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Basic Communications
Systems
Class 8
Today’s Class Topics

Internet Protocols




IP
TCP
Others
Telecommunications Networks




History
Voice Call Control
Business Services
Key Systems, PBX and Centrex
Data Communications and Computer Networks
Chapter 11
Internet Protocols
To support the Internet and all its services, many protocols are
necessary.
Some of the protocols that we will look at:
• Internet Protocol (IP)
• Transmission Control Protocol (TCP)
• Address Resolution Protocol (ARP)
• Domain Name System (DNS)
Data Communications and Computer Networks
Chapter 11
Internet Protocols
Recall that the Internet with all its protocols follows the
Internet model.
An application, such as e-mail, resides at the highest layer.
A transport protocol, such as TCP, resides at the transport
layer.
The Internet Protocol (IP) resides at the Internet or network
layer.
A particular media and its framing resides at the interface
layer.
Data Communications and Computer Networks
Chapter 11
The Internet Protocol (IP)
IP prepares a packet for transmission across the Internet.
The IP header is encapsulated onto a transport data packet.
The IP packet is then passed to the next layer where further
network information is encapsulated onto it.
Data Communications and Computer Networks
Chapter 11
Data Communications and Computer Networks
Chapter 11
The Internet Protocol (IP)
Using IP, a subnet router:
Makes routing decision based on the destination address.
May have to fragment the datagram into smaller datagrams
(very rare) using Fragment Offset.
May determine that the current datagram has been hopping
around the network too long and delete it (Time to Live).
To perform these functions, an IP header is encapsulated onto
each transport layer packet.
Data Communications and Computer Networks
Chapter 11
Data Communications and Computer Networks
Chapter 11
The Transmission Control Protocol
(TCP)
The TCP layer creates a connection between sender and
receiver using port numbers.
The port number identifies a particular application on a
particular device (IP address).
TCP can multiplex multiple connections (using port numbers)
over a single IP line.
Data Communications and Computer Networks
Chapter 11
The Transmission Control Protocol
(TCP)
The TCP layer can ensure that the receiver is not overrun with
data (end-to-end flow control) using the Window field.
TCP can perform end-to-end error correction (Checksum).
TCP allows for the sending of high priority data (Urgent
Pointer).
Data Communications and Computer Networks
Chapter 11
Data Communications and Computer Networks
Chapter 11
Internet Control Message Protocol
(ICMP)
ICMP, which is used by routers and nodes, performs the error
reporting for the Internet Protocol.
ICMP reports errors such as invalid IP address, invalid port
address, and the packet has hopped too many times.
Data Communications and Computer Networks
Chapter 11
User Datagram Protocol (UDP)
A transport layer protocol used in place of TCP.
Where TCP supports a connection-oriented application, UDP
is used with connectionless applications.
UDP also encapsulates a header onto an application packet
but the header is much simpler than TCP.
Data Communications and Computer Networks
Chapter 11
Address Resolution Protocol (ARP)
When an IP packet has traversed the Internet and encounters
the destination LAN, how does the packet find the destination
workstation?
Even though the destination workstation may have an IP
address, a LAN does not use IP addresses to deliver frames.
A LAN uses the MAC layer address.
ARP translates an IP address into a MAC layer address so a
frame can be delivered to the proper workstation.
Address Resolution Protocol (ARP)

The ARP Protocol allows any machine to
determine the 6-byte LAN address of any other
machine on the same subnet if it knows the IP
address of that machine


Each PC keeps an ARP Table that stores IP address
and LAN address of devices on local subnet
ARP Table entries are kept up-to-date automatically by
sending ARP Broadcast messages when necessary
to determine addresses
Two Addresses??

In an IP LAN environment, every PC will
have two addresses:

4-byte IP address

6-byte LAN (Ethernet) address
Two Addresses??

Why??

LAN address can only deliver data to
destination on the same LAN (that is,
the same IP subnet)

IP address is used to deliver data to any
destination, regardless of destination
network.
Two Addresses??

The 6-byte destination address in the
Ethernet header is used to deliver the data
to the next device:

If the packet is not on the final LAN, then
Ethernet destination address identifies the
address of the next router

If the packet is on the final LAN, then Ethernet
destination address identifies the address of the
destination host (server).
IP Routing Example
IP Address 140.192.55.10
IP Address 140.86.22.5
Client
Server
Token Ring LAN
IP Network 140.86
Ethernet LAN
IP Network 140.192
NFS
File Services
Software
TCP/IP
Networking
Software
NIC
Driver SW
NIC Card
Create OPEN FILE
Request in NFS format
Add TCP/IP Headers
with Server IP Address
Add Ethernet Info
with Router MAC Address
Look up IP Dest.
Network in
Routing Table
Recognize MAC Addr.
Remove Ethernet Info
Src: 140.192.55.10
Dest: 140.86.22.5
Src: 15
Dest: 17
Translate Server IP Address to
Server
MAC Address
Routing
Software
NIC
Driver SW
NIC
Driver SW
NIC Card
NIC Card
MAC
#17
TCP/IP Ethernet
Ethernet NFS
Trailer Request Headers Header
Execute OPEN
FILE Operation
IP Router
Add Token Ring Info
with Server MAC Address
Check TCP/IP Info.
Remove TCP/IP Headers
Recognize MAC Addr.
Remove Token Ring Info
NFS
File Services
Software
TCP/IP
Networking
Software
NIC
Driver SW
NIC Card
MAC
#22
NFS
TCP/IP Token
Token
Request
Ring
Headers
Ring
Header
Trailer
Src: 22
Src: 140.192.55.10
Dest: 23
Dest: 140.86.22.5
Data Communications and Computer Networks
Chapter 11
Tunneling Protocols
The Internet is not normally a secure system.
If a person wants to use the Internet to access a corporate
computer system, how can a secure connection be created?
One possible technique is by creating a virtual private
network (VPN).
A VPN creates a secure connection through the Internet by
using a tunneling protocol.
Data Communications and Computer Networks
Chapter 11
BOOTP
Every workstation attached to the Internet needs:
• Its IP address
• Its subnet mask
• The IP address of a router
• The IP address of a name server
Data Communications and Computer Networks
Chapter 11
BOOTP
Thin client workstations do not have a disk drive, and its
ROM does not contain the previous four pieces of
information.
How do we tell the machine this information? BOOTP.
Data Communications and Computer Networks
Chapter 11
BOOTP
There are two types of BOOTP operations:
REQUEST – A workstation asks a server for the information
(source IP address = all 0s, destination IP address = all 1s).
REPLY – The server returns the information to the
workstation.
Data Communications and Computer Networks
Chapter 11
Dynamic Host Configuration Protocol
(DHCP)
BOOTP is not dynamic (when a client requests its IP address,
it is retrieved from a static table).
DHCP is a dynamic extension of BOOTP.
When a DHCP client issues an IP request, the DHCP server
looks in its static table. If no entry exists, the server selects
an IP address from an available pool.
Data Communications and Computer Networks
Chapter 11
DHCP
The address assigned by the DHCP server is temporary.
Part of the agreement includes a specific period of time.
If no time period specified, the default if one hour.
DHCP clients may negotiate for a renewal before the time
period expires.
Data Communications and Computer Networks
Chapter 11
Network Address Translation (NAT)
NAT lets a router represent an entire local area network to the
Internet as a single IP address.
Thus it appears all traffic leaving this LAN appears as
originating from a global IP address.
All traffic coming into this LAN uses this global IP address.
This security feature allows a LAN to hide all the workstation
IP addresses from the Internet.
Data Communications and Computer Networks
Chapter 11
NAT
Since the outside world cannot see into the LAN, you do not
need to use registered IP addresses on the inside LAN.
We can use the following blocks of addresses for private use:
•10.0.0.0 – 10.255.255.255
•172.16.0.0 – 172.31.255.255
•192.168.0.0 – 192.168.255.255
Data Communications and Computer Networks
Chapter 11
NAT
When a user on inside sends a packet to the outside, the NAT
interface changes the user’s inside address to the global IP
address. This change is stored in a cache.
When the response comes back, the NAT looks in the cache
and switches the addresses back.
No cache entry? The packet is dropped. Unless NAT has a
service table of fixed IP address mappings. This service table
allows packets to originate from the outside.
Data Communications and Computer Networks
Chapter 11
Locating a Document on the Internet
When a user, running a web browser, enters a URL, how is
the URL translated into an IP address?
The Domain Name System (DNS) is a large, distributed
database of URLs and IP addresses.
The first operation performed by DNS is to query a local
database for URL/IP address information.
If the local server does not recognize the address, the server at
the next level will be queried.
Domain Name System

The Internet utilizes a system of
Domain Name System (DNS)
servers.

These allow a user to refer to a device by
DNS Name (i.e. brewster.cs.depaul.edu)
rather than by IP address (i.e.
140.192.32.9)
brewster.cs.depaul.edu
Host
subdomain
domain
top-level
domain
Domain Name System

Top-Level Domains (TLDs):








2-letter top-level domains for each country - .fr
for France, .de for Germany, etc.
.edu - Educational Institutions
.com - Commercial
.gov - Government
.mil - Military
.org - Organizations
.net - Network providers
.int - Internet organizations
ICANN

The Internet Corporation for Assigned
Names and Numbers (ICANN) has
responsibility to:



Determine what new Top-Level Domains will
be created (.biz? .jobs?)
Determine what groups can register domain
names
Determine some policy on copyright and
name ownership issues
Who owns what names?

Domain Name registrars can assign
ownership of domain names one level
below the TLDs




One master database of all domain name
owners is kept
Example: DePaul U. owns “depaul.edu”
Registrars charge a nominal fee ($15-$30 per
year) for names
Networksolutions.com is the best known
registrar
Types of TLDs

Uncontrolled TLDs


Controlled TLDs


Anyone can register any name under these TLDs.
Examples of uncontrolled TLDs: .com, .net, .org
Only certain qualifying groups can register a
name under these TLDs – example: .edu.
National TLDs

Most countries have a national registrar that
handles all names for that country’s TLD.
Domain Name System



Top-Level DNS Servers store address of
each Domain Server for a particular top-level
domain (i.e. “.edu”)
Domain DNS Servers store names and IP
addresses for organization (i.e. depaul.edu)
Domain Names are registered with the
ICANN and associated with IP address
prefixes.

Example (depaul.edu <=> 140.192.x.x)
Domain Name System

Example: You type “http://www.ibm.com”






Your PC sends DNS request to DePaul DNS server
DePaul DNS server sends DNS request packet to “.com”
top-level DNS server
Top-level DNS server sends back address for “ibm.com”
Domain DNS Server at IBM Corp.
DePaul DNS server sends DNS request packet to
“ibm.com” Domain DNS Server
IBM DNS server looks up IP address and sends result back
to DePaul DNS Server.
DePaul DNS server sends IP address back to your PC.
Data Communications and Computer Networks
Chapter 11
IP Addresses
All devices connected to the Internet have a 32-bit IP address
associated with it.
Think of the IP address as a logical address (possibly
temporary), while the 48-bit address on every NIC is the
physical, or permanent address.
Computers, networks and routers use the 32-bit binary
address, but a more readable form is the dotted decimal
notation.
Data Communications and Computer Networks
Chapter 11
IP Addresses
For example, the 32-bit binary address
10000000 10011100 00001110 00000111
translates to
128.156.14.7
There are basically four types of IP addresses: Classes A, B,
C and D.
A particular class address has a unique network address size
and a unique host address size.
Network and Host parts

So – the first fixed part of the IP
address is the Network address or
Network Prefix

Determines what network address is on

Used by routers to determine which
direction to send the packet
Network and Host parts

The rest of the IP address is the
Host address or Host Number

Determines which particular device on
the network is addressed

Used by the last router in the path to
deliver the packet to the specific
machine
Data Communications and Computer Networks
Chapter 11
Data Communications and Computer Networks
Chapter 11
IP Addresses
When you examine the first decimal value in the dotted
decimal notation:
All Class A addresses are in the range 0 - 127
All Class B addresses are in the range 128 - 191
All Class C addresses are in the range 192 - 223
Data Communications and Computer Networks
Chapter 11
IP Subnet Masking
Sometimes you have a large number of IP address to manage.
By using subnet masking, you can break the host ID portion
of the address into a subnet ID and host ID.
For example, the subnet mask 255.255.255.0 applied to a
class B address will break the host ID (normally 16 bits) into
an 8-bit subnet ID and an 8-bit host ID.
IP Address Subnetting

Subnetting splits the bits of the IP
address into three address fields:

IP Network – identifies network prefix
assigned by the ICAAN

IP Subnet – identifies internal subnet
number determined by local network
administrator

IP Host – identifies individual machine
IP Packet Delivery

IP routers deliver data packets as follows:

Internet routers only look at IP Network address to
determine destination network

Once inside destination network, local routers look at
IP Subnet address to determine destination LAN

The last router attached to the destination LAN looks
at IP Host address to determine which particular
destination machine.
Subnet Masks


How do Host machines and routers know which
bits are Subnet Bits?
A Subnet Mask is entered into every Host
machine and every router to tell them: Out of
each 32-bit IP address:

How many bits are Network bits?

How many bits are Subnet bits?

How many bits are Host bits?
Subnet Masks

An IP subnet mask is a 32-bit value defined as
follows:



Every IP address bit position that should be a Network
bit or a Subnet bit position is marked with a 1 bit
Every IP address bit position that should be a Host bit
is marked with a 0 bit
Subnet masks are written in dotted decimal
notation, just like IP addresses – but they are
not IP addresses.
Subnet Mask Example
Network
Address:
Subnet
Mask:
Host
140
192
30
100
10001100
11000000
00011110
01100100
11111111
11111111
11111111
00000000
Subnet Mask: 255.255.255.0
Data Communications and Computer Networks
Chapter 11
The Future of the Internet
Various Internet committees are constantly working on new
and improved protocols.
Examples include:
• Internet Printing Protocol
• Internet fax
• Extensions to FTP
• Common Name Resolution Protocol
• WWW Distributed Authoring and Versioning
Data Communications and Computer Networks
Chapter 11
IPv6
The next version of the Internet Protocol.
Main features include:
• Simpler header
• 128-bit IP addresses
• Priority levels and quality of service parameters
• No fragmentation
Data Communications and Computer Networks
Chapter 11
Internet2
A new form of the Internet is being developed by a number of
businesses and universities.
Internet2 will support very high speed data streams.
Applications might include:
• Digital library services
• Tele-immersion
• Virtual laboratories
Data Communications and Computer Networks
Chapter 11
The Internet In Action: A Company
Creates a VPN
A fictitious company wants to allow 3500 of its workers to
work from home.
If all 3500 users used a dial-in service, the telephone costs
would be very high.
Data Communications and Computer Networks
Chapter 11
Data Communications and Computer Networks
Chapter 11
The Internet In Action: A Company
Creates a VPN
Instead, the company will require each user to access the
Internet via their local Internet service provider.
This local access will help keep telephone costs low.
Then, once on the Internet, the company will provide
software to support virtual private networks.
The virtual private networks will create secure connections
from the users’ homes into the corporate computer system.
Data Communications and Computer Networks
Chapter 11
Data Communications and
Computer Networks: A
Business User’s Approach
Chapter 12
Telecommunication Systems
Data Communications and Computer Networks
Chapter 12
Introduction
Students used to go into either data communications or voice
communications.
Today, the two fields are merging.
Most voice systems are computer controlled and data
networks support voice.
Anyone studying the field of data communications and
networks must learn some basic telecommunications too.
Data Communications and Computer Networks
Chapter 12
Basic Telephone Systems
POTS is the plain old telephone system that connects most
homes and small businesses.
POTS lines were designed to transmit the human voice,
which has a bandwidth less than 4000 Hz.
A telephone conversation requires two channels, each
occupying 4000 Hz.
Data Communications and Computer Networks
Chapter 12
Basic Telephone Systems
The local loop is the telephone line that runs from the
telephone company’s central office to your home or business.
The central office is the building that houses the telephone
company’s switching equipment and provides a local dial
tone on your telephone.
If you place a long distance call, the central office passes your
telephone call off to a long distance provider.
Data Communications and Computer Networks
Chapter 12
Data Communications and Computer Networks
Chapter 12
Basic Telephone Systems
The country is divided into a few hundred local access
transport areas (LATAs).
If your call goes from one LATA to another, it is a long
distance call and is handled by a long distance telephone
company.
If your call stays within a LATA, it is a local distance call and
is handled by a local telephone company.
Data Communications and Computer Networks
Chapter 12
Basic Telephone Systems
A trunk is a special telephone line that runs between central
offices and other telephone company switching centers.
A trunk is usually digital, high speed, and carries multiple
telephone circuits.
A trunk is typically a 4-wire circuit, while a telephone line is
a 2-wire circuit.
Voice Network Structure
IXC
POP
C
IX
k
un
Tr
Ameritech
Central Office
em
Tand
Ameritech
T andem Office
k
Trun
Direct
IXC Trunk
IXC
POP
Ameritech
Central Office
Inter-Office T runks
s
X
PB
nk
T ru
PBX
Centrex Lines
Subscriber Lines
POP = Point of Presence
PBX = Private Branch Exchange
Regulatory History
1876
Hubbard, Gray and Bell all file
telephone patents
1877
Bell Telephone Company formed
1878
BTC files patent suit against
American Speaking Telephone
1879
National Bell Telephone (NBT)
formed via mergers
NBT becomes American Bell
1880
Regulatory History
1881
American Bell buys Western
Electric from Western Union
1885
AT&T formed from American
Bell
1910
Western Union joins AT&T
1913
AT&T divests Western Union
in anti-trust suit. Agrees to
interconnect independent
telcos
Regulatory History
1934
FCC formed and universal
telephone service is made a
national goal (Fed. Comm. Act)
1940
Anti-trust suit against AT&T
postponed due to WWII
1949
Anti-trust suit filed against AT&T
1956
AT&T settles by limiting Western
Electric manufacturing to Bell
companies (Consent Decree)
Regulatory History
1966
Computer Inquiry I - Bell System
cannot provide data processing
1968
Carterphone decision Compliant non-Bell devices
must be allowed to connect.
1974
Anti-trust suit filed against AT&T
1982
AT&T settles by agreeing to
divest 22 local operating
companies (Consent Decree)
Regulatory History
1983
Regional Bell Operating Companies
(RBOCs) formed
1983
FCC rules telcos can only provide
equipment (CPE) through separate
subsidiary.
1984
Consent decree is implemented as
modified by Judge Greene (Modified
Final Judgement).
Modified Final Judgement
1984



7 Regional Holding Companies (RHCs) or
Regional Bell Operating Companies
(RBOCs) formed
Each RHC composed of multiple Local
Exchange Companies (LECs) (ex.
Ameritech)
Interexchange Carriers (IXCs) provide
long-distance service. (ex. AT&T)
Data Communications and Computer Networks
Chapter 12
Modified Final Judgement
(1984)




231 Local Access and Transport
Areas (LATAs) defined in US.
LECs can provide only intra-LATA
service.
IXCs provide only inter-LATA service.
LECs must give customers equal
access to all IXCs.
Problems with MFJ of 1984:



No single company (LEC or IXC) can
provide all communications services for
its customers. This is inefficient.
LATA boundaries limit LEC services.
Many other companies (Cable TV,
Competitive Access Carriers, etc.) are
not constrained by MFJ.
The Solution? Deregulation!

Telecommunications Act of 1996




Allows IXCs to offer local service
Allows LECs to offer long-distance service (when
they can show competition)
Allows Competitive LECs (CLECs) to share LEC
facilities (for fair competition).
Allows combination of voice, data, video service
packages.
Telecommunications Act (1996)

LECs must give competitors:




All services at wholesale prices.
Access to telephone numbers, operator
services, directory listings.
Access to poles, ducts, and right-of-ways.
Physical co-location of equipment within
LEC buildings.
Telecommunications Act (1996)

A LEC can offer inter-LATA services when
substantial competition exists on its
network.

The courts are still deciding exactly what this
means...
Merger-Mania!! - LECs

SBC Communications buys:



Qwest Communications buys:


U.S. West – 6/14/96
Bell Atlantic buys:



Pacific Telesis – 4/1/96
Ameritech – 5/11/98
NYNEX Corporation – 4/22/96
GTE Corporation – 7/28/98
BellSouth is only original LEC still unmerged
Merger-Mania!! - IXCs


WorldCom Inc. buys:

MFS Communications – 8/26/96

MCI Communications – 10/1/97

Sprint Corporation – 10/5/99 (failed)
AT&T buys:

McGaw Cellular – 8/17/83

TCI, Inc. (cable) – 6/24/98

MediaOne Group Inc. (cable) – 4/22/99
Telecommunications Act (1996)

December 21, 1999: Bell Atlantic is the
FIRST regulated Local Exchange Carrier to
be approved for long distance services by
the FCC.

Decision was immediately challenged by AT&T
– more court dates pending…
Problems - Telecom Act 1996:
Competition is slow to arrive

IXCs are moving slowly on providing local
service due to wiring costs.


CLECs/LECs are still in court deciding:

Exactly how they must share their facilities

Exactly when they can offer long-distance service.
Cable TV carriers find it expensive to offer
voice services over cable.
Problems - Telecom Act 1996:
Competition reduces quality


Local telephone service is getting worse

Time to get a new phone installed

Time to fix problems

Number of outages
Some Reasons:

Too many telephony carriers are competing for too
few technical personnel

Emphasis is on business customers ($$$)
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
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

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
Service
Circuits
Com m on Data
Contr ol Store
Switch Matrix
Switch Matrix
Line
Cards
Trunk
Cards
Trunk
Cards

Line
Cards
MDF
MDF
Inter-Office Trunk
PBX
Com m on Data
Contr ol Store
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.