NETWORKING DEFINITIONS

Standards

OSI (Open Systems Interconnection) model defines seven levels to a network connection; the lowest three layers are:

Physical layer: Ethernet is the most common physical protocol, e.g. 10 Mbs baseband carrier on 50 ohm thick or thin coaxial cable; different Ethernet LANs can be connected by repeaters. The IEEE 802.3 committee is currently reviewing proposals for a 100 Mbs standard. FDDI (Fiber Distributed Data Interface) is a front- runner. Some proposals are based on multiple UTP (unshielded twisted pair) to take advantage of installed telephone wiring.
Data-link layer: handles transmission, reception and error correction. IEEE 802.3 and the earlier Xerox/DEC Ethernet are the dominant (and very similar) data-link protocols; the access control method is CSMA/CD (carrier-sense mutliple-access with collision detection); most software, except Link Level Access (LLA) software which directly accesses the computers LAN hardware driver, can use either protocol. Another common type of protocol is token ring, of which various proprietary version exist, involves handing control from node to node. LANs which adhere to the same protocol can be connected by bridges, even though their physical layers differ; X.25 is a CCITT standard for encapsulating data with routing information, characterized by robust error checking at each of the switches.
Network layer: handles the packet routing. TCP/IP (Transmission Control Procedure/Internet Protocol) is the most common network protocol; the X.25 standard applies to this OSI layer as well; LANs with a common network protocol can be connected by routers; brouters function both as bridges and routers.

Gateways are computers which can link LANs with nothing in common at these lowest layers.

The remaining layers are transport (creates packets), session (establishes communication), presentation (compression and encryption), and application (the actual program).

Connections

WAN (wide area network): a group of geographically distributed LANs (local area networks) connected by

modem using dial-up or leased dedicated phone lines (up to 14.4 kbs); SLIP (serial line Internet protocol) can be used for networks connections over such lines.
DDS (Dataphone Digital Service) leased lines
PSN (packet-switched network), which can be a private or nstitutional network or a PDN (public data network); throughput depends on the amount of traffic; 1 MBs is considered good.
T1 is a 1.5 Mbs switched circuit (like a phone connection) between LANs; T3 circuits support 45 MBs.
Geosynchronous satellite circuits, typically leased in 128 kbs chunks.
ISDN (Integrated Services Digital Network) service: 64 kbs/channel; for faster throughput, packet switching under ISDN uses a link layer frame relay technology, with error checking only at the source and destination, instead of the X.25 standard.
FDDI (Fiber Distributed Data Interface) with a bandwidth of 100 Mbs can achieve 10 Mbs throughput.
ATM (synchronous transfer mode) takes any kind of data (such as Ethernet or FDDI packets) and breaks it up into 53-byte packets, each of which is transferred over a hardware digital switch network (like a very short phone call) at 155 Mbs minimum. Packets can be transmitted in parallel for higher throughput.

IPC (InterProcess Communication) services provide for exchange of data and control information between processes running on different networked computers. Software entities which handle the communications are called sockets.

References: Byte, 84Dec 131-142; 88Jul 278-286; 91Jul 161-190; 93Aug 111-126.