Customer Premise Equipment
COMT 220
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Definitions
• CPE - Customer Premise Equipment
– Correctly defined as “anything the
customer connects to the network”
– In practice we use CPE to mean
“equipment other than simple terminals”
(phones, PCs, etc.)
• We will discuss mainly voice, but the
arguments apply to video and data also
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Rationale
• Placed between End-Users and CO
connections
• Provide on-premise functions
• Make cost-efficient use of CO connections
User
User
CPE
CO
User
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Different Types…
• PBX - Private Branch Exchange
– Full Premise Based Switching
• Key System
– Smaller System, usually non-switching
– Difference to PBX often unclear
• CENTREX
– PBX functionality, but implemented in the
Central Office Switch
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Switching Principles and
Methods
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Cross-Point Switching
“Space Division Switching ”
1
4
1
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6
Three-Stage Switch
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From Stallings, Voice and Data Communications.
7
Time Division Switching
• Each Line Card “Samples” the inbound signal
(8000 times per second, each sample is taken
over 0.125 msec)
• While the cards collect the next sample
(during the next 0.125 msec), the Time Slot
Interchanger collects all samples and delivers
them to the correct line card outputs
• The TSI must process 8000*2*(number of
conversation) samples
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Time Division Switching
TSI
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PBX “Block Diagram”
Analog
Phones
Digital
Phones
L
I
N
E
S
Special
Workstations
SMDR
IXC
Special
Access
Private
Lines
Administration
Maintenance
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PBX
Switching
System
T
R
U
N
K
S
Local
Switched
Access
CTI
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Packet Switching?
Analog
Phones
Digital
Phone
L
I
N
E
S
PBX
Switching
System
T
R
U
N
K
S
Digital
Phone
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Categories of Features
• Features for the System User (Internal
Customer)
• Features for the System Administrator
• Features for the Caller (External
Customer)
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Handling Outbound Calls
• Call destination restrictions
• Least cost route selection
• Automatic redial
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Handling Inbound Calls
• Routing Calls
– Attendant
– Direct Inward Dialing
• Handling Unanswered Calls
–
–
–
–
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Call Forwarding -- General
Call Forwarding -- Busy/No-Answer
Hunt Groups
Queuing and Message Services
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Handling Unanswered Calls
• Hunt Groups
• Queuing
• UCD
– Universal Call Distributor
• ACDs
– Specialized PBX software for inbound call
handling
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PBX Capacity
• Traffic (usually measured in call
minutes per hour)
– How Many Simultaneous Calls
• Processor
– Total Calling Volume
– Busy Hour Call Attempts
• Physical Interfaces
– Stations and Trunks
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“Programming a PBX”
• Moves, Adds, and Changes (MAC)
– Establish New Service
– Disconnect Service
– Change Features On Existing Service
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Adding an Analog Station
• Physical Install
• Assign Extension Number
• Assign Class of Service and Other
Features
– Often uses a template (or “Table”)
A
B
C
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Adding a Digital Phone
• Physical Installation
• Define Feature Buttons on the Digital
Phone
• Define Line Appearance Buttons
• Assign Extension(s) to Line
Appearances
• Assign Class of Service
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Button Assignments
A
B
1
C
2
4
1
2
5
7
*
3
6
8
0
9
#
3
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Button Assignments
Hold
A
B
Xfer
1
C
4
Line 2
1
2
Line 1
2
5
7
*
3
6
8
0
9
#
3
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Extension Assignments
A
B
Hold
1234
5678
Xfer
C
1
4
1234
1
2
5678
2
5
7
*
3
6
8
0
9
#
3
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Extra Slides
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Multi-Stage Crosspoint
Switching - Concept
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