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+ + 2.1 + WAN Technologies Overview 
  + 2.1.5 + Packet and circuit switching +  
+ + + + + + +
+ + + Packet-switched networks were developed to overcome the expense of + public circuit-switched networks and to provide a more cost-effective + WAN technology.

When a subscriber makes a telephone call, the dialed + number is used to set switches in the exchanges along the route of the + call so that there is a continuous circuit from the originating caller + to that of the called party. Because of the switching operation + used to establish the circuit, the telephone system is called a + circuit-switched network. If the telephones are replaced with modems, + then the switched circuit is able to carry computer data. +

+

The internal path taken by the circuit between exchanges is shared + by a number of conversations. Time division multiplexing (TDM) is + used to give each conversation a share of the connection in turn. TDM + assures that a fixed capacity connection is made available to the + subscriber.

+

If the circuit carries computer data, the usage of this + fixed capacity may not be efficient. For example, if the circuit is + used to access the Internet, there will be a burst of activity on the + circuit while a web page is transferred. This could be followed by no + activity while the user reads the page and then another burst of + activity while the next page is transferred. This variation in usage + between none and maximum is typical of computer network traffic. + Because the subscriber has sole use of the fixed capacity allocation, + switched circuits are generally an expensive way of moving data.

+

An alternative is to allocate the capacity to the traffic only when + it is needed, and share the available capacity between many users. + With a circuit-switched connection, the data bits put on the circuit + are automatically delivered to the far end because the circuit is + already established. If the circuit is to be shared, there must be + some mechanism to label the bits so that the system knows where to + deliver them. It is difficult to label individual bits, therefore they + are gathered into groups called cells, frames, or packets. + The packet passes from exchange to exchange for delivery through the + provider network. Networks that implement + this system are called packet-switched networks. +

+

The links that connect the switches in the provider network belong + to an individual subscriber during data transfer, therefore many + subscribers can share the link. Costs can be significantly lower + than a dedicated circuit-switched connection. Data on packet-switched + networks are subject to unpredictable delays when individual packets + wait for other subscriber packets to be transmitted by a switch.

+

The switches in a packet-switched network determine, from + addressing information in each packet, which link the packet must be + sent on next. There are two approaches to this link + determination, connectionless or connection-oriented. Connectionless + systems, such as the Internet, carry full addressing information in + each packet. Each switch must evaluate the address to + determine where to send the packet. Connection-oriented systems + predetermine the route for a packet, and each packet need only carry + an identifier. In the case of Frame Relay, these are called Data Link + Control Identifiers (DLCI). The switch determines the onward route by + looking up the identifier in tables held in memory. The set of entries + in the tables identifies a particular route or circuit through the + system. If this circuit is only physically in existence while a packet + is traveling through it, it is called a Virtual Circuit (VC).

+

The table entries that constitute a VC can be established by + sending a connection request through the network. In this case the + resulting circuit is called a Switched Virtual Circuit (SVC). Data + that is to travel on SVCs must wait until the table entries have been + set up. Once established, the SVC may be in operation for hours, days + or weeks. Where a circuit is required to be always available, a + Permanent Virtual Circuit (PVC) will be established. Table entries are + loaded by the switches at boot time so the PVC is always available.

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+ + Web Links

+ + +

Packet vs Circuit Switching

+ + http://www.cina.org/ac051599/sld011.htm

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