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OSI Layer 2: Data-Link Layer Protocols and Applications

The Data field contains the information that will be handed to the upper-

layer protocols. It is a variable-length field. Following the data field is the
FCS

. Like the other protocols, it is used for CRC calculation.

Synchronous Data Link Control (SDLC)

Synchronous Data Link Control (SDLC) is based on a synchronous, more
efficient, faster, and flexible bit-oriented format. SDLC has several deriva-
tives that perform similar functions with some enhancements: HDLC, LAPB
(Link Access Procedure, Balanced), and IEEE 802.2, just to name a few.

SDLC is used for many link types. Two node types exist within SDLC: pri-

mary nodes and secondary nodes. Primary nodes are responsible for the con-
trol of secondary stations and for link management, such as link setup and
teardown. Secondary nodes talk only to the primary node when they fulfill
two requirements. First, they have permission from the primary node; sec-
ond, they have data to transmit. Even if a secondary node has data to send,
it cannot send the data if it does not have permission from the primary node.

Both stations can be configured together in four different topologies:

Point-to-point This topology requires only two nodes--a primary and a
secondary.

Multipoint This configuration uses one primary station and multiple
secondary stations.

Loop This configuration uses one primary and multiple secondary sta-
tions. The difference between loop and multipoint setups is that here the
primary station is connected between two secondary stations, which
makes two directly connected secondary stations. When more secondary
stations are added, they must connect to the other secondary stations that
are currently in the loop. When one of these stations wants to send infor-
mation to the primary node, it must transit the other secondary stations
before it reaches the primary.

Hub go-ahead This configuration is different. It also uses one primary
and multiple secondary stations, but it uses a different communication
topology. The primary station has an outbound channel. This channel is
used to communicate with each of the secondary stations. An inbound
channel is shared among the secondary stations and has a single connec-
tion into the primary station.

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