The Network Layer

   Basic Data Transmission Methods
   (continued)

Packet Switching

Packet-switched networks, on the other hand, work more like the real highway system. If you want to visit your friend, you do not need to reserve an entire highway lane between you and your destination. All you really need is enough space for your vehicle. Other vehicles can share the same lane, and everyone will get where they are headed. It is a very flexible approach, because lots of data packets can share the same physical transmission link. This type of network is illustrated on the Packet-Switched Network Diagram.

Packet-Switched Network

Packet-Switched Network

Packet-switched networks also include two subcategories:

Connection-oriented

Connectionless

Connection- Oriented Networks

Connection-oriented packet networks combine the features of packet switching and circuit switching. They break a transmission into packets, but then send all of those packets, in sequence, over the same virtual circuit through the network. And, like circuit-switched networks, a complete connection between source and destination must be established before any packets can begin to move.

Routing is established only once, when the virtual circuit is set up. For example, after you establish a telephone connection, you do not need to keep dialing the telephone number.

However, if a node in the virtual circuit fails, the circuit is broken and must be reestablished through some other route before data transfer can resume. (When a telephone line goes down, it must be fixed, unless the telephone company has another line across which it can route the call.)

Because the same virtual circuit is used for all packets in a transmission, and intermediate nodes know in advance where to send each packet, individual packets need not contain addresses. Thus, connection-oriented networks can carry data efficiently.

Because a virtual circuit is dedicated to each connection, packets are transmitted in sequence and arrive in order at the receiving end. The process of setting up the virtual circuit usually also reserves enough bandwidth for the transmission. Thus, service is predictable and will not vary much due to other traffic in the network. This makes connection-oriented transmission good for time-sensitive applications, such as voice or video.

Connectionless Networks

In contrast, connectionless packet networks send data packets over any available path. Packets may change paths during the same transmission, and may even be sent over multiple parallel paths. As packets reach the destination node, it puts them back into their original sequence.

Because packets may follow different paths, each packet must contain the full Network Layer address of the destination node. Each intermediate node that handles a packet uses that address to decide where to transfer the packet next. This approach increases the "overhead" data that each packet must carry, and requires each node to do more processing on each packet. However, if a node goes down, the network can immediately reroute messages along an alternate route.

Because physical links are shared by the packets of many transmissions, service is degraded by high levels of traffic, just as highway travel slows down at rush hour. However, this shared approach allows data links to be more fully utilized.

On a connectionless network, packets can easily get out of sequence as they travel different paths. Thus, real-time applications, such as audio or video, do not perform well using this type of transmission. However, applications that move data in chunks, such as file transfers, World Wide Web (Web) page requests, and e-mail transmissions, perform quite well with connectionless transmission. Because of this, the Internet is the biggest example of a connectionless packet-switched network.