2.3 Leased Lines
A leased line is a dedicated and permanent connection between two points. This differs from PTSN because there are no telephone numbers needed.
Depending on the country there are different types of leased line which are indicated by a code. The U.S. uses the T series of codes and Europe uses the E series of codes.
Common examples of leased lines and the bandwidth provided are:
T1 = 1.544 Mbps
T3 = 44.736 Mbps
E1 = 2.048 Mbps
E3 = 34.368 Mbps
This is an early time tested WAN set of protocols used in packet switching networks. X.25 networks utilises routes as and when they are available to select the best route at that time.
Early X.25 implementations used analogue telephone lines for data transmission.
X.25 is an ITU-T standard protocol suite that works at the first three layers of the OSI model: Physical, Data Link and Network layers.
It has extensive error-checking capabilities inherited from the days when WAN links were more prone to errors. This high level of error checking makes X.25 reliable but limits its bandwidth. The inbuilt and extensive error checking of X.25, although an advantage due to the unreliability of early media used has now turned into a disadvantage and a burden because it is no longer required with a more reliable infrastructure.