Understanding the network.pdf - Back to Home

The endpoint, which can be a router, modem, or any properly equipped DTE, is
connected to a buffered PSN packet switch, which functions as the station's gateway
to the PSN. When two endpoints exchange information over a PSN, the do so over a
Virtual-Circuit (VC) connection. VCs are established between PSN endpoints (much
like a phone call, in some cases), using their PSN addresses to establish and
maintain the "call" over the shared bandwidth infrastructure provided by the
network of interconnected buffered switches. A PSN endpoint can support single or
multiple VCs between different PSN endpoints. When the PSN endpoints have
established a VC, they can exchange data. The actual data exchange is handled by
formatting the data into a frame format appropriate to the PSN type. After they are
formatted, the frames that comprise the data are handed to the PSN switch
(indicating in the frame which VC should be used to deliver the data) which then
provides the transport over PSN.
The advantage of PSNs is efficiency. PSNs use STDM, and share the total available
network bandwidth between all the connected endpoints, so the bandwidth is
always being used to transmit data. The available bandwidth of the PSN is
determined by the number of subscribers; the trick for the carrier is to provide
enough bandwidth to facilitate adequate performance for all the connected
subscribers. The downside is that PSNs operate with a certain amount of inherent
latency. This is because each data request is first sent to a PSN switch that buffers
the request and then transmits it when there is enough bandwidth available to
complete the request. The bandwidth rate of the PSN circuit defines how much
bandwidth the PSN endpoint can sustain through its interface. The PSN carrier,
however, does not guarantee dedicated bandwidth, only that the bandwidth rate
will be supported. For many data applications, error-free communication is what is
important, not speed, so the additional latency does not present a problem. How the
actual bandwidth of the PSN is managed depends on which PSN technology is used
to provide the transport service.
X.25
The first large-scale PSN implementation was X.25. This is an ITU-T networking
standard that operates at OSI-RM Layers 1, 2, and 3. X.25 networks first began to
appear in the 1970s, and they provided only moderate data transmission rates.
Nevertheless, their lack of speed was offset by network-level error-checking and
recovery services, making X.25 a very reliable connection-oriented data transport
option. X.25 provides delivery acknowledgement for each packet sent, if required.
After a packet is transmitted, an acknowledgement must be received before any
more are sent. This approach, needless to say, adds significant overhead to the data
transmission process. These services were needed mainly because the original
PSTN transport network for X.25 was based on electromechanical switches and
analog transmission facilities, which were considered unreliable for data
transmission.