user has a frame to send, it simply transmits the frame.

If collision occurs, it waits for a random period of time and re-sends it again
Pure Aloha had about a 18.4% max throughput. This means that 81.6% of the total available
bandwidth is basically being wasted due to stations trying to talk at the same time. In the Slotted
Aloha protocol the throughput could be increased up to 36.8% with the provision that a station
could not send anytime. It could send just at the beginning of a timeslot, and thus collisions are
reduced. You should know that Slotted Aloha is not out of use today and it finds its usage on low
bandwidth tactical satellite communications networks by the US Military. In order to mitigate
the problem of contention, a number of ways were proposed. These are given below:
Frequency Multiplexing: In this case, each node are required to use a different radio frequency.
However this would require each node added to able to be tuned in by all of the other machines.

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