Bandwidth Management
DigiPoints, Volume 1
Page 4.1
Student Workbook
DigiPoints Volume 1
Student Workbook
Module 4 – Bandwidth Management
Summary
This module will cover Time Division Multiplexing (TDM). TDM technology allows
many users to access a particular media simultaneously separated only by “time”. At the
end of this module, the student will be able to describe what a multiplexer does, the
meaning of TDM, the concept of time slots, how synchronization is maintained between
terminals, and the relationship between input and output lines in a system. T1 lines, the
most common example of TDM, as well as concentrators will also be covered. The
purpose is to learn the appropriate application of each and to understand the advantages
and disadvantages of each type of device.
Module Objectives
Upon successful completion of this module the student should be able to:
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Describe the function of a multiplexer.
Describe a timeslot.
Describe how the output line rate is related to the input line rates.
Calculate bit rates, line rates, and bit duration time.
Explain the importance of clocking in a digital system.
Explain TDM, the characteristics of a TDM frame, and one of the inefficiencies
found in a TDM system.
Describe the function of a Statistical Multiplexer.
Compare and contrast a TDM versus a STATMUX
Describe a T1 in terms of input bit rates, output bit rate, frames, framing bits, and
control and supervision bits.
Describe the concept of bit robbing.
Identify a T1 Superframe and describe what it provides to the service provider.
Describe a T1 Extended Superframe and explain what it provides the service provider
and the customer.
Describe a 64 kb/s clear channel.
Explain what AMI is, and why it is used.
Explain the concept and reasons for BNZS.
Explain what a concentrator does; compare and contrast it with a TDM multiplexer.
SCTE
Bandwidth Management
DigiPoints, Volume 1
Page 4.2
Student Workbook
Prerequisites
Read DigiPoints, Volume 1, Modules 1 – 4.
SCTE
Bandwidth Management
DigiPoints, Volume 1
Page 4.3
Student Workbook
What are examples of shared or community usage that save money?
Why is this done?
How much bandwidth can a pair of copper wires carry?
How much bandwidth can a coaxial cable carry?
What type of device can we use to efficiently use this bandwidth? In other words, how
can we get multiple channels or customers on one “big pipe”?
What were the modulation techniques that were discussed in Module 2?
SCTE
Page 4.4
Student Workbook
Amplitude
Bandwidth Management
DigiPoints, Volume 1
Sample Interval:
125 µsec
or _______
What are the steps in converting an analog signal to a digital signal?
How often is a sample taken?
How long does the sampling time last?
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Bandwidth Management
DigiPoints, Volume 1
Page 4.5
Student Workbook
TIMESLOTS
Time
Timeslot Timeslot Timeslot
1
2
3
Timeslot Timeslot
4
N
What can be carried in these time slots?
How many times per second would a voice grade signal have a Timeslot?
The PAM signal was output as a PCM signal. How many bits did it require to encode the
PAM sample?
If there are 8000 samples of 8 bits each, for a voice grade line, how many bits are
transmitted each second?
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Bandwidth Management
DigiPoints, Volume 1
Page 4.6
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If 24 voice grade signals are being sampled, what is the output line rate in b/s?
So, how many bits are in one cycle, that is, how many bits are being formed into a frame
during each sampling cycle?
If one frame bit is added to every frame, how many bits per frame?
What is the output line rate now?
SCTE
Bandwidth Management
DigiPoints, Volume 1
Page 4.7
Student Workbook
A
B
C
A4
B4
C4
A3
B3
C3
MUX
INPUT LINES
s1
Control and
Timing Unit
CTU
C2
B2
A2 FR
C1
B1
OUTPUT LINE
Why is the clock important?
What are the functions of the CTU?
What is the purpose of the clock? What does it do?
What is the relationship between the input and output line rates?
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A1 FR
Bandwidth Management
DigiPoints, Volume 1
Page 4.8
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Synchronous TDM Line
Samples from the “A” Input Line
C4
B4
A4 FR C3 B3 A3 FR C2 B2 A2 FR C1 B1 A1 FR
Frame 4
Frame 3
Frame 2
Frame 1
Output Line
What is the difference between a framing bit and an information bit?
How does the far end know when a frame begins?
What prevents cross talk between the different messages?
Why would this technique be more efficient then using asynchronous transmission?
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Bandwidth Management
DigiPoints, Volume 1
Page 4.9
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Variable Bit Rates
on
Synchronous TDM
A
B
C
D
A4
B4
C4
C P4
A3
B3
C3
C P3
MUX
I
D
L
E
INPUT LINES
s1
Control and
Timing Unit
CTU
CP2 C2
B2
A2 FR
CP1 C1
B1
A1 FR
OUTPUT LINE
What was one of the limitations we discussed last time with synchronous TDM?
What is being carried in the idle timeslot?
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Bandwidth Management
DigiPoints, Volume 1
Page 4.10
Student Workbook
Scary Thought
Usage studies have found that, on average, most
buffered terminals require actual communications
line time less than 1% of the time they are in use.
On average, 99% of the time the input lines are
sending 0s.
What does this mean, what is the significance of it?
SCTE
Bandwidth Management
DigiPoints, Volume 1
Page 4.11
Student Workbook
STATISTICAL MULTIPLEXER
the STATMUX
How does it do this:
• Dynamically allocate timeslots based on
need
What is needed:
• Buffers
• Processing Power
What does the STATMUX do that the TDM does not?
What is another name for STATMUX?
What two things does a STATMUX need in order to do what it does?
What could be the disadvantages of a STATMUX?
What are the advantages of using an Asynchronous TDM?
Which input signal to a STATMUX, synchronous or asynchronous, would have priority
and why?
What is the relationship of the output rate to the input rate for a synchronous TDM? How
does that compare with an asynchronous TDM?
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Bandwidth Management
DigiPoints, Volume 1
Page 4.12
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STATISTICAL MULTIPLEXER EVALUATION CRITERIA
Overall Operation
Transmission Efficiency A
Delay
Protocol handling
Throughput
Routing
Synchronous and Asynchronous operation
Programmability
Switching capability
Port contention
Automatic speed changing
Multiple output lines for backup or sharing
Pricing
Ease of Operation
Buffer Management
Size of buffer pool: Should be 6-8 times the output line capacity for
dynamic allocation of input channels to timeslots.
Buffer allocation by channel or group of channels
Buffer capability to absorb peak transmission bursts
Operations/Maintenance
Local and remote loopbacks
Generation of test messages
Memory and component checks.
Self tests on boot up
Fault isolation to circuit board level
Understandability of indicators to an operator
Availability of messages and statistics for user analysis
The following formulas provide a way to quantify transmission efficiency:
Gross Efficiency = (Total bit carrying capacity of incoming lines/bit carrying capacity of
output line) x 100%
User Protocol Efficiency = (Total single channel bit carrying capacity/bit carrying
capacity of output line) x 100%
High speed Link Efficiency = (Data actually being sent over the output line/bit carrying
capacity of the output line) x 100%
True Efficiency = (Total bit carrying capacity actually being input on the low speed
channels/actual bit carrying capacity of the output line) x 100%
SCTE
Bandwidth Management
DigiPoints, Volume 1
Page 4.13
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T-1 Frame
8 bits
per
channel
1 Framing
Bit
Channels 1-24
Each channel has
8 bits
When was T1 technology first implemented?
What types of traffic or information were the first T1 systems designed to carry?
How many bits were used to encode the PAM sample into a PCM signal?
What is the size of the T1 frame, how many bits in a frame?
What is the bit rate of a T1?
What is the bit rate of each of the 24 channels?
What is bit robbing, why is it used, and what are the disadvantages of using it?
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Bandwidth Management
DigiPoints, Volume 1
Page 4.14
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T-1 Superframe
Channels 1-24
Each channel has
8 bits
1 Framing
Bit
1 Frame = 24 channels + 1 framing bit = 193 bits
Superframe
First of
12 frames
(193 bits)
A
B
Signalling Frames
(Bit robbing occurs
in these frames)
How big is a T1 superframe?
What is the problem for data now?
What is the advantage of having a superframe?
Can it provide 64 kb/s clear channel service?
What is significant about the 6th and 12th frame of a Superframe?
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Bandwidth Management
DigiPoints, Volume 1
Page 4.15
Student Workbook
T-1 Extended Superframe
Channels 1-24
Each channel has
8 bits
1 Framing
Bit
1 Frame = 24 channels + 1 framing bit = 193 bits
Extended Superframe
A
First of
24 frames
(193 bits)
B
C
D
Signalling Frames
(Bit robbing occurs
in these frames)
What was the driving force for the development of the T1 Extended Superframe (ESF)?
Bit robbing takes place in which frames?
What is the advantage of the ESF? Does it give 64 kb/s clear channel?
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Bandwidth Management
DigiPoints, Volume 1
Page 4.16
Student Workbook
Extended SuperFrame (ESF) Format
Use of the Framing Bit
1 2
Frame Number
Sequence
m
Messaging
3
1 m
m
6
7
8
2 m 0
m
m
0
1
Signal Frame
5
0 m
m
Synchronization
CRC-6
4
9
m
10
11
3
m
m
1
m
0
2
12
13
14
m
4
m
A
16
17
18
19
20
21 22
23
24
0
m
5
m
1
m
m
1
m
m
0
4
B
m
m
1
3
15
m
5
C
How can the signaling frames 6, 12, 18, & 24 also be used to carry CRC and
synchronization information? How can a bit be robbed twice?
What does the Facility Data Link do?
Where is the framing sequence found?
SCTE
m
1
Message Bit (m) for the Facility Data Link
Framing Pattern Sequence (001011)
Cyclical Redundancy Check (CRC-6) (6 BITS)
What is the purpose of CRC?
6
1
6
D
Bandwidth Management
DigiPoints, Volume 1
Page 4.17
Student Workbook
Alternate Mark Inversion
1
0
1
1
0
0
1
1
What is the average voltage on the T1 line?
Why would it be desirable to have an average voltage = 0 on the line?
How does the system utilize the bits for timing?
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Bandwidth Management
DigiPoints, Volume 1
Page 4.18
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Line Code Restrictions
B8ZS
Digital signal given to the carrier by
the customer:
8
7
6
5
4
3
2
1
8
0 0 0 0 0 0 0 0 1
Digital signal as modified by the
carrier equipment, to create
bipolar violations
V
V
The substituted B8ZS Code
How many consecutive 0s can a T1 receiver tolerate before losing synchronization?
How does the B8ZS line code restriction work?
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Bandwidth Management
DigiPoints, Volume 1
Page 4.19
Student Workbook
Multiplexers
and
Concentrators
in Series
Concentrator
Multiplexer
Multiplexer
Concentrator
Concentrator
OUTPUT
INPUTS
What does a concentrator do?
What is the advantage of the arrangement shown above?
SCTE
Bandwidth Management
DigiPoints, Volume 1
Page 4.20
Student Workbook
Study Questions
1. What function does a multiplexer perform?
2. What is a timeslot?
3. How is the output line rate related to the input line rates?
4. What is the importance of clocking in a digital system?
5. Explain what TDM is, the characteristics of a TDM frame, and one of the
inefficiencies found in a TDM system.
6. Explain why a telephone party line is not multiplexing.
SCTE
Bandwidth Management
DigiPoints, Volume 1
Page 4.21
Student Workbook
7. Give the constraints on the amount of bandwidth that can be gained by multiplexing.
8. What must be added to a time division multiplexer to make it a statistical multiplexer,
and what is the function of this addition?
9. What is the function of bit robbing in a T1 frame?
10. In the T1 Superframe, where are the signaling bits contained?
11. List what has been gained by going to the Superframe and the Extended Superframe.
SCTE