The Network Core

The Network Core

Having examined the Internet's edge, let us now look into more deeply inside the network core - the network of packet switches and links that interconnects the internet's end systems. The following figure highlights the network core with thick, shaded lines.

Circuit Switching and Packet Switching

There are two basic approaches to moving data through a network of links and switches: circuit switching and packet switching. In circuit-switched networks, the resources required along a path (buffers, link transmission rate) to provide for communication between the end systems are reserved for the duration of the communication session between the end-systems. In packet-switched networks, these resources are not reserved; a session's messages use the resources on demand, and as a result, may have to wait (that is, queue) for access to a communication link. As a simple analogy, consider two restaurants, one that requires reservations and another that neither requires reservations nor accepts them. For the restaurant that requires reservations, we have to go through the trouble of calling before we leave home. But when we arrive at the restaurant we can, in principle, immediately communicate with the waiter and order our meal. For the restaurant that does not require reservations, we don't need to bother to reserve a table. But when we arrive at the restaurant, we may have to wait for a table before we can communicate with the waiter.

The ubiquitous telephone networks are examples of circuit-switched networks. Consider what happens when one person wants to send  information (voice or facsimile) to another over a telephone network. Before the sender can send the information. the network must establish a connection between the sender and the receiver. This is a genuine connection for which the switches on the path between the sender and receiver maintain connection state for that connection. In the terminology of telephony, this connection is called a circuit. When the network establishes the circuit, it also reserves a constant transmission rate in the network's links for the duration of the connection. Since bandwidth has been reserved for this sender-to-receiver connection, the sender can transfer the data to the receiver at the guaranteed constant rate.

The network core

Today's Internet is a typical packet-switched network. Consider what happens when one host wants to send a packet to another host over the Internet. As with circuit switching, the packet is transmitted over a series of communication links. But with packet switching, the packet is sent into the network without reserving any bandwidth whatsoever. If one of the links is congested because other packets need to be transmitted over the link at the same time, then our packet will have to wait in a buffer at the sending side of the transmission link, and suffer a delay. The Internet makes its best effort to deliver packets in a timely manner, but it does not make any guarantees.

Not all telecommunication networks can be neatly classified as pure circuit-switched networks or pure packet-switched networks. However, this basic classification into packet- and circuit-switched networks is an excellent starting point in understanding telecommunication network technology.


circuit switching, packet switching, transmission rate

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