SUMMARY
LOCAL AREA NETWORK
A local area network (LAN) provides peer-to-peer communication among
independent devices located within a moderately sized geographical
area.
DEVICES
The computer devices attached to the local area network and intended
for running application programs are called stations, hosts, or end
systems. Examples are LAN Manager workstations and servers.
PHYSICAL MEDIUM
The cable system used to carry data from station to station is called
the physical medium. Examples are twisted wire pair, coaxial, and
fiber optic cable.
INTERMEDIATE SYSTEMS
Intermediate systems equipment passes information between physical
medium segments. Examples include repeaters, bridges, routers and
brouters.
NODE
A network connected byte-level information processor. Stations,
bridges, routers and brouters are nodes, but repeaters are not.
FUNCTIONS
Functions are specific network tasks such as:
- Remote file access
- Password encryption and decryption
- Inter-process session establishment
- Message segmentation and de-segmentation
- Frame routing to off-segment networks
- Frame delimiting and media access arbitration
- Pulse encoding of bits
To reduce network design complexity, functions are organized into
groups, which are then allocated to a series of layers implemented in
software that runs on the network nodes.
LAYER SERVICES
The purpose of each network layer is to offer specific services to the
higher layers and to shield them from the details of how services
are implemented. The services provided by a given layer are a product
of the network functions allocated to and performed by that layer.
LAYER RELATIONSHIPS
Peer Relationship--Protocols
The rules and conventions that allow layer N on one node to carry on
a conversation with layer N on another node are collectively known as
the layer N protocol.
Vertical Relationship--Interfaces
Except at the lowest level (the physical medium) no data moves
directly from layer N on one node to layer N on another node. Instead,
each layer passes data to the layer below it, until the physical
medium is reached. The data is then transmitted to the destination
node and back up through each higher layer to layer N. Control
information attached to the data is used to navigate the path through
the layers.
Between each pair of adjacent layers there is a well defined interface
that defines which services the lower layer offers to the upper one,
and how those services are accessed.
For example, peer processes in layer 4 communicate "horizontally"
using the layer 4 protocol. Each side is likely to have the procedures
SendToOtherSide and GetFromOtherSide, even though these procedures
actually communicate with lower layers across the 3 - 4 interface, not
directly with the other side.
NETWORK ARCHITECTURE
A network's complete set of layers, allocated functions, peer
protocols, and vertical interfaces constitutes its architecture.