Maintenance strategy
Protocols
32
Maintenance Procedures
December 2003
OSI layers
The Open System Interconnect (OSI) model for data communications contains seven layers, each with a
specific function. Communications to and through the system concern themselves only with Layers 1 and
2 of the model.
•
Layer 1, or the physical layer, covers the physical interface between devices and the rules by
which bits are passed. Among the physical layer protocols are RS-232, RS-449, X.21, DCP, DS1,
and others.
•
Layer 2, or the data-link layer, refers to code created and interpreted by the DCE. The originating
equipment can send blocks of data with the necessary codes for synchronization, error control, or
flow control. With these codes, the destination equipment checks the physical link’s reliability,
corrects any transmission errors, and maintains the link. When a transmission reaches the
destination equipment, it strips any layer-2 information the originating equipment may have
inserted. The destination equipment passes to the destination DTE equipment only the
information sent by the originating DTE equipment. The originating DTE equipment can also add
Layer-2 code to be analyzed by the destination DTE equipment. The DCE equipment treats this
layer as data and passes it along to the destination DTE equipment as it would any other binary
bits.
•
Layers 3 to 7 (and the DTE-created layer 2) are embedded in the transmission stream and are
meaningful only at the destination DTE equipment. Therefore, they are shown in
port and Inter-port data transmission states,
on page 33 as “user-defined,” with no state changes
until the transmission stream reaches its destination.
Summary of Contents for CMC1
Page 1: ...Maintenance Procedures 555 245 103 Issue 1 1 December 2003 ...
Page 14: ...Contents 14 Maintenance Procedures December 2003 ...
Page 416: ...Additional maintenance procedures IP Telephones 416 Maintenance Procedures December 2003 ...
Page 426: ...Index X 426 Maintenance Procedures December 2003 ...
