Transcript Architectures.

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 N1.
• 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 ASAP and tagged as ASOMETHING.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
• 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.