Page 61 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes 4.3.1 Digital Channel

Original telephone networks were based on analog voice connections through manual
switchboards. Gradually, digital switch technologies were used to connect digital circuits between
exchanges with analog two-wire circuits to connect to most telephones. The basic digital circuit
uses Digital Signal 0 (DS0) that is 64 kilobits per second channel to carry a typical phone call
from a calling party to a called party. It uses 8-bit Pulse Code Modulation (PCM) with 8 kHz
sample rate to digitize the audio sound ranging from 30 Hz to 3300 Hz. In addition to this a
guard band of 70 Hz is used to keep the voice clean and clear. Thus, audio sound ranges from 0
Hz to 4,000 Hz. According to sampling theorem, a sampling of twice of the audio signal is
required to reproduce the sound. The sound signal is sampled at the rate of 8000 times per
second.

4.3.2 Trunk Lines

The trunk lines in telecommunications that are always digital in nature provide high-speed
connection between central offices in the PSTN system. The twisted pair wiring between central
offices were prone to crosstalk and noise. The twisted pair was also expensive to lay from CO to
CO. Gradually development in telecommunication field led to the development of TDM
techniques to carry the data over the existing copper lines and subsequently on the optical fiber
cables. The fiber cables uses statistical time-division multiplexing, synchronous digital hierarchy,
coarse or dense wave division multiplexing and optical switching to further improve
transmission speeds. The trunk lines therefore contain thousands of simultaneous calls that
have been combined using TDM to carry them from one CO to another CO. A signaling protocol
SS7 is used to transmit call from one telephone exchange to another. At CO, they are
de-multiplexed and switched through digital access cross connecting switches to reach the proper
exchange and local phone number. Some of the examples of trunk lines are T1, T3, DS1, DS3, Tie
line, Tie trunk, etc.

T1: A T1 that is considered a dedicated circuit is available as full T1, channelised T1 and fractional
T1. A T1 circuit is composed of the local loop of the local service provider and the carrier circuit
provided by the same company or different service providers. A full T1 service often referred to
as a digital trunk line is usually available as a complete circuit of up to 1.544Mbps total speed
either as data or voice but not both. A channelised T1 as it name indicates contains 24 individual
channels which are capable of carrying voice or data. The full set of channels provides the same
speed as a full T1. The individual channels may be divided into voice lines for telephone services
or data lines for Internet services using a device called a Channel Service Unit/Data Service Unit
or CSU/DSU. A fractional T1 is available less than a full T1’s bandwidth in which one or more
channels bundled together. Similar to the channelised T1, individual channels can be voice or
data and a CSU/DSU is used to split the channels. A T1 circuit always remains on and therefore
is referred to as private lines or dedicated data line.

DS1: Digital Signalling Level 1 (DS1) contains 24 voice TDM into 192-bit frames across single
physical connection providing 1.544 Mbps data throughput across a T1 Physical Layer digital
voice connection.

DS3: Refers to a telecom circuit that carries multiple calls from one central office to another and
also termed as tie trunks or tie lines. A DS3 contains the equivalent of 28 T1/DS1 circuits by
decreasing the time slicle allotted for each sample of data and multiplexes the T1’s together to
form the final DS3 data stream.

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