Wan Switching Concept


Circuit Switching
A circuit-switched network is one that establishes a dedicated circuit (or channel) between nodes and terminals before the users may communicate.
As an example, 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 caller to 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) gives 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 is 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.
PSTN and ISDN are two types of circuit-switching technology that may be used to implement a WAN in an enterprise setting.
Packet Switching
In contrast to circuit switching, packet switching splits traffic data into packets that are routed over a shared network. Packet-switching networks do not require a circuit to be established, and they allow many pairs of nodes to communicate over the same channel.
The switches in a packet-switched network determine which link the packet must be sent on next from the addressing information in each packet. 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 only has to carry an identifier. In the case of Frame Relay, these are called Data Link Control Identifiers (DLCIs). 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).
Because the internal links between the switches are shared between many users, the costs of packet switching are lower than those of circuit switching. Delays (latency) and variability of delay (jitter) are greater in packet-switched than in circuit-switched networks. This is because the links are shared, and packets must be entirely received at one switch before moving to the next. Despite the latency and jitter inherent in shared networks, modern technology allows satisfactory transport of voice and even video communications on these networks.
Virtual Circuits
Packet-switched networks may establish routes through the switches for particular end-to-end connections. These routes are called virtual circuits. A VC is a logical circuit created within a shared network between two network devices. Two types of VCs exist:
Permanent Virtual Circuit (PVC)-A permanently established virtual circuit that consists of one mode: data transfer. PVCs are used in situations in which data transfer between devices is constant. PVCs decrease the bandwidth use associated with establishing and terminating VCs, but they increase costs because of constant virtual circuit availability. PVCs are generally configured by the service provider when an order is placed for service.
Switched Virtual Circuit (SVC)-A VC that is dynamically established on demand and terminated when transmission is complete. Communication over an SVC consists of three phases: circuit establishment, data transfer, and circuit termination. The establishment phase involves creating the VC between the source and destination devices. Data transfer involves transmitting data between the devices over the VC, and the circuit termination phase involves tearing down the VC between the source and destination devices. SVCs are used in situations in which data transmission between devices is intermittent, largely to save costs. SVCs release the circuit when transmission is complete, which results in less expensive connection charges than those incurred by PVCs, which maintain constant virtual circuit availability.
Connecting to a Packet-Switched Network
To connect to a packet-switched network, a subscriber needs a local loop to the nearest location where the provider makes the service available. This is called the point-of-presence (POP) of the service. Normally this is a dedicated leased line. This line is much shorter than a leased line directly connected to the subscriber locations, and often carries several VCs. Because it is likely that not all the VCs require maximum demand simultaneously, the capacity of the leased line can be smaller than the sum of the individual VCs. Examples of packet- or cell-switched connections include:
X.25
Frame Relay
ATM

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