Networking Explained, 2nd Edition

156 Networking Explained, Second Edition

missions (i.e., different sender–receiver pairs) to use the same communications channels.

There is one caveat, though. Sharing cannot occur at the same time because, during a par-

ticular transmission, the communications channel is reserved exclusively for that specific

sender–receiver pair.

Packet-Switching In a packet-switched network, instead of using a dedicated physical

circuit for every node-to-node communication, nodes share a communications channel via

a virtual circuit. A virtual circuit is a nondedicated connection through a shared medium

that gives the high-level user the appearance of a dedicated, direct connection from the

source node to the destination node. A virtual circuit is created by multiplexing a physical

link so that the physical link can be shared by multiple network programs or data transmis-

sions. This concept is extremely valuable for providing low-cost communications capabil-

ities because it is very expensive to provide dedicated links for every data transmission, as

in circuit-switched networks. A definition of virtual has been memorably coined in this

way: “If you can see it and touch it, it’s physical; if you can see it but can’t touch it, it’s

virtual; if you can’t see it and can’t touch it, it’s gone.”

In a packet-switched network, messages are partitioned into smaller messages called

packets, which may contain only a few hundred bytes of data, accompanied by addressing

information and sequence numbers. A packet represents the smallest unit of data that can

be transferred via a given network. Packets are sent to the destination node one at a time,

at any time, and not necessarily in a specific order. The network hardware delivers the

packets through the virtual circuit to the specified destination node, which is responsible

for reassembling them in the correct order. Unlike circuit-switched networks, where a ded-

icated link is established a priori, every packet in a packet-switched network must carry

the destination node’s address. In a circuit-switched network, only the first data message

carries the destination address, which is needed to initially set up the link. Most data com-

munications networks are packet-switched.

As mentioned in Chapter 2, packet-switching can be implemented using either a vir-

tual circuit or a datagram service. The difference between these two transport schemes is

that with virtual circuit packet-switching all the packets are transported along the same

virtual path as if the path were a dedicated circuit. Furthermore, virtual-circuit packet-

switching employs a store-and-forward transmission in which complete packets are stored

first and then forwarded. In datagram packet-switching. however, packets are transmitted

independently of each other. This implies that packets can travel along separate paths,

which requires separate routes to be established for each packet transmission. Moreover,

packets can arrive out of order, which requires the destination node to reassemble them in

the correct order. (See Table 2.1 for a summary of the differences between the two switch-

ing techniques.)

46. Now that I have an understanding of some of the physical layer issues, let’s dis-

cuss the different types of network media.

OK. We’ll begin by first describing attributes that are common among all network

media except, of course, wireless media.

All physical media, regardless of their type, share three common physical elements.

First, a conductor serves as a medium for the physical signal. This conductor is composed