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The OSI reference model.

This chapter is historical background information only. During the 1980 ISO, the International Standards Organization, defined a packet network standard, called the Open Systems Interconnect, OSI. ISO is a committee organization and their work is heavily influence of this. Committees have the quality that they produce compromise products, laden with options. OSI is heavily influence by two forces of interest, IBM, and CCITT. IBM should be known to everyone, CCITT may be shrouded a bit more, it has now become apart of a new organization, the ITU. These two represent to opposing forces in that IBM is Big business (the capitalist) and CCITT is a telco standard organization (the socialist.)

IBM had a vested interest since they had a comparable packet network SNA, SNA is very much in the prehistory of MidWay since TP monitors are child of the mainframe. Supposedly OSI has seven layers since SNA also has seven layers.

CCITT, which is a French abbreviation I forget, but means the Committee for standardization of international telephone and telegraph. Again they make committee style standards. CCITT is responsible for many things, among them cabling and plugging, such as V.24 (*almost* RS232, think of it as European RS232) V.35 (Used much on telco modems above 64 kbit/s). More importantly they defined two communication standards called X.21 and X.25. (Actually they made a lot of other standards around these too.). X.21 is a line switching (dialup) protocol with digital signaling. X.25 is a packet switching network, which originally was intended to use X.21 between X.25 nodes. (In OSI model terms, V24 and V.25 is layer 1, X.21 is layer 2 and X.25 is layer 3.) Linux users might be amused to know that the MIDI standard which is a standard for connecting music equipment together digitally is modified X.21. Linux users may also may have heard about ax.25 (amateur radio X.25) Pretty much all the nodes in the AX.25 net are Linux boxes. X.25 was designed for error rates in the order of one error in 105 errors, with made it ideal for make a packet network on amateur radio bands which is rather noisy. However in the OSI network, CCITT envisioned using X.21 and X.25 as the link (2) and packet (3) layers, respectively.

OSI was also a response to TCP/IP which was during the 80 regarded somewhere between a worthy research test-bed and a toy. The main criticism of TCP/IP was that it didn't do properly data representation and that addressing within a node/machine was primitive. However and a packet network it has at the time regarded as excellent.

Even now when TCP/IP has emerged victorious, layer 5 and 6 in OSI and SNA do live on top of TCP/IP. RFC1006 specifies how OSI shall be run on top of TCP. Ask IBM on how SNA (Usually LU6.2) is run on TCP, some modern implementations of SNA have the capability.

It should be noted that OSI did define a TP monitor, known as OSITP. When TP monitor of different make cooperate they often talk OSITP to each other.

The reference model.

Even if OSI as a packet switching communication stack is dead, the modeling work done by ISO turned out pretty useful in classifications. Everything such as TCP, UPD, IP, ethernet, ISDN, analog modems, http, html, xml do provide service that fit into the OSI reference model. Http is a protocol that provide layer 5, and html provide layer 6. TCP and UDP is layer 4, IP is layer 3.

The layers in the stack can be briefly described as follows.

7. Applications:
Where the user applications software lies. Such issues as file access and transfer, virtual terminal emulation, interprocess communication and the like are handled here.
6. Presentation:
Differences in data representation are dealt with at this level. For example, UNIX-style line endings (LF only) might be converted to MS-DOS style (CRLF), or EBCIDIC to ASCII character sets.
5. Session:
Communications between applications across a net- work is controlled at the session layer. Testing for out-of-sequence packets and handling two-way communication are handled here.
4. Transport:
Makes sure the lower three layers are doing their job correctly, and provides a transparent, logical data stream between the end user and the network service s/he is using. This is the lower layer that provides local user services.
3. Network:
This layer makes certain that a packet sent from one device to another actually gets there in a reasonable period of time. Routing and flow control are performed here. This is the lowest layer of the OSI model that can remain ignorant of the physical network.
2. Data Link:
This layer deals with getting data packets on and off the wire, error detection and correction and retransmission. This layer is generally broken into two sub-layers: The LLC (Logical Link Control) on the upper half, which does the error checking, and the MAC (Medium Access Control) on the lower half, which deals with getting the data on and off the wire.
1. Physical:
The nuts and bolts layer. Here is where the cable, connector and signaling specifications are defined.

There are actually a lesson in connection orientated and connectless communication here. X.25 is connection orientated.  You can either have a switched virtual circuit(SVC) or a permanent virtual circuit. (PVC).  Between two host you have one or more switches, think of them as routers. Every switch must know about every VC going thru it, packets have only the VC number. not the end point address like in IP.  The VC number on the two interfaces on the switch a connection go thru are not identical. Should IP use the same scheme, the Internet backbone routers know about all the millions of TCP connections that go thru them.  They have enough trouble finding space for the routing tables already. (Routing tables in the Internet backbone routers grow 1.5 times the rate RAM chips grow.). In TCP/IP only the end nodes are aware that an connection exist. In all fairness OSI envisioned that only one X.25 VC should exist between two hosts. Sessions should be multiplexed on the single VC.

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© 2000 Terje Eggestad