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Synchronous Data Link Control (SDLC) Sessions 457
Figure 12-24
SDLC
Think of SDLC as being used over point-to-point or multipoint leased lines to devices at remote
sites, whereas LLC2 is used locally over a LAN connection. SDLC provides reliable transport
and sliding window flow control. SDLC links were originally designed to run over low-speed
serial lines and can be used to run over various media, including telephone lines, fiber optic
links, microwave links, and satellite links, among others. LLC2 was designed for fast LAN
connections such as Token Ring.
With traditional SNA, there are two types of SDLC link stations: Primary Link Stations (PLSs)
and Secondary Link Stations (SLSs). Primary nodes poll secondary nodes in a predetermined
order. Secondary nodes then transmit any outgoing data. When configured as primary and
secondary nodes, your routers are established as SDLC stations.
Primary Link Station
The PLS is in charge. It is responsible for issuing commands and is also responsible for error
recovery. It controls the link and also receives responses. The SLS can be considered a slave
station. There are at least four ways to connect a PLS station to one or more secondary stations:
·
Point-to-point--An example of this is a FEP connected over a serial link to a cluster
controller. In Figure 12-24, the cluster controllers can communicate with each other.
·
Multipoint--This is similar to hub and spoke in Frame Relay, with the FEP acting as the
hub. The spokes would be cluster controllers. However, in this situation, the spokes would
not be able to communicate with each other but with only the FEP (PLS). See Figure 12-25.
·
Loop--This is all devices connected in a circle, with the FEP (PLS) having serial
connections to only the first and last cluster controllers (SLS).
IBM 3720/25/45
IBM S/370
IBM 3x74
IBM 3x74
IBM 3x74
SDLC
SDLC
PLS
SLS
SDLC
SDLC
SDLC
87200333.book Page 457 Wednesday, August 22, 2001 2:53 PM