1. Dealing with Different Types of Networks
Data Link Layer Issues
Dealing with Different Types of Networks

2. Types of Networks Network hardware can be categorized into:
Circuit-switched (e.g. telephone)
Prior to communication, the hardware establishes a dedicated end-to-end connection
Since there is a dedicated connection, a continuous stream of bytes can be sent
Frequency or time-division multiplexing can be used to share links in such a network
Packet-switched (e.g. Ethernet, ATM)
Data is divided into packets of limited size, and each is forwarded through the network to the destination
This can be done by routers or switches

3. Types of Networks Disadvantages Circuit-switched Packet-switched
A dedicated connection that has no transmission means wasted bandwidth
A connection is time consuming if short, infrequent, or sporadic communication is to occur
Packet-switched
Forwarding each packet means that each router must decide the next hop for every packet (even for the same destination)
Routers are typically network slowdowns due to the amount of processing, as well as input/output buffering

4. Types of Networks
Circuit-switching is used in a telephone conversation
A connection to the receiver is established by the sender (the caller)
The telephone company reserves a certain bandwidth (64 Kbps for voice communication) for this call
If the bandwidth is not used by the callers, it is wasted
Packet-switching is similar to the postal service
Each message (envelope) is addressed to the recipient individually, and the postal service delivers each message to the recipient
The postal service may deliver these envelopes through different cities and methods of transport (airplane, truck, …)
It can be said that these messages can be delivered using different routes

5. Circuit-Switching A B Talk Call: B Telephone Company Switching System
Disconnect

6. Circuit-Switching A B Telephone Company Switching System

7. Packet-Switching A B Postal Network Quebec, QC Buffalo, NY
Montreal, QC A
Windsor, ON B
Toronto, ON
Niagara Falls, ON
London, ON
Ottawa, ON
Kitchener, ON
Postal Network

8. Types of Packet-Switching
Virtual circuit-switching
A ‘virtual circuit’ is created between source and destination
This VC is used for all subsequent sending of packets
Datagram
Each packet is routed individually

9. Virtual Circuit Packet-Switching
Advantages
After the first message, routing is faster
A route must only be determined once, for the first message
Once the route has been determined, the path used by the router is reused for all messages
As a result, routing tables are much smaller (and can be searched more quickly)
Because a connection is created, the connection identifier can be used (alone) to address packets
Typically, such as with ATM cells, this can reduce the size of a cell/packet’s header
Messages do not arrive out of order
As a result, receivers do not need to reorder the cells

10. Virtual Circuit Packet-Switching
Disadvantages
Connections take some time to create
Routers/switches must intercommunicate in order to create the connection
Infrequent messaging is not suitable for connection-based messaging
The connection may be lost after a timeout, and will have to be recreated again and again
The time delay for creating the connection may outweigh the speed benefits of using connection-based transport
Routing tables will be dynamic, and routing algorithms are more complex

11. Datagram Packet-Switching
Advantages
Connections need not be created
Infrequent messaging is perfect for connectionless messaging
Connectionless messaging can be resumed after any amount of delay, any number of times, without any delays due to the resumption of communication
Routing each message separately allows for load balancing
Some messages may be sent through one route, but when that route becomes saturated, messages may then be sent through a different route in order to achieve the most optimal communication possible

12. Datagram Packet-Switching
Disadvantages
Each message takes a certain amount of time to transmit (including transmission, routing, reception, etc.)
Nodes communicating large amounts of information in a short time will:
Use a lot of bandwidth for things such as header information
Waste a lot of time routing messages to the same destination
Messages may arrive out of order
Messages must be reordered by the recipient