Types of Packet Switching

Connectionless and connection-oriented packet switching

The service actually provided to the user by networks using packet switching nodes can be either be 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.

History of packet switching

image

The concept of packet switching was first explored by Paul Baran in the early 1960s, and then independently a few years later by Donald Davies (Abbate, 2000).

Leonard Kleinrock conducted early research in queueing theory which would be important in packet switching, and published a book in the related field of digital message switching (without the packets) in 1961; he also later played a leading role in building and management of the world's first packet switched network, the ARPANET.

Baran developed the concept of packet switching during his research at the RAND Corporation for the US Air Force into survivable communications networks, first presented to the Air Force in the summer of 1961 as briefing B-265 then published as RAND Paper P-2626 in 1962 , and then including and expanding somewhat within a series of eleven papers titled On Distributed Communications in 1964. Baran's P-2626 paper described a general architecture for a large-scale, distributed, survivable communications network. The paper focuses on three key ideas: first, use of a decentralized network with multiple paths between any two points; and second, dividing complete user messages into what he called message blocks (later called packets); then third, delivery of these messages by store and forward switching.

Baran's study made its way to Robert Taylor and J.C.R. Licklider at the Information Processing Technology Office, both wide-area network evangelists, and it helped influence Lawrence Roberts to adopt the technology when Taylor put him in charge of development of the ARPANET.

Baran's packet switching work was similar to the research performed independently by Donald Davies at the National Physical Laboratory, UK. In 1965, Davies developed the concept of packet-switched networks and proposed development of a UK wide network. He gave a talk on the proposal in 1966, after which a person from the Ministry of Defense told him about Baran's work. Davies met Lawrence Roberts at the 1967 ACM Symposium on Operating System Principles, bringing the two groups together.

Interestingly, Davies had chosen some of the same parameters for his original network design as Baran, such as a packet size of 1024 bits. Roberts and the ARPANET team took the name "packet switching" itself from Davies's work.

Packet switching

Packet switching is a network communications method that groups all transmitted data, irrespective of content, type, or structure into suitably-sized blocks, called packets. The network over which packets are transmitted is a shared network that routes each packet independently from all others and allocates transmission resources as needed. Principal goals of packet switching are to optimize utilization of available link capacity and to increase robustness of communication.

Network resources are managed by statistical multiplexing or dynamic bandwidth allocation in which a physical 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 a network node asynchronously in a first-in, first-out fashion. Alternatively, the packets may be forwarded according to some scheduling discipline for fair queuing or for differentiated or guaranteed quality of service. In case of a shared physical medium, the packets may be delivered according to some packet-mode multiple access scheme. When traversing 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 specific circuit with a limited number dedicated connection of constant bit rate and constant delay between nodes for exclusive use during the communication session.

Packet mode (or packet-oriented, packet-based) communication may be utilized with or without intermediate forwarding nodes (packet switches).

ConnNet

ConnNet was a packet switched data network operated by the Southern New England Telephone Company serving the U.S. state of Connecticut.

ConnNet was the nation's first local public packet switching network when it was launched on March 11, 1985. Users could access services such as Dow Jones News Retrieval, CompuServe, Dialcom, GEnie, Delphi, Eaasy Sabre, NewsNet, PeopleLink, the National Library of Medicine, and BIX. ConnNet could also be used to access other national and international packet networks, such as Tymnet and ACCUNET. Large companies also connected their mainframe computers to ConnNet allowing employees access to the mainframes from home. The network is no longer in operation.

Hardware

The X.25 network was based on hardware from Databit, Inc. consisting of three EDX-P Network Nodes that performed switching and were located in Hartford, New Haven and Stamford. Databit also supplied 23 ANP 2520 Advanced Network Processors each of which provided the system with a point of presence, a network control center and modems. Customers would order leased line connections into the network for host computers running at 4,800 to 56,000 bits per second (bit/s). Terminals would connect over a leased line from 1,200 to 9,600 bit/s synchronous, 300 to 2,400 bit/s asynchronous or using dial-up connections from 300 to 1,200 bit/s. The connection to Tymnet was established over an X.75 based 9,600 bit/s analog link from the ConnNet Hartford node to Tymnet's Bloomfield node.