Switching Fundamentals & Traffic Analysis

SWIT CHI NG FUNDA MEN TAL S Main function of an exchange is to process calls from calling subscribers and make the connection to the called subscriber. The connection can be direct or via another exchange. As exchange handles thousands of calls at the same time, switch has to handle complex relations between the Hardware and Software.

CALL P RO CES SING A RCHI TE CT URE The main function of the exchange is to process subscriber calls. Call Processing involves four basic switching functions: 1. SUPERVISION : Detects & reports service requests, acknowledgements and request to terminate services. 2. SIGNAL LING : Transmits information about lines and trunks and information about other aspects of call handling to control switching equipments. (a) Between two exchanges (b) Between exchange and subscriber 3. ROUTING : Converts address information to the location of the corresponding called line or to the location of the trunk on the way to that call. 4. ALER TING : Notifies a subscriber of incoming calls i.e. ringing .

BA SIC C ALL

TYPE S

INTRA -EX CHAN GE : These are calls between two subscribers served by the same exchange . These calls are normally line to line calls. INTE R-EX CHANG E : These are the calls that involve two or more exchanges . Within a given exchange there are different types of inter-exchange calls. 1. O/G call is a call that goes out of exchange via a trunk. If the call originated in the same exchange it is called an originating outgoing call. 2. I/C call is a call that comes in to the exchange via a trunk .If the call terminates to a line served by this exchange ; it is called an incoming terminating call. 3. Tandem call is a call that comes in to exchange on one trunk and leaves the exchange on another trunk.

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CALL P RO CE SSIN G ST AGE S INTRA -EX CHAN GE CAL L • • • •

IDLE DIGIT RECEPTION & ANALYSIS ROUTING TALKING

IDLE ST AGE Subscribers have a line that connects to SM via MDF or DDF of the exchange .This line is in one of the two states. 1. ON – Hook – IDLE 2. OFF – Hook – BUSY Associated with each line is a current sensing device in origination scanner . Subscriber requests service by lifting the handset. This closes the subscriber loop circuit and causes the current to flow in the loop Idle to Di gi t R ecep tion Transiti on This transition sets up the dialing connection and delivers dial tone to the subscriber line. Further, database terminal information about the subscriber, such as class of service, assigned subscriber services; type of line, and type of digit decoder is retrieved by the system. Digi t R ece ption and Analysis

Stage

The dialed digits are received and analyzed to determine routing. While in this state, many complex hardware and software translations are taking place. 1. Dial tone is released when the first digit is received. 2. Digits are collected and route is determined Ringin g Stage After digit analysis & proper routing, ringing stage is reached. Several functions are performed at this stage • If subscriber is busy, busy tone is given • If subscriber is idle , following tests are made 1. False cross to ground test. 2. Power cross test 3. Leakage test ( to avoid false ringing) 4. Continuity test : detects breaks in the line. If all of these tests pass, the exchange will provide ringing current to the called subscriber, audible ringing tone to calling subs. Also line is scanned for answer signal.

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Ringin g to Talkin g T ran sit ion and T alk ing Stage After answer signal, talking phase is established. The exchange monitors the lines for an on-hook signal from either subscriber. Talking to Idle Transit ion When the exchange detects a disconnect from either subscriber, it starts the procedure to tear down the call connection. Call termination implies that both lines are idled, and all resources are released.

IN TER -EXCHAN GE CALL 1. 2.

Ori ginat ing Terminat ing

Inte rexchange ca ll exchange in volv ed exchange .

( Line is in

to Trunk) ( T run k to Line)

tr eate d the ca ll

as an orig inatin g ca ll by the first flow and terminatin g ca ll by the last

ORIGIN ATIN G C ALL (a) Idle & Digit Reception & Analysis stage : The first two stages of an originating call are the same as the first two stages of an intra-exchange call. (b) Outpulsing : Originating exchange sends the dialed digits to the next exchange .Firstly , originating exchange sends the notification of the digits by sending seizure signal. Next exchange attaches a digit decoder and return a start pulsing signal to indicate that it is ready to receive digits. •

TALKING

ST AGE

After outpulsing of the address digits talking path is connected in the originating exchange and trunk is placed in the talking stage. Terminating exchange is responsible for ringing . Originating exchange after outpulsing is ready for talking stage but terminating exchange advances to talking through ringing stage. •

TALKING

T O IDLE

TRANSITION

Now calling line is supervised for a disconnect signal . The trunk is supervised for an on – hook signal from the called subscriber. Calling subscriber control applies to inter-exchange calls as it does to intraexchange calls.

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TER MINA TING •

IDLE

CALL

ST AGE

Idle incoming trunks are scanned by the exchange . An incoming call is detected when a trunk seizure signal is received. •

DIGIT

RECEPTION

& ANAL YSIS ST AGE

This stage is the result of other exchange and outpulsing the address digits. •

RINGING & TALKING

ST AGES

Same function as in intra-exchange calls. •

TALKING

T O IDLE

TRANSITION

Both the terminating line and the incoming trunk are supervised. If the called subscriber goes on-hook first, an on-hook signal is generated for that line. An onhook signal is also transmitted over the trunk but it will not take down the connection since the calling subscriber has control of the call.

TANDEM CALL Tandem call comes in to an exchange on one trunk and is routed out of the exchange on another trunk. To the originating exchange , the tandem exchange appears to be the terminating exchange . To the actual terminating exchange , the tandem exchange appears to be the originating exchange . Therefore, Tandem exchange has both impulsing & outpulsing but no ringing stage .

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ERL ANGS ERLANG is total usage during a time interval Divided by the time interval , many network management systems measure usage 1 hour intervals. e.g. if the base station logs a total usage of 72,000 seconds between 3600 seconds (1 hour) , then Traf fic intensit y =>

72 00 0 360 0

= 20 Er langs

GRADE OF SE RV ICE GoS defines the probability that calls will be blocked while attempting to seize the circuits. 1% GoS means 1 call out of 100 will be blocked G.001 =>

1 / 1000

be blocked .

Ckts.

GoS (.0 01)

GoS(.00 5)

30

16.7

17.7

60

40.8

44.7

90

66.5

71.8

120

93.0

99.4

CALL CO MPLE TIO N R ATE This count provides the ratio of total answered to the total attempted calls in %. CCR =>

IANS + OAN S ISEIZE + OATTMPT

CALL SUC CES R ATE This count provides the ratio of total seizures to the total attempted calls in % . CSR =>

ISEIZE + OSEIZE ISEIZE + OA TTMP TS

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