architectures. many tasks involved in encoding, protecting and transmitting user application data as...
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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.
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