System information

Troubleshooting IBM 10-213
SRB
• Supervisory (S) frames—These frames provide control information. They request and suspend
transmission, report on status, and acknowledge the receipt of I frames. They do not have an
information field.
• Unnumbered (U) frames—These frames, as the name suggests, are not sequenced. They are used
for control purposes. For example, they are used to initialize secondaries. Depending on the
function of the unnumbered frame, its control field is 1 or 2 bytes. Some unnumbered frames have
an information field.
The frame check sequence (FCS) precedes the ending flag delimiter. The FCS is usually a cyclic
redundancy check (CRC) calculation remainder. The CRC calculation is redone in the receiver. If
the result differs from the value in the sender’s frame, an error is assumed.
A typical SDLC-based network configuration appears in Figure 10-2. As illustrated, an IBM
establishment controller (formerly called a cluster controller) in a remote site connects to dumb
terminals and to a Token Ring network. In a local site, an IBM host connects (via channel-attached
techniques) to an IBM front-end processor (FEP), which can also have links to local Token Ring
local-area networks (LANs) and an SNA backbone. The two sites are connected through an
SDLC-based 56-kbps leased line.
Figure 10-2 A Typical SDLC-Based Network Configuration
SRB
The SRB algorithm was developed by IBM and proposed to the IEEE 802.5 committee as the means
to bridge between all LANs. The IEEE 802.5 committee subsequently adopted SRB into the IEEE
802.5 Token Ring LAN specification.
IBM mainframe
Front-end processor
Local site
Remote site
Token
Ring
Establishment
controller
Terminals
SDLC Link