•What is a telephone and how does it work?•How does the phone network work?•How are calls routed in the phone network?•Newer technologies: IVR, DOV.

Dynamic routing: • Several alternate paths.• Selects route based on the current state of the network.• Considers time of day, and day of week.

Strategies• Centralized versus distributed• Time-dependent versus Adaptive.

X

K

Y

JI

Route aRoute b

Route c

Rou

te d

Denver

New York

ChicagoSeattle

Pittsburgh

Time Period First Route Second Route Third Route Fourth Route

Morning a b c d

Afternoon a d b c

Evening a d c b

W eekend a c b d

OVERVIEW OF TELEPHONE NETWORK,SWITCHING CONCEPTS AND PBXs

•What is a Switch? •Switching Principles?•What is a PBX?•What is a call center?•What are special phone services?

Point to point lines

If we could have point to point lines between all sendersand all receivers, then we would not need switching.

Switching

Imagine point to point lines between all pairs of people:

# of people # of lines2 13 3

10 45 100 4950 1000 0.5 million

In general, for n people, # of lines = n(n-1)2

162 mill. Hosts > 13 Quadrillion

The Good Old Days

The Even Better Present Time

A technician replaces a line card in the telephone switch. A card handles 24 telephones.

Source: http://www.bpa.gov/Corporate/KCC/circuit/99ci/ci0699x.shtml

Denver

NY City

SwitchBob Alice Jane

Dick

JohnBeth

Dave• Lines or wires from everybody's house go into the switch• When a call is placed, the switch creates a temporary link

between these lines.

Bob

Dave

Alice

Dick

Types of switching

CIRCUIT SWITCHING: creates a continuous, dedicated path between sender and receiver.Must set up an end to end path before any data can be sent.

PACKET SWITCHING: No dedicated path between sender and receiver.Examples:

Circuit versus Packet Switching

Circuit Switching

Source: Tannenbaum, Computer Networks, 3rd Ed., 1996

Packet Switching

A simple switch

Circuit switching• Space division switches: Signal paths are

physically separate from one another (i.e. divided in space). – Single-stage – Multi-stage

• Time-division switches: Same path is shared by multiple streams (division in time)

Modern switches are combination switches

Blocking• Blocking means that even if two parties are free

they cannot call one another because of congestion in the switch (“All circuits are busy, try later!!”).

• If one or both parties are busy, then it is not a case of blocking.

• Single stage, space division switches are non-blocking

• Multi-stage, space division switches are blocking• Time-division switches are usually non-blocking.

1

2

3

9

8

7

6

5

4

10

8

7

6

5

4

3

2

1

9

10

Three Stage Space Division Switch

4

8

PBX (Private Branch Exchange)

• First generation : late 1800s, manual operator

• Second generation: 1930, no operator required

• Second and half generation: 1970, Computerized Business Exchange. – computer controlled switches– programmable, more intelligent– least cost routing of phone calls

• 3rd/4th generation: 1980 - , AT&T Definity 75/85.– “All digital” switching – Both computer data and voice– Simultaneous voice and data– Non-blocking operation.

Switchboard for a company or university.

CENTREX

A Centrex (Central Office Exchange Service) is similar to PBX, but owned and maintained by phone company. Cost is 20 to 50% higher than the cost of plain telephone lines.

Call Center• More than $50 Billion business in North America.

• Growing at 20% per year.

• A central place where customer calls are routed by an organization, usually with some amount of computer automation.

• Sophisticated Routing: Ability to handle a considerable volume of calls at the same time, to screen calls and forward them to someone qualified to handle them, and to log calls.

• Call centers are used by: mail-order catalog organizations, telemarketing companies, computer product help desks, and any large organization that uses the telephone to sell or service products and services.

• Achieve economies of scale associated with mass call handling and yet give callers a tailored feel.

Advanced call routing

• Skills based routing instead of “next-available-agent.”

• Key customers can move higher in the queue or be routed to agents assigned to their account.

• Callers who placed service calls within the last 24 hours can be routed to the agent they originally spoke to.

• Calls can be routed to a properly skilled agent, depending on the products or services previously purchased.

• A telemarketing group can route calls to agents with the highest closing ratios in order to increase sales revenues.

Challenges: supporting multiple types of transactions; multiple skills.

Improve speed and quality of call based on information in the database, and input provided by caller (e.g., IVR). Examples:

Bus_136:

IVR=Interactive Voice Response

Bus_136:

IVR=Interactive Voice Response

Summary•Switching is a very important telecommunications technology.

•Switching technology has become very fast (nano-second speeds).

•Switches are essentially computers with millions of lines of software.

•Computer telephony integration (CTI) has made it possible to develop very sophisticated PBXs and call centers.

•Vendors are coming out with newer products and features every day in telephony equipment.

•The telephone system is moving closer to becoming all digital except for the local loop which is still analog.