Understanding the network.pdf - Back to Home

PSN networks are commonly referred to as "clouds." This reference alludes to the
shared approach used to allocate network bandwidth and the open nature of PSN
networks.
X.25 provides for two types of full-duplex VC: switched and permanent. The VCs are
in essence just like telephone calls. They have several operational modes: call setup,
data-exchange, idle, call teardown, and reset. A switched VC (SVC) is a temporary
connection established between two X.25 connected endpoints. The process is as
follows:
A DTE has data to send, and an SVC call-setup request is made.
The X.25 endpoint establishes the SVC.
Data transfer occurs between the two DTEs.
An idle time is reached on the SVC.
The SVC is torn down.
With a permanent VC (PVC) the call is established once and remains active all the
time, so DTEs can send data at any time. X.25 uses the X.121 address format to
establish SVCs. The X.121 address is an International Data Number (IDN) which
consists of a data network identifier code indicating the PSN the X.25 endpoint is
attached to, and a National Terminal Number (NTN) which identifies the X.25
endpoint.
Frame Relay
Frame Relay is fast packet-switching technology based on STDM switching service.
Like X.25, it can support multiple logical VC connections over the same physical
connection. The service is provided mostly over a primary rate (T1/E1/PRI) PSTN
interface, and some carriers offer Frame Relay at data rates above DS1 (but not
many). Frame Relay is a ITU-T/CCITT standard that was developed during the ISDN
standards development process. It is strictly a connection-oriented data-link
protocol for packet-switched networks. Frame Relay was originally developed as a
high-speed transport alternative to X.25. It supports both SVC and PVC operation,
but does not have the error-control and recovery capabilities that X.25 supports.
Because Frame Relay was designed to operate over digital transmission facilities,
these capabilities are not required, as digital facilities have a significantly lower BER
than the analog electromechanical facilities X.25 was designed to use.