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A packet-switched network is a digital communications network 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 which routes each packet independently from all others and allocates transmission resources as needed.
The principal goals of packet switching are to optimize utilization of available link capacity, minimize response times and increase the robustness of communication. When traversing network adapters, switches and other network nodes, packets are buffered and queued, resulting in variable delay and throughput, depending on the traffic load in the network.
The history of such networks can be divided into three eras: early networks before the introduction of X.25 and the OSI model, the X.25 era when many postal, telephone and telegraph (PTT) companies introduced networks with X.25 interfaces, and the Internet era when restrictions on connection to the Internet were removed.
Early networks 
ARPANET and SITA HLN became operational in 1969. Before the introduction of X.25 in 1973, about twenty different network technologies were developed. There was a debate about the merits of two drastically different views as to proper division of labor between the hosts and the network. In the datagram system the host must detect loss or duplication of packets. Transmission Control Protocol /Internet Protocol (TCP/IP) is the best known example of a host to datagram protocol. In the virtual call system, the network guarantees sequenced delivery of data to the host. This results in a simpler host interface with less functionality than in the datagram model. X.25 is the best known virtual call protocol.
Inexpensive minicomputers were an important component in the early networks. In some cases custom I/O devices were added to allow inexpensive or exotic attachments to communication lines.
This is the principal survivor from the early era. TCP/IP, which was an important component of ARPANET2, was chosen for use in NSFNET which eventually became the Internet.
BNRNET was a network which Bell Northern Research developed for internal use. It initially had only one host but was designed to support many hosts. BNR later made major contributions to the CCITT X.25 project. 
This was an experimental network from Nippon PTT. It mixed circuit switching and packet switching. It was succeeded by DDX-2.
EIN nee COST II 
European Informatics Network was a project to link several national networks. It became operational in 1976.
EPSS (Experimental Packet Switching System) was an experiment of the UK Post Office. Ferranti supplied the hardware and software. The handling of link control messages (acknowledgements and flow control) was different from that of most other networks and is not fully explained in the published literature.  
As General Electric Information Services (GEIS), General Electric was a major international provider of information services. The company originally designed a telephone network to serve as its internal (albeit continent-wide) voice telephone network.
In 1965, at the instigation of Warner Sinback, a data network based on this voice-phone network was designed to connect GE's four computer sales and service centers (Schenectady, Phoenix, Chicago, and Phoenix) to facilitate a computer time-sharing service, apparently the world's first commercial online service. (In addition to selling GE computers, the centers were computer service bureaus, offering batch processing services. They lost money from the beginning, and Sinback, a high-level marketing manager, was given the job of turning the business around. He decided that a time-sharing system, based on Kemney's work at Dartmouth—which used a computer on loan from GE—could be profitable. Warner was right.)
After going international some years later, GEIS created a network data center near Cleveland, Ohio. Very little has been published about the internal details of their network. (Though it has been stated by some that Tymshare copied the GEIS system to create their network, Tymnet.) The design was hierarchal with redundant communication links.  
IPSANET was a semi-private network constructed by I. P. Sharp Associates to serve their time-sharing customers. It became operational in May 1976.
Octopus was a local network at Lawrence Livermore National Laboratory. It connected sundry hosts at the lab to interactive terminals and various computer peripherals including a bulk storage system.   
Philips Research 
The PARC Universal Packet (PUP or Pup) was one of the two earliest internetwork protocol suites; it was created by researchers at Xerox PARC in the mid-1970s. The entire suite provided routing and packet delivery, as well as higher level functions such as a reliable byte stream, along with numerous applications. Further developments led to Xerox Network Systems (XNS).
RCP was an experimental network created by the French PTT. It was used to gain experience with packet switching technology before the specification of Transpac was frozen. RCP was a virtual-circuit network in contrast to CYCLADES which was based on datagrams. RCP emphasised terminal to host and terminal to terminal connection; CYCLADES was concerned with host-to-host communication. TRANSPAC was introduced as an X.25 network. RCP influenced the specification of X.25   
"The experimental packet-switched Nordic telecommunication network SCANNET was implemented in Nordic technical libraries in 70's, and it included first Nordic electronic journal Extemplo. Libraries were also among first ones in universities to accommodate microcomputers for public use in early 80's." 
SITA HLN 
SITA is a consortium of airlines. Their High Level Network became operational in 1969 at about the same time as ARPANET. It carried interactive traffic and message-switching traffic. As with many non-academic networks very little has been published about it. 
Systems Network Architecture (SNA) is IBM's proprietary networking architecture created in 1974. An IBM customer could acquire hardware and software from IBM and lease private lines from a common carrier. This allowed construction of a private network. 
Telenet was the first FCC-licensed public data network in the United States. It was founded by former ARPA IPTO director Larry Roberts as a means of making ARPANET technology public. He had tried to interest AT&T in buying the technology, but the monopoly's reaction was that this was incompatible with their future. Bolt, Beranack and Newman (BBN) provided the financing.
It initially used ARPANET technology but changed the host interface to X.25 and the terminal interface to X.29. Telenet designed these protocols and helped standardize them in the CCITT. Telenet was incorporated in 1973 and started operations in 1975. It went public in 1979 and was then sold to GTE.
Tymnet was an international data communications network headquartered in San Jose, CA that utilized virtual call packet switched technology and used X.25, SNA/SDLC, BSC and ASCII interfaces to connect host computers (servers)at thousands of large companies, educational institutions, and government agencies. Users typically connected via dial-up connections or dedicated async connections. The business consisted of a large public network that supported dial-up users and a private network business that allowed government agencies and large companies (mostly banks and airlines) to build their own dedicated networks. The private networks were often connected via gateways to the public network to reach locations not on the private network. Tymnet was also connected to dozens of other public networks in the U.S. and internationally via X.25/X.75 gateways. (Interesting note: Tymnet was not named after Mr. Tyme. Another employee suggested the name.)  
Xerox Network Systems (XNS) was a protocol suite promulgated by Xerox, which provided routing and packet delivery, as well as higher level functions such as a reliable stream, and remote procedure calls. It was developed from PARC Universal Packet (PUP).
X.25 era 
There were two kinds of X.25 networks. Some such as DATAPAC and TRANSPAC were initially implemented with an X.25 external interface. Some older networks such as TELENET and TYMNET were modified to provide a X.25 host interface in addition to older host connection schemes. DATAPAC was developed by Bell Northern Research which was a joint venture of Bell Canada (a common carrier) and Northern Telecom (a telecommunications equipment supplier). Northern Telecom sold several DATAPAC clones to foreign PTTs including the Deutsche Bundespost. X.75 and X.121 allowed the interconnection of national X.25 networks. A user or host could call a host on a foreign network by including the DNIC of the remote network as part of the destination address.
AUSTPAC was an Australian public X.25 network operated by Telstra. Started by Telecom Australia in the early 1980s, AUSTPAC was Australia's first public packet-switched data network, supporting applications such as on-line betting, financial applications — the Australian Tax Office made use of AUSTPAC — and remote terminal access to academic institutions, who maintained their connections to AUSTPAC up until the mid-late 1990s in some cases. Access can be via a dial-up terminal to a PAD, or, by linking a permanent X.25 node to the network.
Datanet 1 
Datanet 1 was the public switched data network operated by the Dutch PTT Telecom (now known as KPN). Strictly speaking Datanet 1 only referred to the network and the connected users via leased lines (using the X.121 DNIC 2041), the name also referred to the public PAD service Telepad (using the DNIC 2049). And because the main Videotex service used the network and modified PAD devices as infrastructure the name Datanet 1 was used for these services as well. Although this use of the name was incorrect all these services were managed by the same people within one department of KPN contributed to the confusion.
Deutsche Bundespost operated this national network in Germany. The technology was acquired from Northern Telecom.
Hitachi designed a private network system for sale as a turnkey package to multi-national organizations. In addition to providing X.25 packet switching, message switching software was also included. Messages were buffered at the nodes adjacent to the sending and receiving terminals. Switched virtual calls were not supported, but through the use of "logical ports" an originating terminal could have a menu of pre-defined destination terminals. 
JANET was the UK academic and research network, linking all universities, higher education establishments, publicly funded research laboratories. The X.25 network was based mainly on GEC 4000 series switches, and run X.25 links at up to 8 Mbit/s in its final phase before being converted to an IP based network. The JANET network grew out of the 1970s SRCnet (later called SERCnet) network.
PSS was the UK Post Office (later to become British Telecom) national X.25 network with a DNIC of 2342. British Telecom renamed PSS under its GNS (Global Network Service) name, but the PSS name has remained better known. PSS also included public dial-up PAD access, and various InterStream gateways to other services such as Telex.
Transpac was the national X.25 network in France. It was developed locally at about the same time as DataPac in Canada. The development was done by the French PTT and influenced by the experimental RCP network. It began operation in 1978.
Venepaq is the national X.25 public network in Venezuela. It is run by Cantv and allow direct connection and dial up connections. Provides nationalwide access at very low cost. It provides national and international access. Venepaq allow connection from 19.2 kbit/s to 64 kbit/s in direct connections, and 1200, 2400 and 9600 bit/s in dial up connections. For more information, call (toll free) 0-800-EMPRESA (3677372) inside Venezuela from any land phone.
Internet era 
When Internet connectivity was made available to anyone who could pay for an ISP subscription, the distinctions between national networks blurred. The user no longer saw network identifiers such as the DNIC. Some older technologies such as circuit switching have resurfaced with new names such as fast packet switching. Researchers have created some experimental networks to complement the existing Internet.
National LambdaRail 
National LambdaRail is a high-speed national computer network in the United States that runs over fiber-optic lines, and is the first transcontinental Ethernet network working to establish a direct line of communications between international parties.
See also 
- Taylor, Steve; Jim Metzler (2008). "Vint Cerf on why TCP/IP was so long in coming".
- Martel, C. C.; J. M. Cunningham and M. S. Grushcow. "THE BNR NETWORK: A CANADIAN EXPERIENCE WITH PACKET SWITCHING TECHNOLOGY". IFIP Congress 1974. pp. 10–14. http://rogerdmoore.ca/PS/BNR/BNRnet.html.
- Bright, Roy D.; Smith, Michael A. (1973). "EXPERIMENTAL PACKET SWITCHING PROJECT OF THE UK POST OFFICE". Proceedings of the NATO Advanced Study Institute on Computer Communication Networks. Sussex, United Kingdom: Noordhoff International Publishing. pp. 435–44. http://rogerdmoore.ca/PS/EPSSB.html.
- Pearson, D J; Wilkin, D (1974). "Some Design Aspects of a public packet switching network". Proceedings of the 2nd ICCC 74. pp. 199–213. http://rogerdmoore.ca/PS/EPSSFer/EF.html.
- Kirstein, Peter T. (1973). "A SURVEY OF PRESENT AND PLANNED GENERAL PURPOSE EUROPEAN DATA AND COMPUTER NETWORKS". Proceedings of the NATO Advanced Study Institute on Computer Communication Networks. Sussex, United Kingdom: Noordhoff International Publishing. http://rogerdmoore.ca/PS/Kirs1973/Ki.html#GEISCO.
- Schwartz, Mischa; Boorstyn, Rober R. and Pickholtz, Raymond L. (November 1972). "Terminal-Oriented Computer-Communication Networks". Proceedings of the IEEE 60 (11): 1408–23.
- Scantlebury, R. A.; Wilkinson, P.T. (1974). "The National Physical Laboratory Data Communications Network". Proceedings of the 2nd ICCC 74. pp. 223–228. http://www.rogerdmoore.ca/PS/NPLPh/NPL1974A.html.
- Mendicino, Samuel F. (1972). "1970 OCTOPUS: THE LAWRENCE RADIATION LABORATORY NETWORK". COMPUTER NETWORKS (Englewood Cliffs, N.J.: Prentice-Hall Inc.): 95–100.
- Pehrson, David L. (1970). "AN ENGINEERING VIEW OF THE LRL OCTOPUS COMPUTER NETWORK".
- Fletcher, John G. (1975). "Principles of Design in the Octopus Computer network".
- Burnett, D.J.; Sethi, H.R. "Packet Switching at Philips Research Laboratories". Computer Networks (North-Holland Publishing Company) 1: 341–348.
- Després, R. (1974). "RCP, THE EXPERIMENTAL PACKET-SWITCHED DATA TRANSMISSION SERVICE OF THE FRENCH PTT". Proceedings of ICCC 74. pp. 171–85. http://rogerdmoore.ca/PS/RCPDEP/RD.html.
- Bache, A.; Matras, Y. (1976). "Fundamental Choices in the Development of RCP, the Experimental Packet-Switching Data Transmission Service of the French PTT". Proceedings of ICCC 76. pp. 311–16. http://rogerdmoore.ca/PS/RCPBAC/RB.html.
- Bache, A.; L. Guillou, H. Layec, B. Long and Y. Matras (1976). "RCP, the Experimental Packet-Switched Data Transmission Service of the French PTT: History, Connections, Control". Proceedings of ICCC 76. http://rogerdmoore.ca/PS/RCPHCC/RH.html.
- Alarcia, G.; Herrera, S. (1974). "C.T.N.E.'s PACKET SWITCHING NETWORK. ITS APPLICATIONS". Proceedings of 2nd ICCC 74. pp. 163–170. http://rogerdmoore.ca/PS/CTNEA/CTA.html.
- Cuenca, L. (1980). "A PUBLIC PACKET SWITCHING DATA COMMUNICATIONS NETWORK: EIGHT YEARS OF OPERATING EXPERIENCE". Conference Record of ICC 80. IEEE. pp. 39.3.1–39.3.5. http://rogerdmoore.ca/PS/CTNEC1.html.
- Lavandera, Luis (1980). "ARCHITECTURE, PROTOCOLS AND PERFORMANCE OF RETD". Conference Record of ICC 80. IEEE. pp. 28.4.1–28.4.5. http://rogerdmoore.ca/PS/RETDB.html.
- Haarala, Arja-Riitta. "Libraries as key players at the local level".
- Chretien, G.J.; Konig, W.M. and Rech, J.H. (1973). "The SITA Network". Proceedings of the NATO Advanced Study Institute on Computer Communication Networks. Sussex, United Kingdom: Noordhoff International Publishing. pp. 373–396. http://rogerdmoore.ca/PS/SITAB.html.
- Sundstrom, R.J.; G.D. Schultz (1980). "1980 SNA'S First Six Years: 1974-1980". Proceedings of 5th ICCC 80. pp. 578–585. http://rogerdmoore.ca/PS/SNA6Y/SNA6.html.
- TYMES, LA ROY W.. "TYMNET — A terminal oriented communication network". Proceedings of the SJCC 1971. 38. pp. 211–16. http://rogerdmoore.ca/PS/TYMNET/TY.html.
- TYMES, LA ROY W. (APRIL 1981). "Routing and Flow Control in TYMNET". IEEE TRANSACTIONS ON COMMUNICATIONS. COM-29 (4): 392–98.
- H.J. Steneker: Graduation Report on X.25 data services in GSM network Electrical Engineering - TUE, 16 May 1991. Chapter 3: page 20 and further, Retrieved 15 June 2011
- Tomaru, K.; T. Kato and S.I. Yamaguchi (1980). "A Private Packet Network and Its Application in A Worldwide Integrated Communication Network". Proceedings of ICCC '80. pp. 517–22. http://rogerdmoore.ca/PS/HIPA/HIA.html.
- "X.25 Virtual Circuits - Transpac in France - Pre-Internet Data Networking".
- "The Guide to Hacking & Phreaking, Issue #2", Liquid Jesus
- "Hobbes' Internet Timeline v8.1", Robert H'obbes' Zakon, Zakon Group LLC
- 20+ articles on packet switching in the 70s
- "An Introduction to Packet Switched Networks", Phrack, 05/3/88
- EPSS Pictures of the EPSS exchanges in London, Manchester and Glasgow.