DEFINITY Enterprise Communications Server Release 7
Maintenance for R7r
555-230-126 Issue 4
June 1999
Maintenance Architecture
1-29Protocols
1
Layers
The Open System Interconnect (OSI) model for dat a communications contains
seven layers, each with a spec ific function. Communic ations 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 d evices and
the rules by which bits are passed. Among the p hysical layer protoc ols 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 c an send block s of data with the necessary c odes for
synchronization, error control, or flow c ontrol. With these codes, the destination
equipment chec ks the physical-link reliab ility, corrects any transmission errors,
and maintains the link. When a transmission reaches the d estination equipment,
it strips any layer-2 information the orig inating equip ment may have inserted. The
destination equipment only passes to the destination DTE equip ment the
information sent by the originating DTE equipment. The originating DTE
equipment can also ad d layer-2 code to b e analyzed by the d estination DTE
equipment. The DCE equipment treats this layer as d ata 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 embed ded in the transmission
stream and are meaningful only at the d estination DTE equipment. Therefore,
they are shown in the figure as ‘‘user-defined,’’ with no state c hanges until the
transmission stream reaches its destinatio n.