Page 13 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes (a) Broadcast networks: These networks have a single communication channel shared by all
the machines on the network. They work as follows:

All the others receive packets sent by any machine.
An address field within the packet specifies for whom it is intended.
Upon receiving a packet, a machine checks the address field. If it is intended for
itself, it processes the packet; otherwise, it is just ignored.

It is also possible to address all broadcasting or multicasting a subset of the machines.
A common scheme:
(i) The address consisting of all 1 bits is reserved for broadcast.

(ii) All addresses with the high-order bit set to 1 are reserved for multicasting.
(iii) The remaining addresses bits form a bit map corresponding to groups.
(iv) Each machine can subscribe to any or all of the groups.
(b) Point-to-point networks consist of many connections between individual pairs of machines.

Multiple routes and intermediate machines may exist between a pair of machines; so routing
algorithms play an important role here.

Notes A general rule (with many exceptions): smaller and localized networks tend to use
broadcasting, whereas larger networks usually are point-to-point.
An alternative criterion for classifying networks is their scale, which is as follows:

1.5.1 Local Area Networks(LAN)

Three distinguishable characteristics for LANs:
(a) Size: usually a diameter of not more than a few kilometers, with bounded and known
worst-case transmission time, making special design and simple management possible.
(b) Transmission technology: usually a shared cable running at speeds of 10 to 100 Mbps (and
even higher), with delay of tens of microseconds and few errors.
Allocation of the shared channel:
Each machine is statically allocated a time slot to transmit, and gets its turn by round robin.
Each machine is dynamically allocated a time slot on demand.

Centralized method uses an arbitration unit to determine who goes next.
Decentralized method allows each machine to decide for itself.

1.5.2 Metropolitan Area Networks(MAN)

MAN is a bigger version of a LAN and uses similar technology. It uses one or two cables but
does not contain switching elements. It covers an entire city and may be related to the local cable
TV network.
A MAN standard - DQDB (Distributed Queue Dual Bus) IEEE 802.6.
(a) Two unidirectional buses.

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