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 e-mail, 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.

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.

• 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.