Normally
the earlier PSTN is designed for carry voice, but after has been developed to
carry data as well as voice.
There
are many methods used for data communication over the PSTN. They are PSDN, ISDN,
Dial up and X DSL
PSDN (Public
switched data Network) is
runs over normal PSTN line. It is a Packet Switched data network that based on
packet switching as opposed to circuit switching that used in PSTN. When we are
connecting to the internet (WAN) we use normal PSTN twisted pair cabled. In
this technology packets are routing according to the source and destination
address.
Integrated
Services Digital Network (ISDN) turns the local loop into a TDM digital
connection. It is purely digital network but runs on analog twister pair lines. Speed is one advantage ISDN has over
telephone line connections. There are two ISDN implementations. They
are,
Basic Rate Interface (BRI)
B channels –
Bandwidth of 64kbps
D channels – Bandwidth of
16 kbps
Designed as 2B + D
Primary rate Interface
(PRI)
B & D –
Bandwidth of 64kbps
The design varies
according to the nation.
Ex – North America
& Japan 23B+D
SDN Carries voice calls and
data simultaneously on one line. It uses time division multiplexing (TDM) over
two wire pairs. TDM Divides single channel into multiple
channels.
Dial-up connection
also uses the PSTN for connect with WAN. It provides
a low cost need based access. We can achieve bit rate up to 56kbps.In this
method Modem is connected to a Telephone Line on the Customer End. Remote Access Server (RAS) is connected to
Telephone Lines (33.6 Kbps connectivity) or E1/R2 Line (56 Kbps connectivity)
on the Service Provider End. Then RAS provide dialing connectivity,
authentication and metering. Disadvantage of this technology is achievable
bandwidth depends on the line quality and the traffic of the network.
Digital Subscriber Line (DSL) technology is a broadband
technology that uses existing twisted-pair telephone lines to transport
high-bandwidth data to service subscribers. The term XDSL covers a number of
similar yet competing forms of DSL technologies.DSL technology allows the local
loop line to be used for normal telephone voice connection and an always-on
connection for instant network connectivity. The two basic types of DSL
technologies are asymmetric (ADSL) and symmetric (SDSL). All forms of DSL
service are categorized as ADSL or SDSL and there are several varieties of each
type. Asymmetric service provides higher download or downstream bandwidth to
the user than upload bandwidth. Symmetric service provides the same capacity in
both directions.
Multiple
DSL subscriber lines are multiplexed into a single, high capacity link by the
use of a DSL Access Multiplexer (DSLAM) at the provider location. DSLAMs
incorporate TDM technology to aggregate many subscriber lines into a less
cumbersome single medium, generally T3/DS3 connection techniques to achieve
data rates up to 8.192 Mbps.
