Specifications

2-5
TOKEN RING OVERVIEW
Application (Layer Seven)
This layer provides access to the OSI environment and provides
communication based services to the end users. Typical of the services it
provides are file-transfer services, file-directory operations, and electronic
messaging. All the subordinate layers of the model exist to support, and
make possible, the activities of this layer.
The OSI basic reference model was designed to enable communication
between heterogeneous systems using traditional wide area networking
technology. The result is that LAN standards, such as those defined by the
IEEE, do not readily fit into the OSI structure. This is particularly true of
the Physical and Data Link Layers, where the diversity of physical media
and network topologies used in LANs requires that different physical
components be used and a different data link protocol be used for each
type. For this reason, when applying LAN standards to the OSI Reference
Model, the Physical Layer and the Data Link Layer are often divided into
sublayers.
The Data Link Layer has two sublayers - Media Access Control (MAC)
and Logical Link Control (LLC). The MAC sublayer corresponds to a
particular type of LAN, for example, CSMA/CD or Token Ring. It is
responsible for receiving data from the LLC sublayer and encapsulating it
into a packet ready for transmission. The MAC sublayer must also
recognize and generate addresses and the generation and verification of
frame-check sequences. Its primary function is the delivery of frames and
to ensure that transmission onto the network is controlled. The LLC
sublayer is used to provide a consistent service to the Network Layer
irrespective of the MAC sublayer in use. Therefore, the upper layers need
not worry about whether they are connected, for example, to a Token Ring
or Ethernet network.
Communications appear within a network as direct peer-to-peer
communications to the user. Data appears to go from the sending
application layer directly to the receiving application layer as if the
devices were attached locally.
In actuality, the user message is routed from the sending application layer
down through the other layers of the system. Each layer adds to or
modifies the message according to its protocol (e.g., adding address and
error checking information). The message passes through all the layers of
the system before appearing on the data channel (or communication
media) at the Physical Layer.
From the data channel the message passes upward through the same
layers at the destination device. As the message progresses from layer to
layer, each layer changes the message according to its protocol
(e.g., stripping address information and performing error checking). The
end result is the same message as was originally sent, arriving at the top of
the destination Application Layer.