Expand Configuration and Management Manual (G06.24+)

Planning a Network Design
Expand Configuration and Management Manual—523347-008
3-2
Satellite Connections
Satellite Connections
The satellite-connect line-handler process implements the satellite-efficient version of
the HDLC protocol, HDLC Extended Mode. HDLC Extended Mode allows a maximum
window size of 61 frames (the maximum window size is the number of outstanding
frames that can be sent before an acknowledgment is required) and implements the
selective-reject feature. Selective reject causes only frames that arrive in error to be
retransmitted.
The major benefits of satellite connections are
•
Price-to-performance ratio. Satellite channels can add a large amount of
transmission capacity, significantly reducing the cost of long-distance
communications.
•
Fault-tolerance. You can use the multi-line path feature to enhance the reliability of
satellite connections. Using this feature, you can configure up to eight parallel lines
between nodes.
The major disadvantage of satellite connections is potentially long propagation delays
(approximately 240 milliseconds) when sending data to the satellite and then to the
destination node. The reliability of satellite connections can also be adversely affected
by weather and other atmospheric conditions.
X.25 Connections
X.25 is a standard for networks that use packet-switching technology. These networks
include SPRINTNET and TYMNET in the United States; DATAPAC in Canada; DATEX-
P in Germany; TRANSPAC in France; and PSS in Great Britain.
Expand-over-X.25 connections are provided with the HP X.25 Access Method
(X25AM) subsystem. X25AM supports line speeds of up to 256,000 bps, depending on
the X.25 network used, although speeds above 56,000 bps are not always available.
The major benefits of using X.25 are
•
Low cost. The cost of X.25 connections may be less than dedicated lines,
depending on traffic volume. Cost may be further reduced if multiple applications
share the same X.25 link. The Expand subsystem’s implementation of X.25 also
has an additional cost-savings benefit: connections can be automatically
disconnected when not in use and then automatically reconnected when traffic
appears.
•
Flexibility. Virtually any point in the world can be reached by a packet-switched
data network (PSDN). A PSDN can easily accommodate changing
communications requirements and network expansion.
Note. You can configure terrestrial lines to use satellite-efficient HDLC Extended Mode. This
type of configuration can enhance the effective capability of terrestrial lines that carry small
messages at high speeds.