basic communications systems class 9. today’s class topics wan data services dsl cable modems t1...
TRANSCRIPT
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
1 b it 8 b its 8 b its8 b its8 b its
F DS0 #1 DS0 #24DS0 #3DS0 #2 . . .
Frames 6, 12:
Frames 1-5, 7-11:
1 b it 7 b its 7 b its7 b its7 b its
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
ClientClientFile Server
TokenRingHub
Router
ClientFile Server
TokenRingHub
Router
ClientFile Server
EthernetHub
Router
Client ClientFile Server
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
ClientClientFile Server
TokenRingHub
Router
ClientFile Server
TokenRingHub
Router
ClientFile Server
EthernetHub
Router
Client ClientFile Server
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
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
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
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 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.
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
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
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 Switch Matrix can
interconnect Line/Trunk cards and Service Circuits
Line Cards perform per-line processing (BORSCHT functions).
Trunk Cards perform per-trunk processing.
Main 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 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
Main 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
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.
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 ServicesCentral Office
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.
Private Line Services
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)
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 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.
Private Line Services
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
A Call using a Key SystemFrank calls Sam in New York
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
A Call using a Key SystemFrank calls Sam in New York
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
A Call using a Key SystemFrank calls Sam in New York
Jane hears a special intercom ring
Jane and Frank talk
Central Office
KSUBusiness
Lines
Custom er Prem ises
A Call using a Key SystemFrank calls Sam in New York
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
M ulti-ButtonKey Sets
Custom er Prem ises
A Call into a Key SystemJoe calls Frank
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
M ulti-ButtonKey Sets
Custom er Prem ises
A Call into a Key SystemJoe calls Frank
Frank answers phone and talks to Joe
Central Office
KSUBusiness
Lines
M ulti-ButtonKey Sets
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
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 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
A Call into a PBXJoe calls Frank
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
A Call into a PBXJoe calls Frank
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
A Call into a PBXJoe calls Frank
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
AmeritechCentral Office
PBXBusinessLines orTrunks
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
AmeritechCentral Office
PBXBusinessLines orTrunks
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
AmeritechCentral Office
PBXBusinessLines orTrunks
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.