DigiPoints Volume 1. Student Workbook. Module 4 Bandwidth Management

Transcription

1 Bandwidth Management Page 4.1 DigiPoints Volume 1 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: 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.

2 Bandwidth Management Page 4.2 Prerequisites Read, Modules 1 4.

3 Bandwidth Management Page 4.3 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?

4 Bandwidth Management Page 4.4 Amplitude 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?

5 Bandwidth Management Page 4.5 TIMESLOTS Time Timeslot 1 Timeslot 2 Timeslot 3 Timeslot 4 Timeslot 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?

6 Bandwidth Management Page 4.6 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?

7 Bandwidth Management Page 4.7 A B C A4 B4 C4 INPUT LINES A3 B3 C3 MUX s1 Control and Timing Unit CTU C2 B2 A2 FR C1 B1 OUTPUT LINE A1 FR 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?

8 Bandwidth Management Page 4.8 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?

9 Bandwidth Management Page 4.9 Variable Bit Rates on Synchronous TDM A B C D A4 A3 B4 B3 C4 C3 C P 4 C P 3 I D L E INPUT LINES MUX s1 Control and Timing Unit CTU C P 2 C2 B2 A2 FR C P 1 C1 OUTPUT LINE B1 A1 FR What was one of the limitations we discussed last time with synchronous TDM? What is being carried in the idle timeslot?

10 Bandwidth Management Page 4.10 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?

11 Bandwidth Management Page 4.11 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?

12 Bandwidth Management Page 4.12 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%

13 Bandwidth Management Page 4.13 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?

14 Bandwidth Management Page 4.14 T-1 Superframe 1 Framing Bit Channels 1-24 Each channel has 8 bits 1 Frame = 24 channels + 1 framing bit = 193 bits Superframe A B First of 12 frames (193 bits) 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?

15 Bandwidth Management Page 4.15 T-1 Extended Superframe 1 Framing Bit Channels 1-24 Each channel has 8 bits 1 Frame = 24 channels + 1 framing bit = 193 bits Extended Superframe A B C D First of 24 frames (193 bits) 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?

16 Bandwidth Management Page 4.16 Extended SuperFrame (ESF) Format Use of the Framing Bit Frame Number Sequence m 1 m 0 m 2 m 0 m 3 m 1 m 4 m 0 m 5 m 1 m 6 m 1 Messaging m m m m m m m m m m m m Synchronization CRC Signal Frame A B C D Message Bit (m) for the Facility Data Link Framing Pattern Sequence (001011) Cyclical Redundancy Check (CRC-6) (6 BITS) 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 is the purpose of CRC? What does the Facility Data Link do? Where is the framing sequence found?

17 Bandwidth Management Page 4.17 Alternate Mark Inversion 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?

18 Bandwidth Management Page 4.18 Line Code Restrictions B8ZS Digital signal given to the carrier by the customer: 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?

19 Bandwidth Management Page 4.19 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?

20 Bandwidth Management Page 4.20 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.

21 Bandwidth Management Page 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.