Packet switching networks

Packet switching is a digital network communications method that groups all transmitted data – irrespective of content, type, or structure – into suitably-sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams (sequences of packets) over a shared network. When traversing network adapters, switches, routers and other network nodes, packets are buffered and queued, resulting in variable delay and throughput depending on the traffic load in the network.

Packet switching contrasts with another principal networking paradigm, circuit switching, a method which sets up a limited number of dedicated connections of constant bit rate and constant delay between nodes for exclusive use during the communication session. In case of traffic fees, for example in cellular communication, circuit switching is characterized by a fee per time unit (per minute) of connection time, also when no data is transferred, while packet switching is characterized by a fee per unit of information (per Megabyte).

Two major packet switching modes exist; connectionless packet switching (also known as datagram switching) and connection-oriented packet switching (also known as virtual circuit switching). In the first case each packet includes complete adressing or routing information. The packets are routed individually, sometimes resulting in different paths and out-of-order delivery. In the second case a connection is defined and preallocated in each involved node before any packet is transfered. The packets includes a connection identifier rather than address information, and are delivered in order. See below.

Packet mode communication (or packet-oriented, packet-based) may be utilized with or without intermediate forwarding nodes (packet switches). In all packet mode communication, network resources are managed by statistical multiplexing or dynamic bandwidth allocation in which a communication channel is effectively divided into an arbitrary number of logical variable-bit-rate channels or data streams. Each logical stream consists of a sequence of packets, which normally are forwarded by the multiplexors and intermediate network nodes asynchronously using first-in, first-out buffering. Alternatively, the packets may be forwarded according to some scheduling discipline for fair queuing or for differentiated or guaranteed quality of service, such as pipeline forwarding or time-driven priority (TDP). Any buffering introduces varying latency and throughput in transmission. In case of a shared physical medium, the packets may be delivered according to some packet-mode multiple access scheme.

The service actually provided to the user by networks using packet switching nodes can be either connectionless (based on datagram messages), or virtual circuit switching (also known as connection oriented). Some connectionless protocols are Ethernet, IP, and UDP; connection oriented packet-switching protocols include X.25, Frame relay, Asynchronous Transfer Mode (ATM), Multiprotocol Label Switching (MPLS), and TCP.

In connection oriented networks, each packet is labeled with a connection ID rather than an address. Address information is only transferred to each node during a connection set-up phase, when an entry is added to each switching table in the network nodes.

In connectionless networks, each packet is labeled with a destination address, and may also be labeled with the sequence number of the packet. This precludes the need for a dedicated path to help the packet find its way to its destination. Each packet is dispatched and may go via different routes. At the destination, the original message/data is reassembled in the correct order, based on the packet sequence number. Thus a virtual connection, also known as a virtual circuit or byte stream is provided to the end-user by a transport layer protocol, although intermediate network nodes only provides a connectionless network layer service.

Packet switching is used to optimize the use of the channel capacity available in digital telecommunication networks such as computer networks, to minimize the transmission latency (i.e. the time it takes for data to pass across the network), and to increase robustness of communication.

The most well-known use of packet switching is the Internet and local area networks. The Internet uses the Internet protocol suite over a variety of Link Layer protocols. For example, Ethernet and frame relay are very common. Newer mobile phone technologies (e.g., GPRS, I-mode) also use packet switching.

X.25 is a notable use of packet switching in that, despite being based on packet switching methods, it provided virtual circuits to the user. These virtual circuits carry variable-length packets. In 1978, X.25 was used to provide the first international and commercial packet switching network, the International Packet Switched Service (IPSS). Asynchronous Transfer Mode (ATM) also is a virtual circuit technology, which uses fixed-length cell relay connection oriented packet switching.

Datagram packet switching is also called connectionless networking because no connections are established. Technologies such as Multiprotocol Label Switching (MPLS) and the Resource Reservation Protocol (RSVP) create virtual circuits on top of datagram networks. Virtual circuits are especially useful in building robust failover mechanisms and allocating bandwidth for delay-sensitive applications.

MPLS and its predecessors, as well as ATM, have been called “fast packet” technologies. MPLS, indeed, has been called “ATM without cells.” Modern routers, however, do not require these technologies to be able to forward variable-length packets at multigigabit speeds across the network.

Both X.25 and Frame Relay provide connection-oriented packet switching, also known as virtual circuit switching. A major difference between X.25 and frame relay packet switching are that X.25 is a reliable protocol, based on node-to-node automatic repeat request, while Frame Relay is a non-reliable protocol, maximum packet length is 1000 bytes. Any retransmissions must be carried out by higher layer protocols. The X.25 protocol is a network layer protocol, and is part of the X.25 protocol suite, also known as the OSI protocol suite. It was widely used in relatively slow switching networks during the 1980s, for example as an alternative to circuit mode terminal switching, and for automated teller machines. Frame relay is a further development of X.25. The simplicity of Frame relay made it considerably faster and more cost effective than X.25 packet switching. Frame relay is a data link layer protocol, and does not provide logical addresses and routing. It is only used for semi-permanent connections, while X.25 connections also can be established for each communication session. Frame relay was used to interconnect LANs or LAN segments, mainly in the 1990s by large companies that had a requirement to handle heavy telecommunications traffic across wide area networks.[6] (O’Brien & Marakas, 2009, p. 250) Despite the benefits of frame relay packet switching, many international companies are staying with the X.25 standard. In the United States, X.25 packet switching was used heavily in government and financial networks that use mainframe applications. Many companies did not intend to cross over to frame relay packet switching because it is more cost effective to use X.25 on slower networks. In certain parts of the world, particularly in Asia-Pacific and South America regions, X.25 was the only technology available.[7] (Girard, 1997)