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Introduction to Telecommunication
Concepts
Lecture 1
BTN 302 Broadband Network
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Content
Basic concepts PCM and Voice Coding
Primary Level Multiplexing
Multiplexor and Transmission Hierarchy concepts
Telecom transmission concepts Switching and traffic engineering
Signaling and billing
Data
Enterprise networks
Cell based communications Packet based communications
Regulatory and business consideration
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Basic Concepts
Subscriber connection to the PSTN
A subscribers handset or Terminal Equipment (TE) is
connected to the Public Switched Telecommunication Network
(PSTN) by means of a twisted pair made up of two insulated
copper wires twisted together.
PSTN is represented by a cloud for simplicity
The purpose of twisting the insulated copper wires is to reduce
cross-talk, a type of electrical interference caused by proximityto similar signal carrying conductors
The infrastructure made up of the copper twisted pairs to each
subscribers premises is know as the local loop.
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Basic Concepts
Hybrid Circuits The twisted pair is connected to a hybrid circuit at the local
telephone exchange
The hybrid circuit takes the twisted pair from the local loop
and presents two pairs of wires, that is a 4 wires to theexchange
One pair carries the transmit signal (Tx), the second pairscarries the receive signal (Rx)
A circuit that utilizes two separate paths for Tx and Rx is called
a full-duplex circuit. The purpose of a hybrid is to reduce the cost of copper in the
local loop and to provide some isolation of exchangeequipment from the local loop.
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Basic Concepts
Voice PSTN was solely designed to carry voice, which is an analogue signal.
It has been determined through experiments that acceptable tollquality voice us possible by utilizing a bandwidth of less than 4khz
Humans perceives frequencies in terms of pitch High pitch tones will be composed of higher frequencies and low
pitch tones will be composed of lower frequencies
Human ears is particularly sensitive to certain frequencies
To allow for greater intelligibility certain parts of the voice signal are
amplified more than others The weighting given to each frequency is called psophometric
weighting
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Digitizing Voice
In a digital system the voice signal is digitized by ananalogue to digital converter at the exchange
Digitizing the signal allows it to be exactly replicated at thefar end of a network
The A to D converter uses Pulse Code Modulation (PCM) toconvert the signal from analogue to digital
It requires 8000 samples a second each sample being 8 bitsto accurately depict a human voice
The digitized voice signal requires a bit rate of 64 kbps
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Multiplexing
There are two types of multiplexing in common use:
Frequency Division Multiplexing (FDM) each analogue voice
message is carried on a different frequency within the carrier wave
Time Division Multiplexing (TDM) each 64 kbps message is
positioned in a different, individual time slot
The reason for multiplexing is to allow one transmission line
carry several signals, thereby reducing the cost and
complexity of the PSTN
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Primary Level Multiplexing
Time Division Multiplexing (TDM)
TDM is a technique for interleaving data from severalusers onto a single serial channel
Each user is given a distinct time period in which theirdata can be carried
Each distinct period of time is called a Time Slot (TS)
TDM is the method used throughout the PSTN to carrydigitized voice.
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Primary Level Multiplexing
Time Division Multiplexing (TDM)
Consider multiple signal sources each producing an 8 bitquantized sample every 125 us as per a PCM signal
In order that the far end receiver can extract theinformation in an intelligible form each quantized samplemust be presented to the far end every 125us
Whatever format for our TDM channel we decide on interms of number of voice channels, the format must repeat
every 125us or 8000 times a second
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Primary Level Multiplexing
Time Division Multiplexing (TDM)
TDM signal is made up of distinct signals each containedin separate TS, each TS contains 8 bit quantized sample.
Now if the TDM contains 32 TSs of 8 bits and therequired repetition rate is 8000 times per second, werequire a TDM signal with a bit rate of 2,048,000 or2.048Mbps
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Primary Level Multiplexing
E1
Primary rate for TDM is at 2.048Mbps (Europeanstandards).
This contains frame structure of 32 distinct TSs
This format is more commonly known as an E1
In an E1 TS0 is used for synchronization, alarm transportand to transport international bits
TS16 is used for signaling purposes This leaves 30 TSs to carry voice or data traffic at 64 kbps.
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Primary Level Multiplexing
T1
In North America and countries where TDM was adoptedearly, TDM frame consists of 24 8 bits TSs plus a specialbit called single framing bit
The frame repeats 8000 times a second or every 125us
The bit rate of a T1 signal is 1.544 Mbps
One bit from each TS is reserved for signaling and voice isquantized using 7 bits
Voice is carried over 56 kbps instead of the 64 kbps in anE1
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Telephone Numbers
In order to connect the calling party to the called party it
must be possible for the PSTN to automatically route calls
through a network
The calling party provides the routing information to the
PSTN when dials a telephone number
A telephone number is constructed hierarchically as
follows:
International access code (optional)
Country codes and regional codes (optional) Called parties telephone number (mandatory)
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Switches
The routing machines that makes routing decisions are
called switches
Switches are arranged in hierarchy to make call processing
easier and quicker Each switch is aware of the national and international
numbering plan.
Numbering plans are regulated at a national and an
International telecommunications Unions (ITU)
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PCM and Voice Coding
Why digitize voice?
Digitizing the signal allows it to be exactly replicated at
the far end of the network
Quality is not affected by the number of elements in the
network as the signal is regenerated at each node in the
network
Cross-talk which is caused by adjacent cables is almost
eliminated
Manual adjustment of analogue equipment is eliminated
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PCM and Voice Coding
How many samples are required?
In order to represent an analogue signal we need to
sample at twice the highest frequency contained in the
analogue signal, this requirements is known as Nyquists
theorem.
Sampling at higher rate does not add any useful
information
Given that the PSTN is based around a voice signal
where the highest frequency components is less than 4khz it is adequate to sample the signal 8000 times a
second for one sample every 125us.
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Telecom Transmission Concepts
Key transmission Technologies Twisted pairs
Coaxial Transmission Lines
Microwave Radio
Fiber Optics
Satellite communications
Each technology has a specific role to play in thedesign of the core network, the back-haul and thelocal loop.
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Coaxial Cable
Coaxial Cable consists of an inner conductor surrounded bya dielectric with an outer conductor, which operates as ashields
It is an unbalanced transmission line
Coaxial cables are generally confined to the back-haulnetwork and for connecting multiplexors at all levels of themux hierarchy
Before the advent of the fiber optic technology, coaxial
cables were used in the backbone network and for theintercontinental undersea cables
Coaxial cable transmission systems require significantlymore repeaters than the equivalent fiber optic system
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An optical fiber consists of a strand of silica glasswhich has been doped to modify the refractive indexof the glass
Light travels down the center of the fiber, the center
of the fiber is the core Transmitter sends light down the fiber, it received at
the other end by a detector
There are two types of fiber optic cable in commonuse: Single mode very small core
Multimode has a larger core
Fiber Optic Cable System
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Microwave Radio
Microwave radio transmission system are used in thebackbone, the backhaul and the local loop, they are flexible,economic and can carry high capacity traffic.
Used radio wave in the 1Ghz to 50Ghz range
One of the main characteristics of microwave radio is thatLine of Sight (LOS) between the transmitting antenna andthe receiving antenna is required
Generally higher frequency systems are more prone
to rain and atmospheric attenuation Microwave systems have to be carefully planned to
avoid RF interference and rain attenuation
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Satellite System
Satellite transmission system use radio frequencies calledmicrowave
A ground station communicates with a spacecraft, thespacecraft appears to be stationary from the earth but
moves through space in sync with the rotation of the earth Only spacecraft at a distance of 36,000 km from the earth
can orbit in this manner, this orbit is called geosynchronous
The spacecraft carries several radio relays, called
transponder, satellite communication can cover vastdistances at a fixed cost.
It is used primarily for communicating internationally orwith extremely remote communities
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Switching and Traffic Engineering
Circuit Switching
First circuit switches were manual and required operator tomanually connect calls with jumper cables
An undertaker called Almon B. Strowger invented the firstautomatic circuit switching equipment
Example of switches: Crossbar switches
Space Division Switches
Time Division Switches
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Probability Blocking
The interest of the network designer is the number of trunkcircuits needed to satisfy the call demand placed upon thelocal exchange
Although every subscriber is connected to the local
exchange, the same number of egress (outgoing) lines froma switch will not be required
The number of egress lines is chosen such that there is asmall probability of blocking
The probability of blocking is not the same at 3 am as 3 pm.Probability of blocking is calculated based on a busy-hour
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Erlangs
Calls arriving at a rate lambda during the busy hour, withan average holding time T, offers a traffic load of lambdaTErlangs in the absence of blocking
Erlangs are the units in which the average busy hour traffic
load is expressed If we know the traffic load offered in Erlangs and the
number of trunks, the probability of blocking can bedetermine through the Erlang B formula
We can also design a system with a predetermined
probability of blocking, known as the grade of Service(GOS), by providing sufficient trunks for the traffic load inErlangs.
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Enterprise Networks
Voice and PABX Centrex Services
WANs and IP
Data and Voice Convergence Service Provider Networks
ATM
All IP Networks
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Voice and PABX
A large corporation campus will normally have its own
wired network for telecommunications
The wired voice network will be connected to a Private
Automatic Branch Exchange (PABX) PABX will route calls internally to users telephone
extensions and may require an operator on site.
The term PBX is also used instead of PABX
The PABX will then connect with the PSTN of choice
PSTN operator will ensure that all voice calls are processed
in as efficient a manner as possible
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PBX Advance Features
Some of the common feature includes:
Direct dialing (DDD or DDI)
Customized Abbreviated dialing (speed dialing)
Follow-me
Call forwarding on absence
Call forwarding on busy
Call transfer
Music on holdAutomatic ring back
Call distribution (ACD)
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Centrex Service
Some public toll switches can offer feature sets similar to
those on a PBX
These services are commonly marketed by PSTN under
the name of Centrex (CTX) services
Centrex CO indicates that all equipment except the
attendants position and station equipment is located in
the central office
Centrex CU indicates that all equipment, including the
dial switching equipment, is located on the customers
premises
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WANs and IP
Leased Lines play an important part in enterprise data
networks and comprise part of what is commonly known as
a Wide Area Network (WAN)
Enterprise leases a fixed capacity connection between twoor more points and runs either Frame Relay, ATM or X.25
IP runs at a higher level and takes care of addressing of
data and delivery of data
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Data and Voice Convergence
As Enterprise data networks were traditionally designed forthe delivery of data packets, latency or delay was of
minimal concern
A reasonable delay between packet transmission and
reception was deemed acceptable However there is a desire on the part of enterprise to
recoup some of their investments in their networks and one
of the obvious means is to ensure that the same links can
carry data and real time voice and video This move towards a network that can accommodate the
two very different requirements is know as convergence
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Service Provider Networks
each Enterprises data network is designed for therequirements of that particular enterprise
As such, reliability and robustness requirements, are a
purely individual decision
Service provider network (SPN) requirements are different.
As SPNs sell services to other parties normally with some
Service Level Agreement (SLA) robustness becomes a
more important consideration.
SPNs normally design their networks within availabilityfigures recommended by the ITU
Equipment sold to SPNs will normally have high levels of
redundancy with a high Mean Time Between Failure
(MTBF)
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ATM
Asynchronous Transfer Mode (ATM) has been designedwith the requirements of convergence in mind.
It is a connection-oriented protocol that conditioned traffic
according to a traffic contract
The traffic contract can guarantee that delay variation is
within acceptable constraints and can provide bandwidth on
demand
ATM represents a good compromise for PSTN operators
because it can support the traditional cash cow oftelecommunications i.e. voice and accommodate newer
opportunities in the data communication world.
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All IP Networks
advocates of an all IP networks predict that trunks androuters will become so fast in the future that latency will be
only a minor issue that can be overcome by a set of quality
of service labels which will give preference to real time
requirements An all IP network represents a step into the unknown
It also requires huge investments in equipment with
uncertain payback.
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End of Lecture 1
- Telecommunication Overview -