|
URL:
http://www.itu.int
http://www.itu.int
Digital signal X is based on the ANSI T1.107
guidelines. Digital Signal X is a term for the series of standard digital
transmission rates or levels based on DS0, a transmission rate of 64 Kbps, the
bandwidth normally used for one telephone voice channel. Both the North
American T-carrier system and the European E-carrier systems of transmission
operate using the DS series as a base multiple. The digital signal is what is
carried inside the carrier system, typically via time division multiplexing.
DS0 is the base for the digital signal X series. DS1, used as the signal in the
T-1 carrier, is 24 DS0 (64 Kbps) signals transmitted using pulse-code
modulation (PCM) and time-division multiplexing (TDM). DS2 is four DS1 signals
multiplexed together to produce a rate of 6.312 Mbps. DS3, the signal in the
T-3 carrier, carries a multiple of 28 DS1 signals or 672 DS0s or 44.736 Mbps.
Telecom companies have developed transmission services which are essentially a
T1 line with some of the channels turned off. This is to target towards the
niche of cost-sensitive customers. Typical speeds for fractional T1's are 256,
384, 512 and 768kbps.
Main Features of Technology
|
Technology
|
Frame
based transmission over high-speed T1 circuits. Equivalent to X.25 without
network layer functions (node-to-node error checking)
|
|
Operation
|
Allows
customers to select port speed and request permanent virtual circuit (PVC)
with committed information rate (CIR)
|
|
Bandwidth
|
Amount of
bandwidth is adjusted to meet application, but limited
|
|
Capacity
|
Up to
capacity of T1 and multiple T1s
|
|
Coverage
|
Same as
that provided by LECs (local exchange carriers)
|
|
Data rate
|
For each
customer, port speed and CIR from 16Kbps to 256Kbps or higher, up to
the limit of the T1 or fractional T1 installed
|
|
No. of
channels
|
PVC ,
typically 64 channels per T1
|
|
Regulatory
|
None
|
Key Advantages
|
Capacity
|
Multiple
T1s can be provided rapidly by telecommunications providers
|
|
Coverage
|
Very broad
coverage for most areas, as provided by LEC
|
|
Data rate
|
Committed
information rate (CIR)
|
|
Access
|
Faster
network access without latency resulting from node-to-node error checking (in
X.25 network)
|
Key Disadvantages
|
Cost
|
Cost of
local access circuit can be high if the location of the carrier’s POP
(point-of-presence) is not in the same city
|
|
Access
|
End
devices need to perform error checking and request for re-transmission should
error be found. This may slow down overall data transmission
Network
congestion may cause frames to be discarded and will require re-transmission.
|
Keywords:
time division multiplexing, digital signal hierarchy, transmission.
|
