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


Integrated Services Digital Networks (ISDN)


Can provide up to 52 Kbps downstream over subscriber loop.
Provides 144 Kbps over copper telephone line.
Digital Subscriber Line (DSL) Technologies

Can provide up to 52 Mbps over short copper loop.
Data over Copper Loop
Name
Service
Data Rate
Distance
9600 - 28,800 bps
Any Voice Line
Any Voice Line
18,000-24,000 feet
V.32
V.34
V.90
Voice Band
Modems
56K Modem
IDSL
ISDN DSL
24 - 52 Kbps down
33.6 Kbps up
144 Kbps
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 down
Up to 640 Kbps up
12,000 – 18,000 feet
Up to 1.5 Mbps down
Up to 512 Kbps up
12,000 – 18,000 feet
13 Mbps down
4,500 feet
25.82 Mbps down
3,000 feet
52 Mbps down
1,000 feet
ADSL Lite
VDSL
Very High-speed
DSL
Digital Subscriber Lines

Problems

Loop Qualification



Distance



The condition of many copper loops is unknown.
Testing and removal of load coils takes time.
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
DS0
Bit Rate
# Voice
Channels
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
T1
Token
Ring
C.O.
Router
CSU
T1
CSU
Router
Token
Ring
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
Data
+3V
Voltage
-3V
0 0 1 1 0 1 0 0 0 1
T1 Frame Format
1 bit
8 bits
8 bits
8 bits
F
DS0 #1
DS0 #2
DS0 #3
8 bits
. . .
DS0 #24
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


Frames 1-5, 7-11:
1 bit
8 bits
8 bits
8 bits
F
DS0 #1
DS0 #2
DS0 #3
8 bits
. . .
DS0 #24
Frames 6, 12:
1 bit
F
7 bits
1
S
DS0 #1 I
G
7 bits
1
S
DS0 #2 I
G
7 bits
1
DS0 #3
S
I
G
. . .
7 bits
1
DS0 #24
S
I
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:
Client
Client
File Server
Client
Am e rite ch
Sw itch
Ethernet
Hub
File Server
Am e rite ch
Sw itch
Router
Router
Am e rite ch
Sw itch
Client
Token
Ring
Hub
Am e rite ch
Sw itch
File Server
Router
Router
File Server
Token
Ring
Hub
Client
Client
Ethernet
Hub
Number of leased circuits (and cost!) grows very
large as number of sites increases!!
The Frame Relay Solution:
Client
Client
File Server
Client
Am e rite ch
Sw itch
Ethernet
Hub
File Server
Am e rite ch
Sw itch
Router
Router
Am e rite ch
Sw itch
Client
Token
Ring
Hub
Am e rite ch
Sw itch
File Server
Router
Router
File Server
Token
Ring
Hub
Client
Client
Ethernet
Hub
1 Access Line for each site!!
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
(8 bits)
DLCI
DE
(10 bits) (1 bit)
DATA
(up to 4096 bytes)
CRC
(16 bits)
Flag
(8 bits)
•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
Frame
Switch
Frame
Switch
Frame
Switch
Frame Relay Network
Frame
Switch
Leased
Access Line
FRAD /
CSU
Mainframe with
TCP/IP Software
Dial-Up
Access Line
Leased
Access Line
Customer Site
IP Router with
Frame Relay
software
PCs with LAN card
and TCP/IP Software
FRAD /
Modem
IP Router
PCs with TCP/IP Software
LAN
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
Client
Client
File Server
Client
Am e rite ch
Sw itch
56 Kbps
Ethernet
Hub
File Server
Am e rite ch
Sw itch 56
Router
Am e rite ch
Sw itch
Client
Token
Ring
Hub
Router
ps
Kb
6
5
PVC with
16 Kbps CIR
Router
File Server
Am e rite ch
Sw itch
56 K
b
Kbps
ps
Router
File Server
Token
Ring
Hub
Client
Ethernet
Hub
Client
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
Network-to-Network Interfaces


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
Network-to-Network Interfaces

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
Network-to-Network Interfaces

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
Service
Monthly Charge
IntraLATA
56Kbps
$300
IntraLATA T-1
$600
InterLATA 56
Kbps
$900 + $1.50/mile
InterLATA T-1
$2000 + $4/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
16 Kbps
32 Kbps
48 Kbps
56 Kbps
128 Kbps
256 Kbps
384 Kbps
512 Kbps
1024 Kbps
1536 Kbps
Monthly Charge
$25
$40
$50
$60
$110
$230
$330
$410
$1010
$1410
Data Communications and Computer Networks
Chapter 12
Basic Telephone Systems
A 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

697 Hz
770 Hz
852 Hz
ABC
DEF
1
2
3
GHI
JKL
MNO
4
5
6
PRS
TUV
WXY
7
8
9

Dual Tone Multifrequency
(DTMF) or Touch Tone

Oper
941 Hz
*
0
#
1209 Hz 1336 Hz 1447 Hz
After pulling dial for digit ‘N’,
telephone opens and closes
subscriber loop ‘N’ times as dial
returns to its original position.

One frequency assigned to each
row and each column.
Pressing any key sends a 2frequency signal tone.
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
North American
Numbering Plan (NANP)
Prefix + 0/1 + NNX - NXX - XXXX
(N digits are 2-9, X digits are 0-9)

NNX - Numbering Plan Area / Area Code


NXX - Central Office Prefix / Exchange Code


Determines geographic area.
Determines Central Office handling this number.
XXXX - Station Number

Identifies particular end station on C.O.
North American
Numbering Plan (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


101-XXXX (as of 1998)


Selects carrier associated with Carrier Identification
Code (CIC) XXX.
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 / Wire Center

C.O. Switch
Service
Circuits
Common
Control
Line Cards

M ain Distribution Frame (M DF)
Cable Side (Protected)
Cable Vault
Main Distribution
Frame (MDF)
Trunk Cards
Switch Side
Outside
Plant
Switches
Data
Store
Switch Matrix
Central Office

Cable Vault
Wire Center Components
Central Office / Wire Center

C.O. Switch
Service
Circuits
Common
Control
Data
Store
Central Office Switch
Components

Switch Matrix

Line Cards
Trunk Cards
Switch Side
M ain Distribution Frame (M DF)
Outside
Plant
Cable Side (Protected)
Cable Vault

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.
Wire Center Components
Central Office / Wire Center
C.O. Switch
Service
Circuits
Common
Control
Data
Store

Central Office Switch
Components

Switch Matrix
Line Cards
Trunk Cards

Switch Side
M ain Distribution Frame (M DF)
Outside
Plant
Cable Side (Protected)

Cable Vault
Switch Matrix can
interconnect
Line/Trunk cards and
Service Circuits
Line Cards perform
per-line processing
(BORSCHT functions).
Trunk Cards perform
per-trunk processing.
Wire Center Components
Central Office / Wire Center

Distribution Frame

C.O. Switch
Service
Circuits
Common
Control
Data
Store

Switch Matrix
Line Cards
Trunk Cards


Switch Side
M ain Distribution Frame (M DF)
Outside
Plant
Cable Vault
Outside Plant

Cable Side (Protected)

Cable Vault
Patch cables connect
switch interfaces
(Switch Side) to outside
cable (Cable Side)
Fuses protect against
foreign potential
Subscriber loops
Trunks
How is a Voice Call Made?
A Voice Call
Central Office
Service Common
Circuits Control
Data
Store

Originating phone goes
off-hook.

Switch Matrix
Line
Cards
Trunk
Cards

MDF

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
A Voice Call

Customer dials number.

Central Office
Service Common
Circuits Control
Data
Store


Switch Matrix
Dial Tone is disconnected
after 1st digit.
Digit Decoder de-allocated
after all digits dialed.
Common Control:

Line
Cards
Trunk
Cards
MDF



Consults Routing Table.
Determines features of
terminating line.
Connects originating line to
Ringback Generator
Connects terminating line to
Ringing Circuit.
A Voice Call
Central Office
Service Common
Circuits Control
Data
Store

Call is answered.

Switch Matrix

Line
Cards
Trunk
Cards

MDF

Ringing Line Card detects offhook 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.
A Voice Call
Central Office
Service Common
Circuits Control
Data
Store

Phones go on-hook.

Switch Matrix
Line
Cards
Trunk
Cards
MDF

Tear down connection
(after time-out period if
one phone still off-hook).
Store call record.
Inter-Office Calls
Central Office
Central Office
Service Common
Circuits Control
Service Common
Circuits Control
Data
Store
Switch Matrix
Switch Matrix
Line
Cards
Data
Store
Trunk
Cards
Trunk
Cards
Line
Cards
MDF
MDF
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 Services
Central Office
Central Office
Service
Circuits
Com m on
Contr ol
Service
Circuits
Data
Store
Data
Store
Switch Matrix
Switch Matrix
Line
Cards
Com m on
Contr ol
Trunk
Cards
Trunk
Cards
Line
Cards
MDF
MDF
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.
Private Line Services

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
Private Line Services
Central Office
Central Office
Service
Circuits
Service
Circuits
Com m on Data
Contr ol Store
Switch Matrix
Switch Matrix
Line
Cards
Com m on Data
Contr ol Store
Trunk
Cards
Trunk
Cards
Line
Cards
MDF
MDF
Inter-Office Trunk

Foreign Exchange (FX) Line

Provides local switched telephone service from a
Central Office outside of the subscriber’s
exchange area.
Private Line Services
Central Office
Central Office
Service
Circuits
Service
Circuits
Com m on Data
Contr ol Store
Switch Matrix
Switch Matrix
Line
Cards
Com m on Data
Contr ol Store
Trunk
Cards
Trunk
Cards
Line
Cards
MDF
MDF
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)
Private Line Services
Central Office
Central Office
Service
Circuits
Com m on
Contr ol
Service
Circuits
Data
Store
Trunk
Cards
Trunk
Cards

Line
Cards
MDF
MDF
Inter-Office Trunk
PBX
Data
Store
Switch Matrix
Switch Matrix
Line
Cards
Com m on
Contr ol
PBX
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.
Private Line Services
IXC Switch
LEC Switch
Service
Circuits
Service
Circuits
Data
Store
Common
Control
Switch Matrix
Switch Matrix
Line
Cards
Trunk
Cards
Trunk
Cards

Line
Cards
MDF
MDF
PBX
Data
Store
Common
Control
IXC Trunk
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.
Switched Line Services

Direct Distance Dialing (DDD)


Wide Area Telecommunications Service
(WATS)


Plain long-distance telephone service
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


International WATS


Allows customer to charge whatever they want for
calls to this number
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
Customer Premises
Central Office
KSU
Multi-Button
Key Sets
Business
Lines


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.
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 System
Frank calls Sam in New York
Customer Premises
Central Office
KSU
Business
Lines

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.
A Call using a Key System
Frank calls Sam in New York
Customer Premises
Central Office
KSU
Business
Lines


C.O. routes call to New York, rings Sam’s
phone. Sam answers phone.
Frank and Sam talk for a while
A Call using a Key System
Frank calls Sam in New York
Customer Premises
Central Office
KSU
Business
Lines



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
A Call using a Key System
Frank calls Sam in New York
Customer Premises
Central Office
KSU
Business
Lines

Jane hears a special intercom ring

Jane and Frank talk
A Call using a Key System
Frank calls Sam in New York
Customer Premises
Central Office
KSU
Business
Lines

Frank hits LINE #2 button again and now
talks with Sam again.
A Call into a Key System
Joe calls Frank
Customer Premises
Central Office
KSU
Multi-Button
Key Sets
Business
Lines

Joe picks up phone, dials number
associated with LINE #2 of Frank’s Key
System
A Call into a Key System
Joe calls Frank
Customer Premises
Central Office
KSU
Multi-Button
Key Sets
Business
Lines


C.O. rings LINE #2 connected to KSU
KSU sends ringing signal out to all phones
programmed for access to LINE #2
A Call into a Key System
Joe calls Frank
Customer Premises
Central Office
KSU
Multi-Button
Key Sets
Business
Lines

Frank answers phone and talks to Joe
Private Branch Exchange (PBX)
Customer Premises
Central Office
PBX
PBX
Station
Sets
Business
Trunks



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.
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
PBX Features
(not found on Key Systems)


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
PBX Features
(not found on Key Systems)

Class-of-Service


Authentication Codes


Permission to use each PBX feature can be
given or taken away from each user.
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 PBX
Frank calls Sam
in New York
Customer Premises
Central Office
PBX
Business
Trunks



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
A Call using a PBX
Frank calls Sam in New York
Customer Premises
Central Office
FX Trunk
to New York
PBX
Business
Trunks


PBX sends dialed number over FX trunk,
dropping area code (sends “894-6622”).
Call to New York is completed. Frank and
Sam talk.
A Call into a PBX
Joe calls Frank
Customer Premises
Central Office
PBX
PBX
Station
Sets
Business
Trunks


Joe picks up phone.
Dials Frank’s public number


1-312-362-6587
C.O. chooses trunk to PBX (could choose
ANY trunk).
A Call into a PBX
Joe calls Frank
Customer Premises
Central Office
PBX
PBX
Station
Sets
Business
Trunks



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).
A Call into a PBX
Joe calls Frank
Customer Premises
Central Office
PBX
PBX
Station
Sets
Business
Trunks


PBX looks into its station address table and
determines that “6587” corresponds to
Frank’s station set.
PBX rings Frank’s station set.
A Call into a PBX
Joe calls Frank
Customer Premises
Central Office
PBX
Business
Trunks



Frank picks up telephone handset.
PBX sends ANSWER signal back to C.O.
indicating that the call has been answered
Frank and Joe talk
PBX Trunk Signaling
Customer Premises
Ameritech
Central Office
PBX
PBX
Station
Sets
Business
Lines or
T runks


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
Customer Premises
Ameritech
Central Office
PBX
PBX
Station
Sets
Business
Lines or
T runks

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
Customer Premises
Ameritech
Central Office
PBX
PBX
Station
Sets
Business
Lines or
T runks

CO Trunk (1-way outgoing)


WATS (1-way outgoing)


Standard switched service.
Switched service to limited geographic area.
INWATS (1-way incoming)

Trunk for incoming 800 number calls.
PBX Trunk Types
Customer Premises
Ameritech
Central Office
PBX
PBX
Station
Sets
Business
Lines or
T runks

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
Customer Premises
Central Office
Centrex
Electronic
Key Sets
Centrex
Lines


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.
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.