CSIS 625 Week 4

Transmission Media, Multiplexing

For use by students of CSIS 625 for purposes of this class only.

I. Overview

A. Wrap up Transmission Media

1. Wireless

2. Impairments

3. Performance

B. Multiplexing

1. Space Division Multiplexing

2. Frequency Division Multiplexing

3. Wave Division Multiplexing

4. Time Division Multiplexing

a) Synchronous

b) Statistical

(1) Telephony Traffic Engineering

II. Wireless media

A. Wireless communication

1. Using free space or the air as your media. (i.e. not using wire or fiber)

2. Radio waves can be modulated using FM, AM, PM, or QAM

3. Often used for broadcast applications - TV, Radio, etc.

4. Some frequencies bounce off layers in atmosphere allowing for greater distance

5. Higher frequencies = line of sight

B. Frequency Bands

1. 0-300 Hz ELF - Extremely low Freq

2. 300-3000 Hz ILF - Infra Low Freq

3. 3-30 kHz VLF - Very Low Frequency

4. 30-300 kHz LF - Low Frequency

5. 300-3000 kHz MF - Medium Frequency

6. 3-30 MHz HF - High Frequency

7. 30-300 MHz VHF - Very High Frequency

8. 300-3000 MHz UHF - Ultra High Frequency

9. 3-30 GHz SHF - Super High Frequency

10. 30-300 GHz EHF - Extremely High Frequency

11. 300-3000 GHz THF - Tremendously High Frequency

C. Wireless Applications

1. TV and Radio

2. Cellular Telephone

3. Satellite Television

4. Satellite Telephony and Data

5. Wireless LANs

D. Cell Phone Technology

1. Based on the idea of offering mobile phone service

2. Reference:

a) Much of this material taken from:

b) http://www.ee.washington.edu/class/498/sp98/final/marsha/final.html

c) http://www.iec.org/online/tutorials/cell_comm/

d) http://www.iec.org/online/tutorials/tdma/

3. There were early radio to telephone type systems, but cellular technology didn’t start until the early 1980’s.

a) Limited number of channels in a large area

b) 1981the FCC approved the use of a larger number channels.

4. Based on the idea of using radio frequencies in small areas called cells.

a) 806-890 MHz and 1850-1990 MHz

5. Cells are often laid out in a honeycomb type of topology.

a) Frequencies can then be re-used in non-adjacent cells.

b) A cell is 1-2 miles in radius in urban areas

c) up to 20 miles in radius in rural area.

d) Micro-Cell and Pico-Cell are used in very small very high density areas

(1) Such as inside a sports stadium

6. Cellular system components

a) PSTN - Public Switched Telephone System

(1) The “normal” phone system

b) MTSO - Mobile Telephone Switch Office

(1) The switch that controls all of the cell sites

(2) This switch coordinates handoffs between cells

c) Mobile Base Station - The cell site

(1) The antenna, radios, interface equipment, etc.

d) MSU - Mobile Subscriber Units

(1) The cell phone you carry around.

7. Cell phone - Analog vs. Digital

a) Analog technology uses FDM with FM analog modulation

(1) call uses a fixed frequency in its cell for the duration

(2) Older scheme - more coverage in united states than digital technology

(3) Any scanner can listen in on these conversations

(a) Newt Gingrich knows this well

(b) Congress made it illegal to listen in on these conversations, but it is very difficult to enforce unless you come out with a tape

b) Digital technology

(1) Uses PCM to get a digital stream

(2) Then uses various audio compression techniques to get voice down to rates as low as 8kbps.

(a) Compression may reduce the quality of the voice

(3) Newer technology with good coverage in high population density areas.

(4) Allows many more calls for a given bandwidth

(5) May use “silence suppression” to further increase capacity

(a) Silence suppression - not sending data when there is no-one talking

(b) Effectively doubles the amount of data that can be sent.

(c) Need to have comfort noise added

8. Cell - Modulation techniques

a) FDMA - Frequency Division Multiple Access

(1) used for analog phones

(2) The same thing as Frequency Division Multiplexing

b) TDMA - Time Division Multiple Access

(1) Divides a frequency into multiple timeslots

(2) Each call uses one timeslot in one frequency

(3) Increases the number of calls that may be present

(4) Used with digital technology

c) CDMA - Code Division Multiple Access

(1) Use of entire time and frequency

(2) Each call has minimal interference with one another.

(3) Not a hard limit on the number of calls possible

(a) Each call just adds a little more interference

d) GSM - Global System for Mobile Communications

(1) TDMA based system used in Europe.

(2) Voice compressed to 13kbps

e) PCS - Personal Communications Services

(1) use of 1900 MHz frequencies

9. Cell handoff

a) As a phone moves from one cell to another, the call must be handed off with minimal interruption.

(1) When signal strength decreases sufficiently, base station notifies switch

(2) The switch queries the other base stations to determine which has the strongest signal from the phone.

(3) The switch then notifies the new base station to take over the call

(4) Normally less voice is dropped for only 10-100ms

b) CDMA can use a “soft” handoff with no interruption

(1) The use of the entire frequency spectrum means that there is no other channel to use

III. Transmission impairments

A. Attenuation

1. Signal loses strength as it goes through medium

2. All devices that handle a signal cause some attenuation

B. Distortion

1. Signal changes form or shape as it goes through medium

2. Dispersion in fiber optic systems is a prime example

3. The

C. Noise

1. Additional signal merged in

2. The static heard on a radio is one example

3. The ghost images on a TV are an example of a reflection of the original being noise.

D. Signal Strength Measurement

1. Decibel (dB) is a measure of the relative strengths of two signals.

a) dB = 10 * log₁₀ (P₂/P₁)

b) P₁ = Power of signal at point 1

c) P₂ = Power of signal at point 2

2. dB are used because it allows end-to-end signal strength to be determined by adding up attenuation and amplification

3. Signal-Noise Ratio - a dB measurement of signal strength to noise strength

IV. Transmission Performance

A. Throughput

1. How many bits you can put through the system in a unit of time – typically bps (bits per second)

B. Latency

1. How long it takes for bits to get through a system

2. Affected by:

a) The speed of bits through the system

b) The distance the bits travel

c) The processing time at various stops

C. Error Rate

1. How many bits get changed as they go through the system

V. Multiplexing

A. Definition:

1. Multiplexer - (Mux) a device to combine multiple signals to go over one media link

2. Demultiplexer - (Demux) a device to separate the multiple signals from a multiplexer

B. Space division multiplexing

1. Use of multiple paths between one source and one destination

2. Not really multiplexing because it doesn’t use one media link

3. Inverse-Multiplexing - Use of multiple paths between two points for one signal to get greater bandwidth.

C. Frequency Division multiplexing - FDM

1. Use of different carrier frequencies

2. Must make sure that the carriers do not overlap

3. Guard Band - unused bandwidth between signals that provides protection against overlap

4. TV and Radio are most common examples

5. Telephony FDM

a) Telephony before the digital time, used FDM heavily

b) AT&T and CCITT came up with slightly different standards

c) Lower groups multiplex to higher groups

d) Telephony Levels:

# Voice
Channels	Bandwidth	Spectrum	AT&T	CCITT
12	48kHz	60-108kHz	Group	Group
60	240kHz	312-552kHz	Supergroup	Supergroup
300	1.232MHz	812-2044kHz		Mastergroup
600	2.52MHz	564-3084kHz	Mastergroup
900	3.872MHz	8.516-12.388MHz		Supermastergroup
3600	16.984MHz	0.564-17.548MHz	Jumbogroup
10800	57.442MHz	3.124-60.566MHz	Jumbogroup
			Multiplex

6. xDSL - A type of FDM

a) DSL = Digital Subscriber Line

(1) A way of sending digital data over the twisted pair intended for voice traffic

b) ADSL - Asymmetric DSL

(1) Targeted at home users

(a) Asymmetric in that it has more bandwidth to the home than from the home

(b) Based on assumption that home users consume data more than they generate

(c) This may not be true as some peer-to-peer networks take hold

(2) 0-25KHz for POTs (really only 0-4KHz used)

(3) 25-200KHz for Upstream Data

(4) 200-1100KHz for Downstream Data

D. Wave Division multiplexing (WDM)

1. Use of multiple wavelengths of light over a fiber optic system (optical form of FDM)

2. CWDM - Coarse WDM

a) Typically use of 850, 1310nm and 1550nm wavelengths

b) Sometimes use of 4 or 8 wavelengths around 1550nm

3. DWDM - Dense WDM

a) Use of many (16-100+) wavelengths around the 1550nm wavelength.

E. Synchronous Time Division Multiplexing (TDM)

1. Multiple signals are carried by interleaving portions of each signal in time.

2. Each input signal has exactly the same time slot that occurs repeatedly

3. A group of time slots are grouped into a frame

4. May occur at bit level, byte level, or blocks of data

5. May be done in analog systems as well as digital, but typically seen in digital systems

6. The incoming signals must have big enough timeslots so that they never have to buffer data for more than one frame.

7. The outgoing bit rate of a MUX must be ³ the sum of the incoming bit rates.

a) If the incoming bit rates are equal, then typically each source gets one timeslot per frame.

b) If the incoming bit rates are not equal then each source gets a different number of timeslots per frame (but the same in every frame)

8. So that the DEMUX knows when the timeslots are and who gets which data, there is some framing overhead.

a) Typically some extra bytes of data at the start of each frame.

9. If the data rate of the incoming signals does not divide evenly into a timeslot, then extra bits may be inserted by the MUX and discarded by the DEMUX.

a) This is sometimes called bit-stuffing

10. Telephony TDM

a) Telephony uses Synchronous TDM heavily as it always has a constant data rate

b) Named DS or T in North America

(1) DS-1 == T1, DS-3 == T3, etc

	North America			CCITT
Digital	# Voice		Level	# Voice
Signal Number	Channels	Data Rate	Number	Channels	Data Rate
DS-0	1	64kbps	0	1	64kbps
DS-1/T1	24	1.544Mbps	E1	30	2.048Mbps
DS-1C	48	3.152Mbps	E2	120	8.448Mbps
DS-2/T2	96	6.312Mbps	E3	480	34.368Mbps
DS-3/T3	672	44.736Mbps	E4	1920	139.264Mbps
DS-4/T4	4032	274.176Mbps	E5	7680	565.148Mbps

c) DS1 circuit

(1) The DS1 circuit is the most common digital telephony signal

(2) Breakdown of 1.544Mbps

(a) 24 voice timeslots per frame - one byte per timeslot

(b) 1 bit per frame for framing information

(c) 24 timeslots/frame * 8 bits/timeslot + 1 bit/frame = 193 bits/frame

(d) 193 bits/frame * 8000 frames/sec = 1.544Mbps

(3) Fractional T1 –

(a) A T1 where only some of the timeslots are in use.

(b) Still operates at 1.544Mbps

d) T1 - a little more information

(1) Original D1 channel banks

(a) Used alternating 1/0 pattern in framing bit

(b) Could get confused by 1000Hz tone

(c) Used least significant bit of every data byte for signaling.

(2) D2-D4 channel banks

(a) Used 12 bit pattern in framing bit

(b) Used least significant bit data byte for signaling only in the 6^th and 12^th frame

(c) This is AB signaling

(3) SF - Super Frame

(a) Framing format used by D2-D4 channel banks

(b) Also Called D4 Framing

(4) ESF - Extended Super Frame

(a) Groups 24 frames together

(b) Uses 6 of the framing bits for framing

(c) Uses 6 of the framing bits for CRC

(d) Uses 12 of the framing bits FDL - Facility Data link

(i) Allows both ends to communicate

(e) ABCD signaling

(i) 6^th, 12^th, 18^th, 24^th frame least significant bit

(5) T1 - SF Line coding

(a) SF - typically uses AMI line coding

(i) This requires that there are some 1’s every so often.

(ii) This is a problem for pure data.

(iii) Solution - Use HDLC and invert logic levels.

(a) After 5 ones in a row HDLC inserts a 0

(b) When inverted this will create a 1 after every 5 zeros

(iv) Telephony - quiet tone is all 1’s and all 0’s is biggest amplitude.

(a) All 0’s very rarely occurs

(b) No problems with AMI

(6) T1 - ESF Line coding

(i) ESF - typically uses B8ZS line coding

(a) No Data dependencies

(b) B8ZS makes sure that any data pattern can pass without problem.

(7) If you order a T1 from the phone company

(a) Specify ESF

(b) Specify B8ZS

(c) Especially true for data, but true even for modem traffic or voice traffic

(d) You get better protection and CRC error counts

F. Statistical Time Division Multiplexing

1. With Synchronous TDM, if an input has nothing to send, that timeslot is wasted.

2. With Statistical TDM you are betting that at any given time only some of the inputs want to send data

3. The sum of the input bit rates to the MUX may exceed the output bit rate of the MUX

4. If you are “unlucky” some data may be delayed or discarded by the MUX

5. Delaying data because others are using the line requires additional buffers at the MUX

6. A burst of high speed data at the DEMUX may require the DEMUX to buffer data until the lower speed output can accept it

7. Timeslots can be borrowed

8. Some inputs can have priority over others

9. Some systems have variable length timeslots

10. Additional framing overhead required

a) Just knowing the timeslots is not enough

b) Each packet of data in a statistical TDM system must have overhead labeling its source or destination

c) It is best to have relatively large timeslots to minimize overhead relative to data carried

11. Almost all data systems today use statistical TDM at some point.

VI. Traffic Engineering

A. In telephony networks, not all phones are in use at the same time, so trunks between central offices are over-subscribed

1. This is a form of statistical TDM

B. Agner Krarup Erlang (1878-1929)

1. developed equations on how the blocking probability relates to the amount of traffic and number of lines.

C. Traffic Engineering Definitions

1. Trunk - a communication line between two switching systems

2. Poisson Distribution - A mathematical formula that defines the probability of x events occurring in a certain time

3. Busy Hour - The one hour during the day or year that has the most traffic

4. CCS - Centum Call Seconds - amount of traffic offered on a line.

a) 60 * 60 = 3600 seconds or 36 CCS

D. Calculating Traffic

1. Amount of traffic offered can be calculated from the average number of calls and average length.

2. For example: 2 calls / hour * 3 minutes / call = 2 * 180 = 360 call seconds = 3.6 CCS

3. If one phone offers 3.6CCS, then 100 phones offer 360 CCS

4. Often Erlangs are used in describe the amount of traffic offered.

a) 36 CCS = 1 Erlang

E. Different Traffic Engineering models

1. Poisson distribution - simplest

a) Assumes that blocked calls are held.

b) Infinite number of sources

2. Erlang B

a) Assumes that blocked calls never return

(1) Used originally for blocked calls that went to higher cost lines.

b) Infinite number of sources

3. Extended Erlang B

a) Adds a retry probability to Erlang B

4. Erlang C

a) Assumes that blocked calls are delayed

b) Infinite number of sources

c) Used for Call Center applications

(1) “Trunks” are service people

5. There are models for Finite number of sources, but they are used much less often.

a) Even if they should be used - people don’t

6. Equations given are nice, but either look up tables, or calculators are really used.

F. Poisson Distribution

1. Poisson assumes that blocked calls wait forever.

a) This will tend to over estimate the number of trunks needed

b) Equation for Poisson

(1) See Figure 1

(2) N = Number of events to occur in a unit time (Number of trunks)

(3) A = Average number of events occuring per unit time (Traffic in Erlangs)

G. Erlang B

1. Erlang B assumes that blocked calls never retry

a) This will tend to under estimate the number of trunks needed

b) Equation for Erlang B

(1) See Figure 2.

(2) N = Number of trunks

(3) A = Traffic offered in Erlangs

H. Traffic Engineering example problem

a) Given

(1) 100 homes, with average 1.5 phones / home

(2) First line of a home has 3.6CCS

(3) Second line of a home has 30CCS

(4) 50% of blocked calls retry immediately

b) Calculate number of trunks to serve these homes with a blocking probability of 0.02