Understanding Network Architectures: Layers and Standards

1. 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.

2. 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

3. 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).

4. 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)

5. 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.

6. 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.

7. 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.

8. 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).

9. 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

10. 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.

11. 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 e-mail, FTAM or whatever.

12. 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.

13. 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

14. 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.

15. 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.

16. Presentation 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.

17. 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.

18. 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.

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

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

21. 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.

22. 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.