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Telephone network

T-110.300Jouni Karvo, Raimo Kantola, Timo Kiravuo

Telephone network

• Background• Topology• Reference connection• End systems• Access network• Transmission• Switching & Signaling• Layers• User-Network interaction• Liberalization• Business roles• Telephone numbers

Background

• World's largest machine; extends to all countries• Huge economic and social importance• Specialized in voice transmission• Other applications have been created: fax, data etc.• Basic service: full-duplex voice transmission• Small end-to-end delays, small delay variation (more than

150ms delay disturbs discussion --note satellites!)• Call admission control, and accepted calls will complete• Grows all the time, now mostly growth of mobile networks• Although most traffic in the telecom networks is now data,

most of the money comes from voice

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Background cont.

• Telephones are addressed by telephone numbers, that are unique

• There are special numbers or area codes that need translation

• The network formed by the end systems (telephones, faxes, modems etc.) and the hierarchic switching and transmission systems is called Public Switched Telephone Network (PSTN) or informally as Plain Old Telephone System (POTS).

Topology

• Basic structure is tree-like• The core network more fully connected• Direct links for some special busy routes

Reference connection

• ITU-T E.830• The longest possible international telephone connection

model• LE = Local Exchange, ISC = International Switching Center• Delay of more than 20ms requires echo cancellation

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End systems

• Telephones: Voice-electrical signal transducers (microphone+earphone), dialer, ringer, switch hook

• A single twisted pair is used for signal transmission for both directions � echo

• Dialing+ringing: either pulses or tones

The Access Network

• a) distribution panel• b) subscriber terminal

equipment• c) radio link for emergency

traffic (112, 911 etc.)• d) fiber link,

• i) PCM multiplex• e) concentrator• f) telephone exchange• g) fiber link• h) subscriber lines

The Access Network

• The distribution panel connects operator's cables to subscriber lines– E.g. a thick cable of 30 pairs to individual lines to apartments– Usually located in customer premises (building)– The subscriber lines to the apartments are often the

responsibility of the building owner (Finland) – The cable to the telephone network is the responsibility of the

telco operator• The concentrator collects a group of subscriber lines

– I/O cards for subscriber lines. – A/D and D/A circuitry for the analog lines

– Fiber link or twisted pairs carrying PDH/SDH towards the telephone exchange

– May have a radio link for emergency calls (if the main link fails)

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The Access Network

• Static concentrators have PCM streams flowing constantly for each subscriber (even if there is no call going on).

• Dynamic concentrators generate a PCM stream only when necessary, using PDH/SDH towards the telephone exchanges.

• Concentrators do not switch. If a subscriber (A) wants to make a call to subscriber (B) connected to the same concentrator, the call goes through the nearest telephone exchange.

• The only switching type action is routing emergency calls through the radio link in case of failure in the fiber link.

Transmission

• The network elements are connected with transmission links• Link metrics: capacity (usually measured in 64kbps

channels), and delay. • For digital transmission, signal distortion is usually not a

problem, but ITU-T has defined signal quality assessment methods, originally for analog transmission

• Delay can be a problem for digital transmission, too; normally delays over 0.1-0.2s irritate people discussing

• Multiplexing techniques (PDH, SDH) are used to reduce the number of individual lines in cables

• Link costs dominated by installation labor and rights of way• Technologies: twisted pair, coaxial, terrestrial microwave,

satellite microwave, optical fiber

Switching & Signaling

• The way to connect a subscriber to another --- no need for a fully connected topology

• Switch hardware carries the voice• Switch controller controls the hardware• Signaling is the means for switches to communicate control

information with each other• In PSTN, control and data are separated in different

networks• The control network is called signaling network• The signaling network is a packet switched network, and

uses common channel signaling, i.e. signaling for all users uses the same signaling channels.

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Layers

• [7] IN services; answering machines– Need to differentiate, new services, billing

• [3] switches, exchanges– Dimensioning, routing

• [2] transmission– Coverage, capacity, radio design

• [1] twisted pair, coaxial, fiber, radio– Rights of way, re-use, natural monopoly

• Different parts of the system have internally protocols of any layer

User-Network Interaction

• The users of the telephone network act randomly, and generate load that varies through the day.

• The stochastic nature of load leads to the use of probability theory for defining performance properties of the system.

• ITU-T has defined the so-called Grade-of-Service parameters for telephone systems.

• As an example, the standard ITU-T E.543 defines the following GoS parameters for telephone exchanges:

High loadNormal loadParameter

0.010.002Internal loss probability

(X>1s) <= 5%(X>0.5s) <= 5%Through connection delay

(X>1s) <= 5%(X>0.5s) <= 5%Exchange call setup delay

(X>1s) <= 5%(X>0.5s) <= 5%Incoming response delay

Liberalization

• Providing telecommunications services has traditionally been a government or private monopoly– In Finland, local telcos ("puhelinyhdistys'') private– Government long distance monopoly (Tele)

• Liberalization opened the market in the 90's: – Several long distance national and international carriers – Several GSM networks– Local network owners have to provide for other operators

– Local loop still often like a monopoly in practice

• Liberalization is supposed to lower prices, improve customer service and introduce new telecom services

• Current regulation sets requirements for technology and critical services

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Business roles

• The model is an idealistic view, but starting to happen

• Many GSM service operators do not own their own network, but focus on customer management and billing instead

Telephone numbers (ITU-T E.164)

• A telephone number can point to a subscriber or to a service• A telephone number of a subscriber is also an address for

routing of the call– Area code, city areas

• Number portability breaks this connection• Service numbers are "logical'' and require number

translation for use as routing addresses– 112, 118 etc.

• The user must be able to deduce the expected cost of the call from the telephone number. Thus, the number allocation is tied to geography and network topology

Telephone Exchanges

T-110.300Jouni Karvo, Timo Kiravuo

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Telephone exchanges• Subscriber lines are connected to the telephone

exchanges– Originally each customer had a direct connection

(electrical pair) to the exchange

• Current exchanges are complex and expensive– A need to reduce their number

• Physical cabling becomes an obstacle– Concentrators are used to reduce the number of cables

• As the number of telephone exchanges is decreasing, the hierarchy of telephone exchanges is flattening

The structure of a telephone exchange

• a) Control unit• b) I/O• c) Switching

fabric

The most important tasks for telephone exchanges

• Connecting to subscriber terminal equipment and other exchanges

• Telephone number analysis and translation• Collecting accounting data for billing• Logging data for the use of authorities• Switching calls

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I/O

• I/O circuitry connects the telephone exchange to the outside world. The I/O is arranged in hot-swappable I/O cards.

• There are several types of I/O cards (part of which are also used in concentrators)– subscriber line cards (which contains circuits for

connecting several subscribers)– PDH/SDH/ATM cards

Subscriber line cards have several tasks:

• [B] Giving power to the terminal equipment (battery) • [O] Over voltage protection• [R] Ringing• [S] Supervising the status of the terminal equipment (e.g. if

the handle is on or off hook in an old style telephone)• [C] Coder and Decoder (A/D or D/A converters, A-law/µ-law

(de)coding )• [H] Hybrid - a transformer between two half-duplex and one

full-duplex connection• [T] Testing the status of the subscriber line

• Memory rule; BORSCHT, the red soup.

Switching fabric

• The task of the switching fabric is to transmit the PCM streams between two I/O ports.

• Both Time and Space Division multiplexing are used.• One measure of the size of the exchange is the capacity of

its switching fabric.• It is easy to find books describing design of switching fabrics

– in a sense it is one of the "easy'' but interesting parts of thetelephone exchanges. We will cover the structure of the switching fabrics only briefly;

• Two main types of switching fabric are used: the crossbar switch and the interconnection network.

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Crossbar switches

• Often used in small scale exchanges

• Usually has an electronic switch for each line– To connect n lines, n^2

switches are needed– A typical space division switch

– Uses physical space for connections

– Solid state technology

Interconnection Network

• Used in larger switches– Different types available

• Interconnection networks are made from a number of smaller switching elements.

• The idea in interconnection networks is to save in the number of crossbars using special topologies.

Time Division Switching

• Time Division Multiplexing is used in combination with the above mentioned structures. There are different structures: Time-Space, Space-Time, Time-Space-Time etc.

• The idea in Time division Multiplexing is to rearrange the time slots in a PCM frame (E1).

• Time slot interchanger (TSI):

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Time-Space-Time switching

• In Time-Space-Time switching, the internal crossbar works at a higher rate than the input line rate– Any time slot can be switched to any time slot– The crossbar configuration changes for each time slot

SS#7

T-110.300Jouni Karvo, Timo Kiravuo

Signaling

• The network made of exchanges and links must be co-ordinated in order to make calls possible

• The purpose of signaling is just to do this: it– establishes calls,– monitors the calls, and– tears down the calls.

• In-band signaling was used earlier– MF tones waste of resources– Security problems

• A new signaling system was created:– Common Channel Signaling Subsystem #7

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Common Channel Signaling Subsystem #7

• SS#7 is a packet switched network on top of the telephone network

• It is used to transport signaling information– It controls the trunk connections– SS#7 does not transport user data

• SS#7 packets use either a dedicated 56 or 64kbps channel (such as E1 TS16) or dedicated links.

• If signaling does not work, the whole telephone network is "dead''. Thus, stringent reliability requirements.– There is a lot of redundancy; every part of the signaling system

has at least one spare

SS#7 elements

Operator A Operator B

• The actual telephone exchanges switching the actual telephone traffic are called Signal Switching Points (SSP) (circles).

• The packet switches (routers) of the SS#7 network are called Signal Transfer Points (STP) (squares with a diagonal).

• The databases for advanced call-processing capabilities are called Signal Control Points (SCP) (cylinders).

SS#7 overlay network:

SS#7 Protocol Stack

• SS#7 uses a packet switched protocol set to carry its mission.

• The protocol stack generates a packet switched network over a circuit switched one

• MTP-x protocols (Message Transfer Part) protocols provide transport of SS#7 messages– MTP-2 is the link level protocol– MTP-3 provides network level functionality

– Routing to specific elements

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SS#7 Protocol Stack

• The upper parts (layers) provide functions for call management– Telephone User Part (TUP) for traditional telephone signaling– Data User Part (DUP) for datacom users– ISDN User Part (ISUP) and Broadband ISDN User Part (BISUP)

for ISDN and ATM users.

Twisted pair / fiber

PDH/SDH/ATM

MTP-1

MTP-2

MTP-3

SCCP

TCAP

OMAPISUPBISUPDUPTUP

Signaling Connection Control Part (SCCP)

• Provides connections for the parts above it• Incremental routing

– Global Title Translation, GTT (compare to DNS)

• Sub-addressing for applications within an element– SSN = Subsystem Number (compare to ports in UDP

and TCP).

• Four classes of service available: – [0] basic connectionless class,– [1] sequenced connectionless class (messages arrive in

order),– [2] basic connection oriented class, and – [3] flow control connection oriented class.

SS#7 Protocol Stack Above SCCP

• Operations, Maintenance and Administration Part (OMAP) provides services for the operator – Include updating routing tables, and diagnostics

• Transactions Capabilities Application Part (TCAP) provides database transaction service for the upper level protocols (such as INAP).

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SS#7 Environment Adaptation

• SS#7 is used for the core network • For the access network, there are different signaling stacks:• For ISDN: LAPB/LAPD corresponding MTP-2 on top of

which Q.931 corresponding to ISUP.• For B-ISDN:

– MTP layers 1-2 can be replaced by a specific Signaling AAL protocol (SAAL)

– Three layer protocol: bottom: AAL5, then SSCOP (Service Specific Connection Oriented Peer-to-Peer Protocol), then SSCF (Service Specific Coordination Function). SSCOP+SSCF = Service Specific Convergence Sublayer.

– BISUP can be replaced with ATM Forum UNI or ITU-T Q.2931.