Using NS3000/iX Network Services (36920-90008)

Chapter 1 17
Introduction to NS 3000/iX
Network Architecture
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.
In NS 3000/iX, 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 NS 3000/iX 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 IEEE 802.3, 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/iX Link
correspond to the lower four layers of the Open Systems Interconnect
(OSI) model (Figure 1-1), and the NS 3000/iX product corresponds to
layer seven of the OSI model.
For more information on lower-level network functions, see the and the
NS 3000/iX Error Messages Reference Manual.