The Telephone Network. An Engineering Approach to Computer Networking

(1)

The Telephone Network

An Engineering Approach to Computer Networking

(2)

Is it a computer network?

■

Specialized to carry voice

_{Specialized to carry voice}

■

Also carries

◆ ◆ _telemetry_telemetry ◆ ◆ _video_video ◆ ◆ _fax_fax ◆

◆ _{modem calls}_{modem calls}

■

Internally, uses digital

_{Internally, uses digital}

samples

■

Switches and switch controllers are special purpose computers

_{Switches and switch controllers are special purpose computers}

■

Principles in its design apply to more general computer

_{Principles in its design apply to more general computer}

networks

(3)

Concepts

■

Single basic service: two-way voice

_{Single basic service: two-way voice}

◆

◆ _{low end-to-end delay}_{low end-to-end delay} ◆

◆ _{guarantee that an accepted call will run to completion}_{guarantee that an accepted call will run to completion}

■

Endpoints connected by a

_{Endpoints connected by a}

circuit

_circuit

◆

◆ _{like an electrical circuit}_{like an electrical circuit} ◆

◆ _{signals flow both ways (}_{signals flow both ways (}_{full duplex}_{full duplex}₎₎ ◆

(4)

The big picture

■

Fully connected core

_{Fully connected core}

◆

◆ _{simple routing}_{simple routing} ◆

◆ _{telephone number is a hint about how to route a call}_{telephone number is a hint about how to route a call}

✦

✦ but not for 800/888/700/900 numbersbut not for 800/888/700/900 numbers

◆

(5)

The pieces

1. End systems

2. Transmission

3. Switching

4. Signaling

(6)

1. End-systems

■

Transducers

_Transducers

◆

◆ _{key to carrying voice on wires}_{key to carrying voice on wires}

■ ■

Dialer

_Dialer

■ ■

Ringer

_Ringer

■ ■

Switchhook

_Switchhook

(7)

Sidetone

■

Transmission circuit needs two wires

_{Transmission circuit needs two wires}

■

And so does reception circuit

■

=> 4 wires from every central office to home

_{=> 4 wires from every central office to home}

■

Can we do better?

_{Can we do better?}

■

Use

_Use

same

_same

pair of wires for both transmission and reception

_{pair of wires for both transmission and reception}

■

Cancel out what is being said

_{Cancel out what is being said}

■

Ergonomics: leave in a little

_{Ergonomics: leave in a little}

◆

◆ sidetone_sidetone ◆

(8)

Echo

■

Shared wires => received signal is also transmitted

_{Shared wires => received signal is also transmitted}

■

And not completely cancelled out!

■

Leads to echo (why?)

_{Leads to echo (why?)}

■

OK for short-distance calls

_{OK for short-distance calls}

■

For long distance calls, need to put in echo chancellors (why?)

_{For long distance calls, need to put in echo chancellors (why?)}

■

Expensive

_Expensive

■

Lesson

_Lesson

◆

(9)

Dialing

■

Pulse

_Pulse

◆

◆ _{sends a pulse per digit}_{sends a pulse per digit} ◆

◆ _{collected by central office}_{collected by central office}

■

Tone

_Tone

◆

◆ _{key press (feep) sends a pair of tones = digit}_{key press (feep) sends a pair of tones = digit} ◆

(10)

2. Transmission

■

Link characteristics

_{Link characteristics}

◆

◆ _{information carrying capacity (bandwidth)}_{information carrying capacity (bandwidth)}

✦

✦ information sent asinformation sent as symbolssymbols

✦

✦ 1 symbol >= 1 bit1 symbol >= 1 bit

◆

◆ _{propagation delay}_{propagation delay}

✦

✦ time for electromagnetic signal to reach other endtime for electromagnetic signal to reach other end

✦

✦ light travels at 0.7c in fiber ~8 microseconds/milelight travels at 0.7c in fiber ~8 microseconds/mile

✦

✦ NY to SF => 20 ms; NY to London => 27 msNY to SF => 20 ms; NY to London => 27 ms

◆

◆ _attenuation_attenuation

✦

✦ degradation in signal quality with distancedegradation in signal quality with distance

✦

✦ long lines need regeneratorslong lines need regenerators

✦

(11)

Transmission: Multiplexing

■

Trunks

_Trunks

between central offices carry hundreds of conversations

_{between central offices carry hundreds of conversations}

■

Can’t run thick bundles!

■

Instead, send many calls on the same wire

_{Instead, send many calls on the same wire}

◆

◆ multiplexing_multiplexing

■

Analog multiplexing

_{Analog multiplexing}

◆

◆ _{bandlimit call to 3.4 KHz and frequency shift onto higher bandwidth}_{bandlimit call to 3.4 KHz and frequency shift onto higher bandwidth}

trunk trunk

◆

◆ _obsolete_obsolete

■

Digital multiplexing

_{Digital multiplexing}

◆

◆ first convert voice to_{first convert voice to} samples_samples ◆

◆ 1 sample = 8 bits of voice_{1 sample = 8 bits of voice} ◆

(12)

Transmission: Digital multiplexing

■

How to choose a sample?

_{How to choose a sample?}

◆

◆ ₂₅₆₂₅₆ _{quantization levels}_{quantization levels}

✦

✦ logarithmically spaced (why?0logarithmically spaced (why?0

✦

✦ sample value = amplitude of nearest quantization levelsample value = amplitude of nearest quantization level

◆

◆ _{two choices of levels (mu law and A law)}_{two choices of levels (mu law and A law)}

■

Time division multiplexing

_{Time division multiplexing}

◆

◆ _{trunk carries bits at a faster bit rate than inputs}_{trunk carries bits at a faster bit rate than inputs} ◆

◆ _n_n _{input streams, each with a 1-byte buffer}_{input streams, each with a 1-byte buffer} ◆

◆ _{output interleaves samples}_{output interleaves samples} ◆

◆ _{need to serve all inputs in the time it takes one sample to arrive}_{need to serve all inputs in the time it takes one sample to arrive} ◆

◆ _{=> output runs}_{=> output runs} _n_n _{times faster than input}_{times faster than input} ◆

(13)

Transmission: Multiplexing

■

Multiplexed trunks can be multiplexed further

_{Multiplexed trunks can be multiplexed further}

■

Need a standard! (why?)

■

US/Japan standard is called

_{US/Japan standard is called}

Digital Signaling

hierarchy (DS)

_{hierarchy (DS)}

Digital Signal

Number

Number of

previous level

circuits

Number of voice

circuits

Bandwidth

DS0

1 64 Kbps

DS1

24

24 1.544Mbps

DS2

4

96 6.312 Mbps

DS3

7

672 44.736 Mbps

(14)

Transmission: Link technologies

■

Many in use today

_{Many in use today}

◆

◆ _{twisted pair}_{twisted pair} ◆

◆ _{coax cable}_{coax cable} ◆

◆ _{terrestrial microwave}_{terrestrial microwave} ◆

◆ _{satellite microwave}_{satellite microwave} ◆

◆ _{optical fiber}_{optical fiber}

■

Increasing amount of bandwidth and cost per foot

_{Increasing amount of bandwidth and cost per foot}

■ ■

_Popular

◆ ◆ _fiber_fiber ◆ ◆ satellite_satellite

(15)

The cost of a link

■

Should you use the cheapest possible link?

_{Should you use the cheapest possible link?}

■

No!

■

Cost is in installation, not in link itself

_{Cost is in installation, not in link itself}

■

Builders routinely install twisted pair (CAT 5), fiber, and coax to

_{Builders routinely install twisted pair (CAT 5), fiber, and coax to}

every room

■

Even if only one of them used, still saves money

_{Even if only one of them used, still saves money}

■

Long distance

_{Long distance}

◆

(16)

Transmission: fiber optic links

■

Wonderful stuff!

_{Wonderful stuff!}

◆

◆ _{lots of capacity}_{lots of capacity} ◆

◆ _{nearly error free}_{nearly error free} ◆

◆ _{very little attenuation}_{very little attenuation} ◆

◆ _{hard to tap}_{hard to tap}

■

(17)

More on fibers

■

Three types

_{Three types}

◆

◆ _{step index (multimode)}_{step index (multimode)} ◆

◆ _{graded index (multimode)}_{graded index (multimode)} ◆

◆ _{single mode}_{single mode}

■

Multimode

◆

◆ _cheap_cheap ◆

◆ _{use LEDs}_{use LEDs} ◆

◆ _{short distances (up to a few kilometers)}_{short distances (up to a few kilometers)}

■

Single mode

_{Single mode}

◆

◆ expensive_expensive ◆

◆ use lasers_{use lasers} ◆

(18)

Transmission: satellites

■

Long distances at high bandwidth

_{Long distances at high bandwidth}

■

Geosynchronous

◆

◆ _{36,000 km in the sky}_{36,000 km in the sky} ◆

◆ _{up-down propagation delay of 250 ms}_{up-down propagation delay of 250 ms} ◆

◆ _{bad for interactive communication}_{bad for interactive communication} ◆

◆ _{slots in space limited}_{slots in space limited}

■

Nongeosynchronous

_{Nongeosynchronous}

(Low Earth Orbit)

_{(Low Earth Orbit)}

◆

◆ appear to move in the sky_{appear to move in the sky} ◆

◆ _{need more of them}_{need more of them} ◆

◆ handoff is complicated_{handoff is complicated} ◆

(19)

3. Switching

■

Problem:

_Problem:

◆

◆ _{each user can potentially call any other user}_{each user can potentially call any other user} ◆

◆ _{can’t have direct lines!}_{can’t have direct lines!}

■

Switches establish temporary

_{Switches establish temporary}

circuits

_circuits

■

Switching systems come in two parts: switch and switch

_{Switching systems come in two parts: switch and switch}

controller

(20)

Switching: what does a switch do?

■

Transfers data from an input to an output

_{Transfers data from an input to an output}

◆

◆ _{many ports (up to200,000 simultaneous calls)`}_{many ports (up to200,000 simultaneous calls)`} ◆

◆ _{need high speeds}_{need high speeds}

■

Some ways to switch:

_{Some ways to switch:}

◆

◆ _{space division}_{space division} ◆

(21)

Switching

■

Another way to switch

_{Another way to switch}

◆

◆ _{time division (time slot interchange}_{time division (time slot interchange} _or_or _TSI)_TSI) ◆

◆ _{also needs a schedule (why?)}_{also needs a schedule (why?)}

■

To build larger switches we combine space and time division

switching elements

(22)

4. Signaling

■

Recall that a switching system has a switch and a switch

_{Recall that a switching system has a switch and a switch}

controller

■

Switch controller is in the

control plane

control

◆ _{does not touch voice samples}

■

Manages the network

◆

◆ call routing (collect_{call routing (collect} dialstring_dialstring and forward call)_{and forward call)} ◆

◆ _{alarms (ring bell at receiver)}_{alarms (ring bell at receiver)} ◆

◆ billing_billing ◆

(23)

Signaling network

■

Switch controllers are special purpose computers

_{Switch controllers are special purpose computers}

■

Linked by their own internal computer network

◆

◆ _{Common Channel Interoffice Signaling (CCIS) network}_{Common Channel Interoffice Signaling (CCIS) network}

■

Earlier design used

_{Earlier design used}

in-band

tones, but was severely hacked

_{tones, but was severely hacked}

■

Also was very rigid (why?)

_{Also was very rigid (why?)}

■

(24)

Signaling

■

One of the main jobs of switch controller: keep track of

_{One of the main jobs of switch controller: keep track of}

state

_state

of

_of

every endpoint

■

(25)

Cellular communication

■

Mobile phone talks to a

_{Mobile phone talks to a}

base station

_{base station}

on a particular radio

_{on a particular radio}

frequency

■

Aren’t enough frequencies to give each mobile a permanent

frequency (like a wire)

■

Reuse

◆

◆ temporal_temporal

✦

✦ if mobile is off, no frequency assigned to itif mobile is off, no frequency assigned to it

◆

◆ _spatial_spatial

✦

(26)

Problems with cellular communication

■

How to complete a call to a mobile?

_{How to complete a call to a mobile?}

◆

◆ _{need to}_{need to} _{track a mobile}_track

◆ _{on power on, mobile tells base of its ID and home} ◆ _{calls to home are forwarded to mobile over CCIS}

■

How to deal with a moving cell phone?

◆ _{nearest base station changes}

◆ _{need to} _{hand off existing call to new base station} ◆ _{a choice of several complicated protocols}

(27)

Challenges for the telephone network

■

Multimedia

_Multimedia

◆

◆ _{simultaneously transmit voice/data/video over the network}_{simultaneously transmit voice/data/video over the network} ◆

◆ _{people seem to want it}_{people seem to want it} ◆

◆ _{existing network can’t handle it}_{existing network can’t handle it}

✦

✦ bandwidth requirementsbandwidth requirements

✦

✦ burstinessburstiness in traffic (TSI can’t skip input)in traffic (TSI can’t skip input)

✦

✦ change in statistical behaviorchange in statistical behavior

■

Backward compatibility of new services

◆

◆ _{huge existing infrastructure}_{huge existing infrastructure} ◆

◆ _{idiosyncrasies}_{idiosyncrasies}

■

Regulation

_Regulation

◆

(28)

Challenges

■

Competition

_Competition

◆

◆ _{future telephone networks will no longer be monopolies}_{future telephone networks will no longer be monopolies} ◆

◆ _{how to manage the transition?}_{how to manage the transition?} ■

■ _{Inefficiencies in the system}_{Inefficiencies in the system}

◆

◆ _{an accumulation of cruft}_{an accumulation of cruft} ◆

◆ _{special-purpose systems of the past}_{special-purpose systems of the past} ◆

◆ _{‘legacy’ systems}_{‘legacy’ systems} ◆