Line Side Signaling & Digitizationeeichen/spring_2015/class_notes/a_jan_13... · Channelized, bi-directional 64kb/s “In-band” circuit switched network for media ... • Nyquist

Copyright 2003-2011 © by Elliot Eichen. All rights reserved.

Classical Telephony

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"When I was a boy of 14, my father was so ignorant I could hardly stand to have the old man around. But when I got to be 21, I was astonished at how much the old man had learned in seven years." – Mark Twain

"To remain ignorant of things that happened before you were born is to remain a child." – Cicero

"History never looks like history when you are living through it.“ – John Gardner


Prehistory<< A. Graham Bell

- visual signals, drums, horses, pigeons- telegraph

Ancient HistoryBell >> >> WWII

- analog telephony (loop start)- carrier frequencies, modulation around- copper wires& pumps- mechanical co switches

MedievalWWII >> >> 1980

-TDM- microwave & ICs- electronic switches- software

Old Stuff1980 >> >> 2000

- optics (fiber, optoelectronics, amps)- video & broadband (DSL, cable)- wireless (1G, 2G), PCs

New Stuff2000 …

- VoIP (H323, SIP, RTP, ..) - FiOS- video (& telepresence), - security- wireless (3G, 4G – LTE / WiMax, VoLTE) - Smartphones & Tablets !!!!!

Eichen’s fractured “History of Communications.”

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{Aside: Golden Boy - How Competitive Life Is}


By KATHLEEN TELTSCH (The New York Times); Metropolitan Desk August 31, 1981, Monday Late City Final Edition, Section B, Page 3, Column 3, 435 words [ DISPLAYING ABSTRACT ]A painstaking restoration is under way in a West Side warehouse of a 20,000-pound bronze statue that once was a familiar landmark in lower Manhattan. For 64 years, the statue, the ''Spirit of Communications'' stood atop the 26-story headquarters of A.T.&T. at 195 Broadway. It was a heroic-sized sculpture said to be one of the city's largest single architectural figures, possibly second in size only to the Statue of Liberty.


Line Side Signaling & Digitization

Loop start or ground start analog signaling. {very little BRI-ISN digital signaling (Q.931)}. Bi-directional analog transmission on twisted pair => use of “hybrid” at CO for separating out inbound and outbound traffic.

End OfficePSTN Switch

Digitization of analogwaveform to 64 kbpsPCM at switch

Line Side (of the switch) Trunk Side


PRI-ISDN: Digital Line Side

For PRI, Q.931 signaling on the D channel. For in-band PRI, robbed bit signaling. Digitization to 64 kbits/s at the PBX, in the handset, or both (transcoding by PBX). Sometimes end-end digital => no hybrid (not a source for echo).

End OfficePSTN Switch

Line Side Trunk Side

4 wire digital T1 (one circuit for traffic to the switch, one circuit for traffic away from the switch). Eithertraditional repeated T1, or HDSL. Typically carried insame outside twisted pair loop plant as analog traffic.

PBX

Copyright 2003-201` © by Elliot Eichen. All rights reserved.

DSL


Brief Aside: Broadband Access Technology (the last mile, = the barrier to entry. Are there any $$s here?)

• ISDN (It Still Does Nothing)– Just history (It Still Does Nothing refers to basic rate ISDN)

• DOCSIS et.al. (Data over Cable Services Spec) – Often HFC: fiber to the head end with coax distribution– Frequency modulation of sub-carriers, NGen digitial rather than analog

• DSL (Digital Subscriber Loop)– Asymmetric (ADSL) or Symmetric (SDLS). (note also HDSL for T1).

• WiFi (LAN, not really an access technology. Also see Zigbee, and ZMax)• Optical Transport

– Passively split fiber to the home (VZ FiOS), baseband TDM.– Passively split fiber to the curb with VDSL distribution (AT&T). This really looks

more like the Cable Company architecture). • Wireless: 3G, 4G (LTE and WiMax) (ah, WiMAX is now officially dead! –

2011). 4G will be metered, what about fixed broadband? Will 4G disintermeidate wireless voice (do the same for wireless as wireline?)

• Other– Power Lines (probably no traction here for WAN, but there is PLN for LAN). – Satellite: geostationary and now low earth orbit (c.f. O3b Networks Ltd for

distribution, and HugesNet for access.)– LMDS (point to multi-point microwave) –pretty unusual (Kathmandu Island Pk)


Digitization

G711 codec (CCIT standard)8k samples/sec * 8 bits/sample = 64 kbits/s.Companding (think of this a Dolby for telephony) is μlaw in North America, ALaw in Europe, Africa, and Asia. Not sure about South America.{Shannon: 8 khz sampling frequency => 4 kHz max frequency Component. In reality, voice passband is ~ 50Hz to 3 KHz}


PSTN – Network Architecture Within the LATA:

Local Exchange Carrier (LEC) orCompetitive Local Exchange

Carrier (CLEC)

Between LATAs:Inter Exchange Carrier

Within the LATA:Local Exchange Carrier (LEC) or

Competitive Local ExchangeCarrier (CLEC)


SS7 – Out of Band Signaling for PSTNOut of band, packet network for signaling and applications:- highly redudent, X.25 like, network- signaling:

- call setup (calling =>, progress<= alerting {ringing<=} )- call tare down, etc.

- applications:- Call Name, 800 number resolution, LNP, AIN apps, etc.

Channelized, bi-directional 64kb/s “In-band” circuit switched network for media (voice path).


SS7 Network Architecture

End Office Switches

IMT – Inter machine trunks

(voice circuits)

SS7 signaling

a: links

g: links


SS7 Network Architecture

dB apps

(like 800 #s, also AIN)

Signaling messages

X.25 like

Reliable transport at session level (similar function to TCP)


SS7 Example


PSTN Switched Services – 100 years of stuff

• Voice• Fax• Modem• Digital (64 kb/s + bonding): small• Emergency (911, E911)• Toll Free (800), reverse toll (900)• Local Number Portability• Caller ID, Caller Name, Blocking, Original Called

Number, …• Wiretapping (Title 3, CALEA)• Operator Intervention• Fraud Management• Perceived Security (Privacy)


Routing & Telephone Numbers• Routing rules are hard coded – circuit switches don’t

advertise telephone numbers, don’t discover routes, etc.=> lots of administration (npa splits, carrier routing,..)

• E.164 telephone numbers are administered in the US byNANPA – sort of the IANA for the PSTN. DOC awardscontract for administration, currently Nuestar – alsoadminister LNP, and .us/.biz top level domains.

• {TRIP – Telephony Routing over IP – advertisingtelephone number space. Also ENUM – DNS resolution of telephone numbers}

• Why haven’t URI’s replace TNs ??

Everything You Always Wanted to Know About Communications Theory (1)

• Nyquist (aka Whittaker–Nyquist–Kotelnikov–Shannon) theorem:– If an analog signal s(t) is bandlimited (has no frequency content higher than

Bmax), then one can perfectly reproduce the signal by sampling the function at fn = 2 Bmax (or fn >= 2 Bmax ).

– Obviously important for digitizing real information streams – signal in time (t) like speech or neurons firing, etc., or signal in space (x,y) like an image.

– For many practical reasons, you can’t perfectly reproduce anything, but you can get arbitrarily close depending upon how hard you work (over sample, longer windows, better precision, etc.), and how much noise is introduced by the digitization process.

– You can also turn the argument around – if you sample the function at fs, then the highest frequency component that can be reproduced is fs/2.

– An interesting question is how do compression algorithms – audio codecs, image compression, etc. - “get around” Nyquist.


• Wiener–Khinchin (aka Wiener–Khinchin–Einstein or Khinchin–Kolmogorov) theorem:

– The Power Spectrum of a signal is the Fourier Transform of the AutoCorrelation of the signal, which is the convolution of the signal with the complex conjugate of itself, or

– Given that the Fourier Transform of a convolution is the product of the individual Fourier transforms, this can be written (in it’s most useful form) as

– Often folks look at the power spectrum of something on a log scale (for example, on a spectrum analyzer in the lab). From a practical perspective, the power spectrum that you see on the instrument is thus:

– Note that all the usual stuff around Linear Shift Invariant Systems, integerablefunctions, etc. apply to this.



Aside: Power Spectrum of an NRZ signal

• Copyright 2003-2011 © by Elliot Eichen. All rights reserved.

• Baseband, First Zero in the Power Spectrum is 1/T (bit length)

• Need to discuss coding (balancing 0 and 1), etc.


• Queuing Theory: – Erlang (Danish engineer working for the Copenhagen Telephone Company

early 1900s). Offered load on a system due to the random (Poisson distributed) arrival of requests and the length of time each request requires to service.

• Erlang B Model:– Calculates the probability that a request for service will be blocked given a

random (Poisson distributed) arrival of requests at rate r, an average time h required to service the request, and N resources to service the request. Assumes an infinite pool of requestors, but no queuing of a requests that cannot be serviced

– Erlang B is good for calls that go to DSPs, trunks, etc. There is no queuing, either the call gets accepted, or it gets dropped. If dropped, it does not add to the overall arrival rate of requests.

– Erlang C introduces queuing of requests that cannot be serviced (e.g., call center).

– Most folks either use a calculator, or calculate it recursively (converges quickly). Or, for back of the envelope, use the rule of thumb of 52 mou/hour availability.


Everything You Always Wanted to Know About Communications Theory (3a)

• Erlang B Continued:

• Rule of thumb: in the limit of large N, each resource contributes 52 MOU/hour.

• Also, you might be interested in queing theory as it relates to Packet (Statistical) Networks, and to IP in particular. Some of the important contributions are from : Kleinrock (USLA, early Arpanet operations, also early packet queueing more in line with X.25, before IP), Licklider (BBN – he lived in Arlington!, I’m told that he basically put forth almost the complete architecture of an IP based, best effort packet network), Vint Cerf and Bob Kahn (TCP). Many other contributions, look at the Wikidpedia article on the Internet. Also look thepredecessors – X.25, frame relay, SS7.



• Really hard stuff: Voice Coding (Codecs) –– Somewhat simple ideas (like the compandiing in G711 a or mu),– To much more complex methods:

• ADPCM (e.g., G726.)• Hybrid /A BS: GSM. {little known – GSM lowers bit rate as recevied

signal decreases, to maintain an acceptable S/N. } • CLEP (code excited linear predictive – I don’t know what this means):

G729, G723, iLBC– Look at Herssant’s book if you are interested.

• More really hard stuff: Echo Cancelation – OK, this is not quite as hard as CLEP codecs, it’s more or less figuring out

where the echo is (in time), and feeding the negative of received signal back into the signal with the right delay/amplitude/phase.

– It’s still hard.• Signal processing is fun these days (it’s just code). The math is fun (but

hard), and the code is fun (but frustrating). Take a class on signal processing.



Speech Coders: Lots of Code, DSP intensive


Summary: Classical Telephony • Line Side (access) vs. Trunk Side (multiplexed)• In-band signaling (loop start/robbed bit T1) vs. out of band

(SS7 .. and => VoIP)• Digitization (=> digitization + packetization for VoIP).• Broadband/access – barrier to entry, the only game left in

town for wired networks (and wireless soon ..)• Stacks/layers/protocols: {POTS}, {Q.931/ISUP,

SS7/CSS7}, {T1 .. SONET}, {ATM, Frame,} {IP, SIP, RTP}

• Services, Services, Services, Services ……• Classical Communications Theory: Nyquist, Wiener-

Khintchin, Erlang.



RTP

• RTP – carries real time data.• RTPC – control port that carries

information to monitor quality of service.+1 port


Bandwidth requirement: G729 Example

• Two 10 ms frames/packet (RTP default). For G729, voice is 8 kbps.• Packet payload size = 2 X 8 kbps X 20ms = 160 bits = 20 octets/packet.

IP Packet = 60 bytes.

• For Cisco HDLC (cisco-cisco router serial), add 5 bytes header, 3 bytes trailer. Bandwidth = (63 bytes/20ms)*(8 bits/byte) = 25.2 kbps

• For ethernet, 14 byte header, 4 byte trailer =>Bandwidth = (78 bytes/20ms)*(8bits/byte) = 31.2 kbps

IP 20 bytes UDP 8 bytes

RTP12 bytes

G729 Payload20 bytes


RTP12 bytes


HDLC5 bytes

HDLC3 bytes


CODEC table


Summary (PAMS listening score only)


Codecs – “Your mileage may vary”• Large perceived quality difference between PSTN quality

(MOS=4.1) and cell quality (MOS=3.5)• Codecs respond differently to packet loss, delay, noise,

multiple transcodings, etc. It’s not just the MOS/PSQMmeasurement that counts.

• There are also licensing issues ($$), what the call is being used for (messaging, conferencing, etc.), and DSP HP required.

• The number of frames/packet can be dynamically adjusted to tradeoff quality for efficiency.

• Header compression can drastically improve efficiency –at the expense of router processing power.

• VoIP Community has – for the most part – convergedaround G711, G729. Some application for G726 (packsbetter into ATM). EE thinks that GSM interworking is very important!


Bandwidth reduction• IP – almost by definition - is less bw efficient than PSTN.• Bandwidth reduction where the cost for bandwidth is at a

premium – typically tail circuits, or some internationaldestinations.

• VAD (Voice Activity Detection). Rule of thumb is 1/3-1/2reduction in bw. Cost is voice quality (clipping), poorperformance in conferencing.

• Header Compression– Compressed RTP: Hop-to-hop, requires

decompression and compression at each hop. ForG279, = 12.8 kb/s

– VoMPLS: New, not sure of what it takes beyondMPLS. End-to-end (not hop-to-hop), likely to be muchless router CPU intensive.


Header Compression Example

• Conventional frame G729 = 25.2 kbps

• Compressed frame G729 = 12.8 kbps

CRTP4 bytes



RTP12 bytes


HDLC5 bytes

HDLC3 bytes

HDLC5 bytes

HDLC3 bytes


Bandwidth reduction• Voice carried as 64 kbps isochronous (heartbeat!) traffic

using G711.• Signaling (mostly, US, EU,etc.) is out-of-band;

trunk side = ISUP SS7, line side = Q.931 PRI.• Network services include modem, fax, some digital (64

kb/s clear channel) as well as voice. CName (caller id),local number portability, 800 # translations, etc.

• Routing rules are hard coded, based on e.164 telephonenumbers – no RIP in telephony (see PINT). Lots ofadministration (npa splits, carrier routing, etc.)

• Everything is regulated (PUC, FCC, DoC), tarriffed,policed, and taxed!!

• ε business, efficient marketplace, size counts.

Documents

Line Side Signaling & Digitizationeeichen/spring_2015/class_notes/a_jan_13... · Channelized, bi-directional 64kb/s “In-band” circuit switched network for media ... • Nyquist