Lecture 7

The Public Network

Chapter Outline

• Switched Services

• Dedicated Services

• Signaling

Public Switched Network

• The public switched network is:– crucial to a countries national security – central to a healthy, growing economy– essential in times of public emergencies and

natural disasters

Switched Services

• Dial-up: users dial a number to create a temporary network connection

• Anyone on the network can be reached by anyone by dialing a number

• Pay-for-use service for voice, image and data traffic, analog or digital

• When call completed, network free for next person

Dedicated Services

• Dedicated, private lines fixed configuration

• Clearly defined boundaries, users fixed

• Used for voice, video, and data, analog or digital

• Users pay flat monthly fee

• Dedicated to an exclusive set of users

Definition Switching

• Premises equipment which performs the functions of establishing and releasing connections on a per call basis between two or more circuits, services or communication systems

ITU Definition Switching

• The establishment on demand, of an individual connection from a desired inlet to a desired outlet within a set of inlet and outlet for as long as is required for the transfer of information

Dialing

• Anytime dialing is done switching occurs

• Switched services are completed in a central office (CO)– where calls are routed based on the number

dialed

• Switched services have brand names: UniPlan OneRate, Real Solutions

Attributes Real Time Switching

• Addressing

• DTMF

• Pay-as-you-go Postalized Rates

• On Demand

• Immediate

• Analog or digital

• Incoming, outgoing, two-way

Addressing-Flexibility

• The telephone number is an address

• Numbers are sent in dual tone multi-frequency tones (DTMF)

• Tones are decoded at CO to address signals– area codes are assigned to metropolitan areas– exchanges are assigned to a CO– last four numbers assigned to individual users

Number Portability

• Numbers within an exchange assigned to different CO’s if users moves– if a customer changes from their local

telephone company to a CLEC, their phone number will be assigned to a particular central office, the last four digit of their number will be assigned to equipment associated with their carrier

Transmitting Data w/ Switched Service

• Flexible

• Can be used with multiple locations and offers many choices of medium– ISDN– Switched 56– POTS

• Frequently used in teleconferencing

Switched 56

• Switched data service which lets you dial someone else and transmit at 56kilobits p/s

• circuit switched service• full duplex transmission of data• digital synchronous 56kbps for price of

telephone call• most widely used switched digital service in

North America

DTMF

• 1963: Touch tone or Dual Tone

• Prior to 1963: Pulse tone utilized– Touch tone: dials 10 digits in 1 seconds– Pulse tone: dials 10 digits in 11.3 seconds

• Adds efficiency to the network

• Speeds up processing time for CO switches during set up

DTMF

• Used to:– access voice mail– bank accounts– entering orders for home shopping

• from telephones, once connected..

• DTMF tones passed along in system to interface with voice mail, etc..

DTMF Signals

• are a standard established by ATT

• Provides consistent functionality and format

• Continues to be expanded upon

Pay-as-you-go Service

• Dialing addresses a call in the network

• DTMF tones establish a temporary private connection

• Services are charged on the amount of time calls are connected

• Time-of-day rates may vary, peak hours

• Network based on some average usage

Batch or Bulk Usage

• To save on usage charges

• Retailers sent days transactions during off-peak hours, midnight

Usage Costs

• Used to be based on distance of call

• Long distance used to be the most expensive

• No longer distance sensitive

• Flat rate, postal rate for service today in LD

• Now based on time

Immediacy/Urgency

• Given capacity, usage is instantaneous

• Capacity is influenced by extreme weather conditions and power sources

• Safety:– remote alarm monitoring systems– back-up systems

The “Last Mile”

• Copper

• ISDN

• DSL

• Cable Modem Service

• The final cable connection to a home

ISDN

• Switched digital service

• Basic rate ISDN with two paths supporting voice or data and a third slower path dedicated to signaling

DSL

• Digital subscriber line services

• Works on copper cabling

• Not a switched service, is always available in the “last mile” on a dedicated path from the telephone company to the customer

• Connection always on!

Cable Modem Service

• Non-switched connection

• Always on data communications and Internet connection

• High-speed services

• One-way being converted to two-way– Huge investment

Design Assumptions PSTN

• Public Switched Telephone Network– Assumptions:

• Based on voice traffic where at any given time not every telephone user would be on a call

• The average call used to be 3 minutes, today the average call lasts 20 minutes, some last 12-24 hours

• Today there are more frequent calls and longer calls

• Increased demand on public switched network will need expensive upgrading

Inefficiency of Circuit Switching

• Switching of circuits results in use of a connection for the duration of the call

• A path is established between callers

• Not shared

• Capacity dedicated to the call

• Wasteful, can’t share usage when there are pauses in conversation, etc.

Dedicated Services

• Available for exclusive use of owner

• Placed at predetermined locations

• Cheaper than switched circuit use when volume is high

• Flat fee per month, usage not charged by volume

• Very secure transmission of data

Attributes of Dedicated Lines

• Fixed monthly fee

• Fixed routes

• Exclusive use

• 24-hour per day availability

• Voice, video, and data

• Fixed capacity

• Analog or digital

Fixed Routes

• Dedicated circuits are not shared

• Put into place to exclusively transport traffic from one location to the next– Video-conferencing– Retail stores to warehouse– Manufacturing plants to dealers– Global companies

Voice, Video and Data

• Firms often lease T-1 lines to have 24 channels to tie two locations together

• May use 10 data and 14 for voice or video

Security

• No problem transmitting proprietary or financial information

• Added encryption may be added

Applications Dedicated Services

• Video transmission to multiple sites

• Transfer of customer calls between sites within a company

• Transmission of orders to factory

• Bulk transmission of x-ray images

• Database access between different sites

Topologies Dedicated Lines

• Point-to-point: one line, two locations

• Multi-point: more lines connecting several sites

• Star configuration: all locations connect to a central site

• Mesh design: all points on network connect to each other

Pricing

• Based on distance and speed of medium

• Must add in cost of employing technical staff to maintain network

• Organizational expertise needed to design, implement and maintain the system

Outsourcing

• Become a hot business

• Companies hire outside expertise to manage network and equipment– need expertise to select: multiplexers, routers,

modems that connect to computer networks– Vendor then becomes responsible for problems

with the network

Virtual Private Networks

• Easy way to link various sites

• Acts like a private network

• Software defined

• Can be mixed with usage of PSTN

Signaling

• Tones in the network have special meanings:– dial tone– ringing– busy signal– DTMF tones

Signaling

• Process of sending information between two parts of the network to control, route and maintain a telephone call

• Pertains to the transmission of electrical signals to and from the user’s premises and the telephone company central office

Types of Signals

• Supervisory signals: monitor busy or idle condition of the phone

• Alerting signals: bell signals, tones, strobes, lights alerting that a call has arrived

• Addressing signals: touch tones or pulse tones telling network where to send a call

In-band Signaling

• Signal sent over the same channel as the voice channel

• Used extensively prior to 1976• Inefficient use of phone line• Ties up line to set up a call

– 20-30% of attempted calls never reach destination due to busies or no answers

• Slow call set up

Out-of-band Signaling

• Common channel interoffice signaling

• A separate network laid over the switching network

• Signaling done on a separate channel– Includes:

• routing instructions

• data base information

• specialized programs

Signaling System 7 (SS7)

• Signaling system based on CCIS

• Instrumental in lowering barriers to entry

• Routing intelligence migrated from expensive switching equipment to lower cost computer based peripherals

• Most significant innovation in public switched network

Advantages SS7

• Problems can be detected remotely

• Network information sent to centralized network maintenance location

• Network can be dynamically reconfigured

• Central offices not required to maintain sophisticated routing information

Added Features

• Voice-activated dialing for calling cards

• Automated roaming on cellular telephone networks, roaming locations in database

• Custom calling features

• Load balancing by call volume

• Detailed record keeping per call

End and Tandem Offices

• Local phone company has 2 types offices:– Tandem Office:

• connect central office to central office

• central office to inter exchange carrier

• volume high

– End Office:• connect homes to central office equipment

• volume low, last to get switch upgrade

• caller id information sent on this last connection

SS7 Glue between Carriers

• Enables all carriers to work with each other

• Standard protocol approved by ITU

SS7 Components

• Signal transfer points

• Service control points

• Service switching points

Signal Transfer Points

• Packet switches that route signals between central offices and databases

• One link can support multiple call paths

• Function of transferring signaling messages from one signaling link to another and considered exclusively from the viewpoint of the transferer

Service Switching Points

• Enable CO’s to initiate queries to databases and specialized computers

• Software capable of sending specialized messages– 900 # dialed, query sent to 900 database, which

has information on how to route the call

• Encodes and decodes switching information to switches from databases

Service Control Points

• Hold specialized databases with routing instructions for each call based on calling party

• In cellular networks holds roaming information

• Network based voice-mail and fax applications are located on service control points

SS7 in IP-Based Networks

• Networks are being built to support Voice over IP based networks

• Gateways are required to translate and deliver SS7 signaling messages between PSTN and IP based networks