Architectures. Many tasks involved in encoding, protecting and transmitting user application data as bit stream. Network Architecture is how tasks are grouped (into layers) –e.g, the number of layers and layer functionality.

Different architectures (eg. TCP, SNA, Decnet, ISO etc.) have different number, order and composition of layers. ISO and TCP/IP most frequently cited No ideal architecture - see Tanenbaum Also, requirements of architecture change with time

Terminology hosts (terminals, TEs) terminate all layers nodes (stations, exchanges, switches, routers or IMPs) lowest two or three. Nodes connected by channels (pairs, fibre, solid media, microwave, satellite, mobile links).

Standards documents define interface requirements - –not implementation. –interface between layers (up/down) or across. Documents refer to :- –Service Specification - the services provided by a layer to the layer above (up/down flows) –Layer Protocols - how entities at the same layer, but in different locations, exchange information (across)

–Information (data structures) passed between layers (up/down) in form of Service Data Units (SDUs). –Service provided by lower layers accessed at SAPs; addresses, port numbers, entry points etc.

Information passed across network is contained within PDUs (Protocol Data Units.) Protocols operate across the network between entities in logically linked peer layers. Service user (layer N+1) uses Service Primitives to indicate service required of layer N.

The provider (layer N) uses primitives to respond and request services from layer N- 1. If the layer protocol (across the network) is a Confirmed service there are four basic primitives; Confirmed set {request, indication, response, confirm} Unconfirmed and there are only two.

Primitives (abstract concept) used to communicate up/down stack and go between layers via SAPs. The generation of a primitive usually results in the release of a PDU. Primitive types dependent on service (CONS/CLNS).

Applications Layer - (AL) Provides communications services to user application processes which are not part of the model. Some entities in AL provide specific services (SASEs) –e.g MMS Others common services (CASEs). –e.g. ACSE

Components of AL are User Element (UE), which is the actual i/f between applications and the stack, CASEs and SASEs. AL Services accessed using primitives which are issued and accepted by user application. Issued through A- SAP and tagged as A- SOMETHING.request etc.

In AL, CONNECT is ASSOCIATE. ASSOCIATE creates logical link between peer entities in communicating ALs. After ASSOCIATE, specific service (SASE) identified as best suiting needs of user application. ASSOCIATE indicates , FTAM or whatever.

SASEs attempt to make individual properties of host machine OPEN - accessible to all not just similar marks. Files Systems : FTAM makes irrelevant the fact that accesses could be to Unix m/c, PC, IBM or whatever. ftp for UNIX m/cs only. FTAM makes all real file structures look like a standard (virtual) filestore; all remote file systems now look the same.

Machine Tools : MMS for communication within manufacturing environment (cells). All manufacturing devices are made to look the same (Virtual Manufacturing Devices, VMDs). Dialogue between them is standardised - MMS. Eg. Puma robot could talk MMS with any brand of remote machine tool. CIM

Terminals : With VT emulation all terminals/keyboards look the same. Thus 'Break' key made to look as though it works the same on every machine; 'cursor home' code sequence is the same, and so on Key word when referring to SASEs is VIRTUAL; key concept is OPEN - anything to anything.

The standardised descriptions of (virtually) everything are passed to the PL which codes them into a standard (transfer) syntax for transmission across the network. The coding may include compression etc but always includes ASN.1 type conversions.

P resentation Layer. Concerned with syntax (bit encoding) not semantics (L7) Usual topics, compression, encryption, fec etc - all about changing bit patterns. Main element of L6 however is ASN.1 - crops up "everywhere" - OSI, TCP/IP, GSM, INs, CIM etc. A standard (efficient?) way of encoding user messages into bits.

In OSI-RM, ASN.1 split between L7 and L6 - L7 parses user data into standard ASN.1 data structures. L6 encodes data structures. L6 takes in (at PSAP) HL descriptions of user data and outputs (as SSAP) an encoded bit stream.

The other presentation layer services to mention are, –Compression - frequency depended coding (Huffman), run length encoding and string encoding (Ziv Lempel). –Be aware of type differences and overall aim - ie. producing a reduced length bit stream. –Encryption - why needed, overview of principle but not detail. Result - indecipherable bit stream. –Fec - Add bits to bit stream in strategic places so that errors can be corrected not just detected.

Most common technique is Hamming - described in all the references. Any future encoding operations will be here, in L6

Session layer. Very controversial Many think it is not needed See notes for details –e.g. no session layer in TCP/IP architecture

Summary - L7-L5 Application Layer analyses user input - what does it mean? (semantics). Passes a description of user input to PL. Presentation Layer takes standardised descriptions and encodes it into bit stream (syntax) - abstract syntax (compressed, encrypted?) results.

Session Layer takes encoded bit stream and manages bit flow between “application processes”. The three layers are about data processing. Transport Layer (just above the network) now takes bit stream (modified by L5) and gets it ready for transmission.