basic communications systems class 9

Basic Communications

Systems

Class 9

Today’s Class Topics WAN Data Services

DSL Cable Modems T1 Access Lines Frame Relay

Voice Processing Voice Call Control Business Services Key Systems, PBX and Centrex

What can you do with a copper subscriber loop?

Modems Can provide up to 52 Kbps downstream over subscriber

loop.

Integrated Services Digital Networks (ISDN) Provides 144 Kbps over copper telephone line.

Digital Subscriber Line (DSL) Technologies Can provide up to 52 Mbps over short copper loop.

Name Service Data Rate Distance

V.32V.34

Voice Band Modems

9600 - 28,800 bps Any Voice Line

V.90 56K Modem 24 - 52 Kbps down33.6 Kbps up

Any Voice Line

IDSL ISDN DSL 144 Kbps 18,000-24,000 feet

SDSL Single-Pair DSL Up to 1.544 Mbps 12,000 feet

HDSL High Speed DSL 1.544 Mbps

12,000-18,000 feet

ADSL Asymmetric DSL

Up to 7 Mbps downUp to 640 Kbps up

12,000 – 18,000 feet

ADSL Lite Up to 1.5 Mbps downUp to 512 Kbps up

12,000 – 18,000 feet

VDSL Very High-speed DSL

13 Mbps down 4,500 feet

25.82 Mbps down 3,000 feet

52 Mbps down 1,000 feet

Data over Copper Loop

Digital Subscriber Lines Problems

Loop Qualification The condition of many copper loops is unknown. Testing and removal of load coils takes time.

Distance DSL does not work over 18,000 feet More than 20% of subscriber loops in U.S. are

longer than 18,000 feet. Crosstalk

Some types of ADSL and VDSL can interfere with data signals on adjacent pairs (crosstalk)

ADSL

Provides 1.5 Mbps – 7 Mbps downstream, depending

on equipment and distance 16 Kbps – 640 Kbps upstream, depending on

equipment and distance 4 KHz analog voice channel

Currently being aggressively deployed by CLECs and LECs for Internet access and other applications

ADSL Lite A simpler version of ADSL (G.lite)

designed to be installed without any Telco visit to the home.

G.Lite modems will be sold directly to consumers

Usually provides 1.5 Mbps downstream 384 Kbps upstream No analog channel (splitterless)

May allow faster deployment due to simplified installation

Voice over ADSL

“Voice over ADSL” products are now being deployed.

Allow customer to dynamically allocate multiple voice channels on ADSL data channel

Voice allocation choices: static 24 x DS0 allocation as with T1 (i.e. 4 voice

channels and 20x64K = 1.28 Mbps of data) packetized voice over DSL data channel Voice/data over ATM over DSL

VDSL

Very High Speed data channel over short copper loops

Provides access to Fiber-to-the Curb installations Carrier provides fiber to building basement or

to neighborhood Optical Networking Unit (ONU) VDSL provides data access to ONU

Can provide high-speed LAN-to-LAN interconnection at up to 52 Mbps

CATV Telephony CATV advantages:

Coaxial cable offers much greater bandwidth than copper pair

Cable is already installed to ~70% of U.S. homes

CATV problems: Coax systems may need to be upgraded

Hybrid Fiber Coax (HFC) 2-way amplifiers

Cost of telephony equipment is large Return signals are very noisy Not a great perception of reliability

Cable TV Plant

From: Videon Cable Modem Technology Primer

Hybrid Fiber / Coax

From: Videon Cable Modem Technology Primer

Cable Modem Standards

Data Over Cable Service Interface Specification (DOCSIS): Components:

Cable Modem Termination System Hybrid Fiber Coax Network Cable Modem

Connection is achieved through 10 / 100 Mbps Ethernet connector

Downstream speed: 27 Mbps or 40 Mbps.

T-Carrier Systems

Service Circuit Bit Rate # VoiceChannels

DS0 64 Kbps 1

DS1 T1 1.544 Mbps 24

DS2 T2 6.312 Mbps 96

DS3 T3 44.736 Mbps 672

DS4 T4 274.176 Mbps 2016

The T1 System T1 was the first T-carrier system

deployed by the Bell System (in 1962) Bit Rate: 1.544 Mbps:

Digital Information: 1.536 Mbps Framing Bits: 8 Kbps

Originally run over 4-wire (2 pair) copper wire with regenerators every 6000 feet. Can also be sent over fiber.

T1 System Uses T1 Carrier Trunk

Telecommunications companies use T1 trunks between switching offices

T1 Access Circuit Business customers use T1s as:

PBX - CO trunks (24 digital trunks on 1 cable)

Data access lines (1 data channel running at 1.536 Mbps)

Channelized?

Channelized T-1 Circuit T-1 is utilized as 24 DS0 channels of 64

Kbps each. Each DS0 can be allocated to carry any

single service, such as CO trunk, DID trunk, WATS, FX, 56K data, switched 56K, etc.

Unchannelized T-1 T-1 is utilized as a single 1.536 Mbps data

circuit.

Fractional T1 (Channelized)

A customer may request a leased Fractional T1, which means: Customer only sends data on an agreed

subset of the DS0s (example: DS0s 1-6) Carrier only forwards these particular

DS0s to the far-end Customer pays less than full T1 fee Example: 256 Kbps fractional T1 (4 x

DS0) from Chicago-NY could be ~$2500/month

UnChannelized T1:1.536 Mbps Leased Data Service

Router RouterCSU CSUC.O .

T1T1TokenRing

TokenRing

T1 Details Bipolar Representation

T1 uses Bipolar Coding to represent 1 and 0 bits

‘1’ bit represented by alternating +3 volt, -3 volt pulses

‘0’ bits represented by no voltage Framed Format

T1 transmits 8000 frames per second, 193 bits per frame (8000 * 193 = 1,544,000).

Bipolar Representation

0 0 1 1 0 1 0 0 0 1Data

+3V

-3V

Voltage

T1 Frame Format

1 b it 8 b its 8 b its8 b its8 b its

F DS0 #1 DS0 #24DS0 #3DS0 #2 . . .

Each DS0 called a time slot 8000 frames/sec * 8 bits/slot = 64 Kbps 24 * 8 + 1 = 193 bits/frame 8000 frames/sec * 193 bits/frame = 1.544 Mbps 8000 Framing bits sent per second

T1 Framing Bits D4 T1 lines (1972):

Allow receiver to find the start-of-frame (frame synchronization).

Group sets of 12 frames into superframes Indicate that frames 1 and 6 contain signaling

bits (to specify if channel is in use or not) D5 (ESF) T1 lines (1983):

Provide error checking (CRC) (ESF T1) Provide Facilities Data Link channel to transmit

network management messages (ESF T1)

D4 Frame Format


F DS0 #1 DS0 #24DS0 #3DS0 #2 . . .

Frames 6, 12:

Frames 1-5, 7-11:


F DS0 #1 DS0 #24DS0 #3DS0 #2 . . .1 111SI

G

SI

G

SI

G

SI

G

T1 Framing Bits (ESF Frame)

D5 Framing - Extended Superframe T1 (1983) F-bit pattern marks 24-frame extended

superframes F-bit pattern:

Odd frames: Facilities Data Link Every 4th frame: 001011 (Framing pattern) Every 4th frame: CRC for previous ESF

ESF Frame Advantages

Facilities Data Link Network diagnostics and management

messages sent between carrier equipment

Cyclic Redundancy Check (CRC) Allows error detection on T1 lines Carrier can offer Automatic Protection

Switching service to customer (switches to another T1 line if errors detected)

Data over T1

A T1 carries 24 DS0 channels A DS0 may carry a maximum of 56

Kbps or 64 Kbps of data A restricted T1 can carry up to 24 x 56

Kbps = 1.344 Mbps. A clear-channel T1 can carry up to 24 x

64 Kbps = 1.536 Mbps.

T1 Data – 56K or 64K ? T1 capacity depends on:

Line Coding An AMI T1 carries 56 Kbps per DS0 A B8ZS T1 carries 64 Kbps per DS0

Signaling A T1 using robbed bit signaling is limited to

56 Kbps per DS0 for data (to avoid signal bits)

A leased-line T1 (no signaling) or a T1 on a Signaling System 7 (SS7) network does not need robbed bit signaling.

Frame Relay

Telecommunications carriers maintain networks of Frame Relay switches

Customer get access line to nearest switch to get Frame Relay service

Higher data rates than X.25 Lower delays, higher throughputs and

better security than the Internet

Frame Relay Basics

Data is sent over pre-established Virtual Circuits, like X.25.

Frame Assembler/Disassemblers (FRADs) can be used to connect internal devices to the frame relay network FRADs generate Frame Relay

headers/trailers and send data frames into the network

Frame Relay Basics

Data only sent over Permanent Virtual Circuits (PVCs), which are set up by the carrier, not customer - always available

Customer can access network at data rates from 56 Kbps to 45 Mbps

No error control done by network switches (error control is responsibility of customer)

Frame Relay Basics

Fixed monthly cost based on “Line charge” for access line between user site

and frame relay carrier location “Port charge” for each connection into carrier

equipment “PVC charge” for each PVC defined between ports

Frame Relay is a layer 2 protocol, so any layer 3 protocol (like IP, for example) can be carried over a Frame Relay network

Replacing Leased Lines

Typically used to replace leased lines: Customer gets one Frame Relay PVC to

replace each leased line Customer still gets guaranteed delay and

throughput (CIR) similar to leased line Customer uses one access line at each

business location for all frame relay data Customer saves money - PVCs cost less than

leased lines

Leased Line Problem:

Number of leased circuits (and cost!) grows very large as number of sites increases!!

EthernetHub

Router

ClientClientFile Server

TokenRingHub

Router

ClientFile Server

TokenRingHub

Router

ClientFile Server

EthernetHub

Router

Client ClientFile Server

AmeritechSwitch

AmeritechSwitch

AmeritechSwitch

AmeritechSwitch

The Frame Relay Solution:

1 Access Line for each site!!

EthernetHub

Router


TokenRingHub

Router

ClientFile Server

TokenRingHub

Router

ClientFile Server

EthernetHub

Router


AmeritechSwitch

AmeritechSwitch

AmeritechSwitch

AmeritechSwitch

Frame Relay Addressing

The carrier assigns each PVC a 10-bit Data Link Connection Identifier (DLCI).

Customer sets up a table in each access router that maps each possible destination to its DLCI.

Router puts correct DLCI into each frame header before sending frame into network.

Frame Relay Frame Format

•Flag - Fixed bit pattern to start and end frame - set to 01111110

•Data Link Connection ID – PVC Address

•Discard Eligible – Determines whether this frame can be discarded at network switches

•CRC - Allows error detection• Some unused header bits not shown

Flag(8 bits)

Flag(8 bits)

CRC(16 bits)

DATA(up to 4096 bytes)

DLCI(10 bits)

DE(1 bit)

Fram eS w itch

Fram eS w itch

Fram eS w itch

Frame Relay Netw ork

Fram eS w itch

IP Router

C us tom er S iteIP R ou te r w ithF ram e R e lay

so ftw a re

LANPCs w ith LAN card

and TCP/IP Softw are

LeasedA ccess L ine

PCs w ith TCP/IP Softw are

FRAD /CSU

M ain fram e w ithT C P /IP S o ftw are

LeasedA ccess L ine

FRAD /M odem

D ia l-U pA ccess L ine

CIR

For each PVC, customer specifies a Committed Information Rate (CIR): CIR represents a guaranteed throughput for

this PVC Carrier also guarantees limited data delivery

time if customer does not exceed CIR Price of PVC is directly related to CIR

High CIR = high monthly price Low CIR = lower monthly price

CIR

Example: I have a T1 (1.536 Mbps) access line into the

frame relay network in Chicago I ask my carrier to create a PVC from Chicago

to Dallas with CIR = 512 Kbps. If I stay within my CIR (i.e., send less than

512,000 bps Chicago-Dallas, on average): Carrier guarantees 99.99% traffic gets through Carrier guarantees <= 20 ms. Network delay

Frame Relay PVCs with CIRs

EthernetHub

Router


TokenRingHub

Router

ClientFile Server

TokenRingHub

Router

ClientFile Server

EthernetHub

Router


AmeritechSwitch

AmeritechSwitch

AmeritechSwitch

AmeritechSwitch

56 Kbps 56 Kbps

56 Kbps 56 Kbps

PVC with16 Kbps CIR

Frame Relay Pricing

Example: 4 sites, each with T1 access line 6 PVCs providing connectivity between

sites, with 56 Kbps CIR on each PVC

Monthly Costs: 4 x (T1 access port cost) plus 6 x (56 Kbps PVC cost)

What if I exceed my CIR?

Example: I have a T1 (1.536 Mbps) access line

into the frame relay network in Chicago I ask my carrier to create a PVC from

Chicago to Dallas with CIR = 512 Kbps. Isn’t it possible for me to exceed my

CIR (send more than 512 Kbps Chicago-Dallas)?

YES!!!!!

What if I exceed my CIR?

Most carriers will allow customers to exceed CIR up to a fixed Burst Rate (Br) for up to 2 seconds with no penalty.

If customer continues to exceed CIR beyond 2 seconds, carrier sets Discard Eligible (DE) bit in frame headers If network congestion occurs, DE marked

frames are discarded by network switches

The Tradeoff

Low CIR ==> Low cost, but your data may be discarded by the network

High CIR ==> High cost, but data throughput is guaranteed by carrier

Note: Many customer still choose to pay lowest cost by selecting a Zero CIR option that provides no delivery guarantees

Frame Relay vs. The Internet

Frame Relay advantages: Guaranteed throughput and delay (ISPs

generally give no guarantees) Security (hackers cannot break into PVCs

between corporate sites)

Frame Relay disadvantages Price (more expensive than Internet service) Inflexibility (can’t send data to another site

unless PVC is already in place)

Network-to-Network Interfaces

Do you ever want to set up a PVC between sites connected to 2 different Frame Relay providers??

YES!! To connect between LEC networks To set up an extranet with trading

partner To interconnect sites after company

merger


How do you set up an inter-carrier PVC?

The carriers need to set up a Frame Relay Network-to-Network Interface (NNI) which controls traffic between the networks


Carriers don’t like NNIs!! How do they split the fees ($$)?? If the PVC goes down, who is to blame? What are the end-to-end performance

guarantees? Coordinating PVC addresses (DLCIs) for

both networks is a hassle


NNIs: The bottom line: Some carriers refuse to set up NNIs If you have a big enough contract with them,

perhaps you can convince them otherwise. Alternatives

You can move all your FR sites to one carrier You can set up your own router with connections to

PVCs on both carrier networks But this may cause traffic bottleneck at that router.

You can switch to a routed IP network service

Mini-Case Study

GREGCO has a main office in Chicago and branch offices in New York, Miami and Los Angeles.

Branch offices and main office exchange data files in both directions

Average file size is 150 Kbytes Average file transfer time must be less

than 30 seconds

Mini-Case Study

Can GREGCO satisfy its requirements using dial-up modems?

If GREGCO uses leased lines, what are the monthly service costs?

If GREGCO uses Frame Relay service How many PVCs? What are the CIRs? What are the monthly service costs?

Example: Leased Line Pricing

InterLATA T-1 $2000 + $4/mile

Service Monthly Charge

IntraLATA 56Kbps

$300

IntraLATA T-1 $600

InterLATA 56 Kbps

$900 + $1.50/mile

Example: Frame Relay Port Charges

Port Speed Monthly Charge

56Kbps $220

128 Kbps $400

256 Kbps $495

512 Kbps $920

T-1 $1620

Example: Frame Relay PVC Charges

PVC CIR Monthly Charge16 Kbps $2532 Kbps $4048 Kbps $5056 Kbps $60

128 Kbps $110256 Kbps $230384 Kbps $330512 Kbps $410

1024 Kbps $10101536 Kbps $1410

Data Communications and Computer Networks Chapter 12

Basic Telephone SystemsA telephone number consists of an area code, an exchange, and a subscriber extension.

The area code and exchange must start with the digits 2-9 to separate them from long distance and operator services.

Data Communications and Computer Networks Chapter 12

Address Signaling

Rotary Pulse Dial After pulling dial for digit ‘N’,

telephone opens and closes subscriber loop ‘N’ times as dial returns to its original position.

Dual Tone Multifrequency (DTMF) or Touch Tone One frequency assigned to

each row and each column. Pressing any key sends a 2-

frequency signal tone.

ABC

2DEF

31

Oper

0 #*

TUV

8WXY

9PRS

7

JKL

5MNO

6GHI

4

1447 Hz

852 Hz

770 Hz

697 Hz

1209 Hz 1336 Hz

941 Hz

North American Numbering Plan

Administration (NANPA) Determines format for dialed numbers. Chooses new area codes, exchange codes Format before 1995:

Prefix + 0/1 + N(0/1)X - NNX - XXXX (N digits are 2-9, X digits are 0-9)

Format after January, 1995:Prefix + 0/1 + NXX - NXX - XXXX

(N digits are 2-9, X digits are 0-9)

www.nanpa.com


(NANP)Prefix + 0/1 + NNX - NXX - XXXX (N digits are 2-9, X digits are 0-9)

NNX - Numbering Plan Area / Area Code Determines geographic area.

NXX - Central Office Prefix / Exchange Code Determines Central Office handling this

number.

XXXX - Station Number Identifies particular end station on C.O.


(NANP)Prefix + 0/1 + NNX - NXX - XXXX (N digits are 2-9, X digits are 0-9)

Prefix - Can be used to select IXC carrier 10-XXX

Selects carrier associated with Carrier Identification Code (CIC) XXX.

101-XXXX (as of 1998) Selects carrier associated with Carrier Identification

Code (CIC) XXXX. 0/1 - Used to request “operator

assistance”: 0 = Collect, calling card, etc.

Central Office Switch Technologies

Central Office

Wire Center Components

Central Office

Switches

Main Distribution

Frame (MDF)

Cable VaultMain Distribution Frame (MDF)

C.O. Switch

Central Office / Wire Center

Switch Side

Cable Side (Protected)

Switch Matrix

ServiceCircuits

CommonControl

DataStore

Trunk Cards

Cable Vault

OutsidePlant

Line Cards

Wire Center Components

Central Office Switch Components Common Control is

central processor Data Store contains

all customer feature and billing information

Service Circuits are shared circuits for ringing, tones, digit collection, etc.

Main Distribution Frame (MDF)

C.O. Switch


Switch Side


Switch Matrix

ServiceCircuits

CommonControl

DataStore

Trunk Cards

Cable Vault

OutsidePlant

Line Cards

Wire Center Components Central Office Switch

Components Switch Matrix can

interconnect Line/Trunk cards and Service Circuits

Line Cards perform per-line processing (BORSCHT functions).

Trunk Cards perform per-trunk processing.


C.O. Switch


Switch Side


Switch Matrix

ServiceCircuits

CommonControl

DataStore

Trunk Cards

Cable Vault

OutsidePlant

Line Cards

Wire Center Components Distribution Frame

Patch cables connect switch interfaces (Switch Side) to outside cable (Cable Side)

Fuses protect against foreign potential

Cable Vault Outside Plant

Subscriber loops Trunks


C.O. Switch


Switch Side


Switch Matrix

ServiceCircuits

CommonControl

DataStore

Trunk Cards

Cable Vault

OutsidePlant

Line Cards

How is a Voice Call Made?

A Voice Call

Originating phone goes off-hook. Line Card sees current,

notifies Common Control Common Control checks

customer records Common Control sets up

connections to Service Circuits:

Digit Decoder circuit Dial Tone Generator circuit

Central Office

Switch Matrix

ServiceCircuits

CommonControl

DataStore

MDF

LineCards

TrunkCards

A Voice Call Customer dials number.

Dial Tone is disconnected after 1st digit.

Digit Decoder de-allocated after all digits dialed.

Common Control: Consults Routing Table. Determines features of

terminating line. Connects originating line to

Ringback Generator Connects terminating line

to Ringing Circuit.

Central Office

Switch Matrix

ServiceCircuits

CommonControl

DataStore

MDF

LineCards

TrunkCards

A Voice Call

Call is answered. Ringing Line Card detects

off-hook and informs Common Control

Common Control disconnects ringing circuit

Common Control connects originating line card to terminating line card through Switch Matrix

Talking path has been established.

Central O ffice

Switch M atrix

ServiceCircuits

CommonControl

DataStore

MDF

LineCards

TrunkCards

A Voice Call

Phones go on-hook. Tear down connection

(after time-out period if one phone still off-hook).

Store call record.

Central Office

Switch Matrix

ServiceCircuits

CommonControl

DataStore

MDF

LineCards

TrunkCards

Inter-Office Calls

Central Office

Switch Matrix

ServiceCircuits

CommonControl

DataStore

MDF

LineCards

TrunkCards

Central Office

Switch Matrix

ServiceCircuits

CommonControl

DataStore

MDF

TrunkCards

LineCards

Inter-Office Trunk

Business Service Pricing

Fixed Price Services Same monthly cost regardless of how

much service is used Example: Leased Line

Measured Services Monthly cost based on minutes of

usage Example: Direct Distance Dialing (DDD

or long distance calling)

Private Line Services

Private Line Service provides a dedicated circuit between two locations Fixed connection is made on the

distribution frame Central Office provides no dial tone service Customer pays flat rate each month Also called a “dedicated circuit” or “leased

line” service

Private Line ServicesCentral Office

Switch Matrix

ServiceCircuits

CommonControl

DataStore

MDF

LineCards

TrunkCards

Central Office

Switch Matrix

ServiceCircuits

CommonControl

DataStore

MDF

TrunkCards

LineCards

Inter-Office Trunk

Private Line / Leased Line / Dedicated Line Direct circuit between 2 locations No switching service from Central Office (only

transmission service). Flat Rate charge based on distance and

bandwidth.


Foreign Exchange (FX) Line Allows a telephone to get dial tone

from a C.O. switch other than the closest one

Off Premises Extension (OPX) Allows a telephone to connect to a

PBX at a different customer location


Switch Matrix

ServiceCircuits

CommonControl

DataStore

MDF

LineCards

TrunkCards

Central Office

Switch Matrix

ServiceCircuits

CommonControl

DataStore

MDF

TrunkCards

LineCards

Inter-Office Trunk

Foreign Exchange (FX) Line Provides local switched telephone service

from a Central Office outside of the subscriber’s exchange area.


Central Office

Switch Matrix

ServiceCircuits

CommonControl

DataStore

MDF

LineCards

TrunkCards

Central Office

Switch Matrix

ServiceCircuits

CommonControl

DataStore

MDF

TrunkCards

LineCards

Inter-Office Trunk PBX

Off Premises Extension (OPX) Private line service that allows a remote

business telephone to access local PBX services.

FX Line Example

GregCo Corporation in Chicago has many customers in New York DDD Chgo-NY costs 15 cents per minute FX line Chgo-NY costs $800 per month GregCo makes about 25 calls to NY each

day, each call lasting about 10 minutes SHOULD GREGCO GET AN FX LINE?

Private Line Services Tie Line

Used to directly connect PBX switches at 2 customer sites

Dedicated Access Line to IXC POP Directly connects PBX switch to IXC

point of presence. Eliminates LEC access charges for

calls made on this line (replaces them with flat monthly fee for private line)


Switch Matrix

ServiceCircuits

CommonControl

DataStore

MDF

LineCards

TrunkCards

Central Office

Switch Matrix

ServiceCircuits

CommonControl

DataStore

MDF

TrunkCards

LineCards

Inter-Office Trunk PBXPBX

Tie Line A dedicated circuit between two PBXes. Tie lines may be used due to cost savings

(as compared with switched service) or to allow proprietary signaling between PBXes.


Dedicated access to IXC POP A dedicated circuit between customer site

and IXC POP Eliminates LEC access charges for long-

distance calls over this line.

LEC Sw itch

Switch M atrix

ServiceCircuits

CommonControl

DataStore

M DF

LineCards

TrunkCards

IXC Sw itch

Switch M atrix

ServiceCircuits

CommonControl

DataStore

M DF

TrunkCards

LineCards

IXC T runkPBX

Switched Line Services Direct Distance Dialing (DDD)

Plain long-distance telephone service Wide Area Telecommunications

Service (WATS) Reduced rates to certain geographic

areas IN-WATS (800, 888 services)

Business pays for each call received on this line.

Switched Line Services 900 Services

Allows customer to charge whatever they want for calls to this number

International WATS Reduced rates to certain countries

Remote Call Forwarding; Ex: Business in Chicago can advertise local

phone number in New York. May be cheaper than FX line.

RCF Example

GregCo Corporation in Chicago has many customers in New York. GregCo wants to offer a local New York number. DDD Chgo-NY costs 15 cents per minute RCF service costs $50 per month FX line Chgo-NY costs $800 per month Customers in NY make about 10 calls to

GregCo each day, each call lasting about 10 minutes

SHOULD GREGCO GET AN FX LINE?

Virtual Private Networks Virtual Private Networks (VPNs) are

an alternative service, where The carrier does not dedicate an end-to-

end circuit to the customer circuit. Carrier charges fixed rate per month for

calls between 2 locations Carrier guarantees a particular service level

(i.e., 99.99% of calls get through on first try).

Virtual Private Networks

So, Virtual Private Networks look and perform a lot like Private Lines, but they cost less.

Virtual Private Networks Example:

Private line from Chicago - New York Cost: $800 per month Reliability: 99.999% uptime

VPN Service from Chicago - New York Cost: $600 per month Reliability: 99.99% calls get through on first try

Which would you choose?

Private Line Features Fixed cost per month regardless

of usage Typically best value when lots of

calls go between two fixed locations

Can be imitated by Virtual Private Network carrier service

Switched Service Features Variable cost per month depending on

usage May provide special service (like 800

number) Lower cost than leased lines when there

is not much call traffic Price discounts offered for

large call volumes predictable call patterns (for example, WATS

services to particular geographic regions)

Key Telephone Systems

Key System Unit (KSU) ties many multi-line sets into a set of C.O. lines.

User selects outside line by pressing corresponding line button on key set.

Central Office

KSUBusiness

Lines

M ulti-ButtonKey Sets

Custom er Prem ises

Typical Key System Features

Call Transfer Call Hold Music on Hold Conference Calling Speed dialing Intercom calling between key sets Paging Call Pickup and Call Barge-In Speakerphones / Hands-free calling Automatic Call Information Recording

A Call using a Key SystemFrank calls Sam in New York

Frank picks up phone, hits LINE #2 button Key system connects set to C.O. line #2 Frank hears dial tone from the C.O. Frank dials 1-212-894-6622.

Central Office

KSUBusiness

Lines

Custom er Prem ises


C.O. routes call to New York, rings Sam’s phone. Sam answers phone.

Frank and Sam talk for a while

Central Office

KSUBusiness

Lines

Custom er Prem ises


Frank decides to consult with Jane Frank hits HOLD button - Key System

puts current call to Sam on hold. Frank hits INTERCOM button and dials

Jane’s set number

Central Office

KSUBusiness

Lines

Custom er Prem ises


Jane hears a special intercom ring

Jane and Frank talk

Central Office

KSUBusiness

Lines

Custom er Prem ises


Frank hits LINE #2 button again and now talks with Sam again.

Central Office

KSUBusiness

Lines

Custom er Prem ises

A Call into a Key SystemJoe calls Frank

Joe picks up phone, dials number associated with LINE #2 of Frank’s Key System

Central Office

KSUBusiness

Lines


Custom er Prem ises


C.O. rings LINE #2 connected to KSU KSU sends ringing signal out to all phones

programmed for access to LINE #2

Central Office

KSUBusiness

Lines


Custom er Prem ises


Frank answers phone and talks to Joe

Central Office

KSUBusiness

Lines


Custom er Prem ises

Private Branch Exchange (PBX)

PBX is a true intelligent switching system. Provides local switching between station sets

or access to C.O. Users dial access code (like ‘9’) to get C.O. service.

Provides advanced intelligent features to users.

Central Office

PBXBusiness

Trunks

PBXStation

Sets

Custom er Prem ises

PBX Features (not found on Key Systems)

Private Dialing Plans 4-digit, special prefixes for WATS, FX, etc.

Automatic Route Selection PBX collects dialed digits and intelligently

decides how to route this call for lowest cost

Over FX line Over WATS trunk Over DOD trunk


Voice Mail Automatic Call Distribution (ACD)

Routes incoming calls (usually INWATS) to the best station set and location

Voice Response Unit (VRU) Provides recorded messages and

responds to touch-tone requests from callers


Class-of-Service Permission to use each PBX feature

can be given or taken away from each user.

Authentication Codes PBX users can identify themselves with

“passwords” when dialing calls to get special access permissions

Data Services ISDN, Dial-Up digital services

A Call using a PBXFrank calls Sam in New York

Frank picks up the phone and gets dial tone from the PBX.

Frank dials 9-1-212-894-6622. PBX consults routing tables,

determines that this call should go over the FX trunk to New York

Central Office

PBXBusiness

Trunks

Custom er Prem ises

A Call using a PBXFrank calls Sam in New York

PBX sends dialed number over FX trunk, dropping area code (sends “894-6622”).

Call to New York is completed. Frank and Sam talk.

Central Office

PBXBusiness

Trunks

Custom er Prem ises

FX Trunkto New York

A Call into a PBXJoe calls Frank

Joe picks up phone. Dials Frank’s public number

1-312-362-6587 C.O. chooses trunk to PBX (could

choose ANY trunk).

Central Office

PBXBusiness

Trunks

PBXStation

Sets

Custom er Prem ises


C.O. sends OFF-HOOK signal to PBX over the trunk

PBX sends WINK-BACK signal to C.O. C.O. sends last 4 digits - “6587” (this

is a DID trunk).

Central Office

PBXBusiness

Trunks

PBXStation

Sets

Custom er Prem ises


PBX looks into its station address table and determines that “6587” corresponds to Frank’s station set.

PBX rings Frank’s station set.

Central Office

PBXBusiness

Trunks

PBXStation

Sets

Custom er Prem ises


Frank picks up telephone handset. PBX sends ANSWER signal back to

C.O. indicating that the call has been answered

Frank and Joe talk

Central Office

PBXBusiness

Trunks

Custom er Prem ises

PBX Trunk Signaling

AmeritechCentral Office

PBXBusinessLines orTrunks

PBXStation

Sets

Customer Premises

1-way trunks: A call can only be initiated from one end (incoming or outgoing).

2-way trunks: A call can be initiated from either end of the trunk (PBX or C.O.)

PBX Trunk Signaling



PBXStation

Sets

Customer Premises

Trunk signaling for PBX trunks: Loop start : Like subscriber loop. Ground start : Either side can ground a wire to

initiate a call. Better control than loop start. E&M : Separate wire pair used for signaling.

PBX Trunk Types



PBXStation

Sets

Customer Premises

CO Trunk (1-way outgoing) Standard switched service.

WATS (1-way outgoing) Switched service to limited geographic area.

INWATS (1-way incoming) Trunk for incoming 800 number calls.

PBX Trunk Types



PBXStation

Sets

Customer Premises

DID (Direct Inward Dial) Trunk 1-way incoming trunk CO sends dialed number after call connect so PBX

can automatically ring station. DOD (Direct Outward Dial) Trunk

1-way outgoing trunk PBX repeats dialed number to C.O.

Centrex

Centrex is a service that imitates all PBX features at the C.O.

Private dial plans, special features, etc. are all available at a cost from your local telco.

Central Office

CentrexLines

CentrexElectronicKey Sets

Custom er Prem ises

Centrex Advantages (compared with PBX system)

Low up-front cost - no need to buy expensive PBX or Key System equipment

System Maintenance and backups are all done by the LEC carrier rather than customer.

Easy to combine multiple geographic locations into one logical Centrex group

Advanced features are available that may not be included on all PBX or Key systems

Centrex Disadvantages (compared with PBX system):

Customer has less direct control With PBX, customer can modify switch data at any time

needed (i.e. add new telephone set, change phone number, etc.)

With Centrex, customer must ask LEC to make changes using service order.

Centrex may be more expensive if most calls are internal (between phones on PBX)

Fixed costs: Centrex is per-line, PBX is per-trunk. Fewer trunks needed if calls mainly internal.

Usage costs: Lines see much more usage than trunks if calls are mainly internal. Line-to-line calls are “free” with PBX.

basic communications systems class 9

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