HP 3000 Manuals

Using NS3000/XL Network Services

Network Architecture 

A network architecture specifies the transmission tasks of distinct
hardware and software modules or layers.  The architecture of NS3000/XL
is based on the seven-layer OSI (Open Systems Interconnection) model
(Figure 1-1) developed by the International Standards Organization (ISO).
One of the purposes of having a layered architecture is to make the
complexities of data communications transparent to the high-level user.
Some familiarity with the tasks performed at different levels may be
helpful.

The highest layer regulates user services, while the lowest layer
regulates the actual transmission of bits from one node to another.  Each
computer system in the network is called a node.  At each layer one or
more protocols is responsible for carrying out the appropriate tasks.  A
protocol is a set of rules that specify software message format.  From a
logical point of view, the protocol entity at each level communicates
with the corresponding protocol entity at the same level on another node.
In reality, except for the physical transmission of data to another node,
each protocol entity communicates with other protocols at the layer
immediately above and below its own.

When a message is sent from one node to another in a network, it is first
passed down through the architectural levels at the source node.  That
is, it is transferred from the control of one protocol entity to the
control of the next.  At one of the middle layers, the message is broken
down into packets.  At the lowest layer, the packets are sent across the
physical communications link.  The destination node collects the packets
and passes them up to the higher protocol levels where they are
reassembled into the original, complete message.


          Figure 1-1.  OSI Model 

In NS3000/XL, the Application Layer, at the top of the hierarchy,
consists of user-level services such as Virtual Terminal (VT), Network
File Transfer (NFT), Remote File Access(RFA), and Remote Database Access
(RDBA). The next two layers, Presentation and Session, define functions
that contribute to these high-level services, but there is no exact
correspondence between NS3000/XL features and these layers.

The Transport Layer protocols such as TCP, PXP, and UDP handle end-to-end
communications between a source and a destination node, ensuring that a
message from the source arrives at its destination in the proper form.
The fragmentation of messages into packets occurs at this level.

The Network Layer protocols such as IP and X.25 perform an addressing
function, making sure that the packets are acquired by the node to which
they are addressed.

The Data Link Layer protocols such as IEEE802.3, Ethernet, LAPB and X.25
govern the actual transmission of the packets over the communications
link.  At this level the packets are technically known as frames.  The
lowest layer, the Physical, provides electrical and mechanical
specifications for the transmission of bits across the link.

Hewlett-Packard link products such as the ThinLAN 3000/XL Link correspond
to the lower four layers of the Open Systems Interconnect (OSI) model
(Figure 1-1), and the NS3000/XL product corresponds to layer seven of the
OSI model.

For more information on lower-level network functions, see the NS3000/XL 
Configuration Planning and Design Guide and theNS3000/XL Error Messages 
Reference Manual.

MPE/iX 5.0 Documentation