Strictly speaking, a packet network is a network that conveys Layer 3 packets using store-and-forward switches connected by communication facilities that implement statistical time division multiplexing (STDM). Although there have been many different Layer 3 protocols historically (e.g., IPX, X.25, DDP, etc.), modern networks are almost universally based on IP, which means the packet switches are IP routers.
That being said, the term packet network is often loosely applied to any network that implements a store-and-forward switching model, even if they are not operating at Layer 3. We find many references that describe frame relay, ATM, and Ethernet networks as implementing a packet switching model, even though they are considered Layer 2 technologies that convey Layer 2 frames or cells, rather than Layer 3 packets.
The primary benefit of a packet network is that the communications facilities are shared statistically. Because packet-based devices tend to transmit sporadically (e.g., they are said to be bursty), the network can support more total bandwidth at the edges (e.g., to the attached devices or subscribers) than it implements internally. This would never be true in a circuit network, where bandwidth is dedicated to communicating subscribers. Circuit networks (e.g., PSTN are blocking networks; if there is not enough capacity for a new circuit, they reject the new circuit. Packet networks are queueing networks; if there is not enough capacity for a new packet, the packet is held in a waiting line until there required facility becomes available.
Most of the time, the total offered load to a well-designed packet network is less than the network is capable of carrying and all is well. Occasionally, more load is offered than the network, or a part of the network, has the capacity to carry. At that moment, packets will experience additional delay as they in a packet switch somewhere in the network for transmission capacity to become available. If the excess traffic load persists for too long, the packet switches will run out of space to store the arriving packets. This phenomenon is known as congestion.
Historically, packet networks have been pure data networks and considered unsuitable for voice and video applications. Many modern packet networks, however, implement class of service (CoS) strategies to differentiate different types of traffic, and quality of service (QoS) strategies to ensure that different traffic types are treated differently within the network. This provides some degree of control over end-to-end delay (or latency), jitter (or delay variation), bandwidth (or throughput) and packet loss, and makes these packet networks suitable for multimedia applications.
|<mp3>http://podcast.hill-vt.com/podsnacks/2008q1/sf_switch.mp3%7Cdownload</mp3> | Store-and-forward switch|