Week 5
Powerpoint presentaton for Module 4 - View Online or Download
Cabling LANs and WANs
Extra Info - Communication Networks
Types
- Switched Networks
- Circuit Switched Networks
- Message Switched Networks
- Packet Switched Networks
- Broadcast Networks
- Packet Radio
- Satellite
- Local Area Networks
- Integrated Services Digital Network
Switched Networks
Circuit Switching
A dedicated communications path through the network between two end-users is established and maintained for the duration of the data transmission period.
The connection must be established before data can be transfered.
There is a setup, a transfer, and a tear-down phase.
It is very in-efficient as channel capacity is dedicated throughout the duration of the link, even if no data is being transfered.
Once established, the end-to-end link is viewed as "transparent" to the users. Data is transmitted at a rate determined by the channel capacity, with nil delays at the nodes.
Cicuit Switching is very appropriate to the telephone system where connections are of short duration, and is simple to implement. The circuit is established by the call-originator as they input the dial-code of the call-reciever.
Both stations must be available however for the circuit to be established, thus this method is no good for real-time applications or for where there is some urgency involved.
Message Switching
An alternative approach is to foward messages through the network form node to node. E-mail is a good example of this method. Each node on the network must be able to store data for later delivery. No data is forwarded until the complete message is received, in effect, each node "originates" each message anew.
Each node will transmit a message as resources become available. This is also known as a "store-and-forward" system.
Advantages
- High line efficiency as the messages can be queued.
- Sender and Receiver need not be concurrently available.
- Heavy loads can be dealt with easily by simply storing at the nodes until traffic eases.
- One message can go many places.
- Messages can be prioritised and may 'queue jump' at the nodes.
- Error control on total message becomes possible.
- Speed conversion can be dealt with as can code conversion.
- Messages can be re-routed as required.
Disadvantages
- Not suitable for real-time or high-urgency applications.
Packet Switching
This method attempts to combine the advantages of message and circuit switching, while minimising the disadvantages. It is much like message switching but has a restricted packet size, whereas message switching had no -real size limit. Messages on Packet Switching networks must be broken up into these smaller packets.
There are two ways for handling an entire message over the network from one end-user to the another.
Datagram Approach
Each packet is dealt with independently of each other. The packets are individually addressed to the eventual destination and this is the only information available to the switching nodes. Packets may take different routes through the network and arrive out of order. It is up to the receiving station to re-assemble the message.
Virtual Circuit Approach
A Logical connection is made between two end points before any data is sent. A CALL REQUEST packet is sent out through the network. At each node a routing decision is made and the CALL REQUEST packet is sent on to the next node. The CALL REQUEST packet eventually reaches it's destination thus establishing a path through the network. Each packet of that particular message will follow that exact same path through the network. All packets will have a destination address and a Virtual Circuit Identifier. The Virtual circuit is not a dedicated path, more appropriately it is "prescribed path". Packets constituting other messages can share the medium, as they have their own Virtual Circuit Identifier.
The Datagram approach avoids the "Set-Up" phase. For short, single packet messages, this works well as the packet is dynamically re-routed to take account of congestion. A Virtual circuit is limited to a given route through the network and cannot be altered. Datagrams are more reliable, in that if a node fails the datagram can be re-routed whereas this is not possible for the virtual circuit.
Packet Size
Packet size affects the time taken to send a message across the network, and is very noticeable even on a simple case. Smaller packets improve efficiency, until the size of the packet header versus the information content becomes an issue. For any given type of network (medium), there will be an optimim packet size for maximum throughput.
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