1 210 1081_06F9_c1 © 1999, Cisco Systems, Inc. 1 210 1081_06F9_c1 © 1999, Cisco Systems, Inc.

12101081_06F9_c1 © 1999, Cisco Systems, Inc. 12101081_06F9_c1 © 1999, Cisco Systems, Inc.

2© 1999, Cisco Systems, Inc.

2101081_06F9_c1

Advanced Voiceband Advanced Voiceband Modem Configuration and Modem Configuration and

TroubleshootingTroubleshootingSession 210Session 210

32101081_06F9_c1 © 1999, Cisco Systems, Inc.

AgendaAgenda

• Understanding the Anatomy of a V.90 Modem Understanding the Anatomy of a V.90 Modem ConnectionConnection

• What V.90 Wants and Does Not Want

• Hooking up Things Right and Proving It

• Living with Imperfection

• Hand-to-Hand Combat with Individual Connections


2101081_06F9_c1 42101081_06F9_c1 © 1999, Cisco Systems, Inc.

Anatomy of a V.90 Anatomy of a V.90 ConnectionConnection

What a Modem IsWhat a Modem Is


DTE

The CircuitThe Circuit

Modems Are an “Old World” Artifact—They Deliver Data over Circuits

DTE

‘The Network’

DCE DCE

Communications Link—Communications Link—“Old World” Model“Old World” Model


LayerLayer ProtocolsProtocolsApplicationApplication8 Bit Data8 Bit DataStreamStream

PPPPPP SLIPSLIP TTY (Async Terminal)TTY (Async Terminal)

Async Async FramingFraming

1 Startbit, 7–8 Databits, 0–1 Parity Bits (Even, Odd, Mark, Space), 1–2 1 Startbit, 7–8 Databits, 0–1 Parity Bits (Even, Odd, Mark, Space), 1–2 StopbitsStopbits

ModemModem(DCE)(DCE)

CompressionCompression MNP5, V.42bisMNP5, V.42bis (Compression Requires EC)(Compression Requires EC)

Error ControlError Control V.42: MNP4, LAP-MV.42: MNP4, LAP-M (EC Uses Sync Framing with 8-Bit (EC Uses Sync Framing with 8-Bit Words)Words)

ModulationModulationAnalog CarriersAnalog CarriersBell103, Bell212A, V.21, V.22, V.22bis,Bell103, Bell212A, V.21, V.22, V.22bis,V.23, V.32, V.32bis, V.FC, V.34V.23, V.32, V.32bis, V.FC, V.34

Normally 10 Bits Per Word: 1 Startbit, 8 Databits, No Parity, 1 StopbitNormally 10 Bits Per Word: 1 Startbit, 8 Databits, No Parity, 1 Stopbit

Half-Digital: K56Flex, X2, V.90Half-Digital: K56Flex, X2, V.90Client->Server: AnalogClient->Server: AnalogV.34-Like ModulationV.34-Like Modulation

Server->Client: Server->Client: Digital PCM WordsDigital PCM Words

PSTN PSTN CircuitCircuit

POTS Circuit—3–4 KHz POTS Circuit—3–4 KHz Analog (~200–~3600Hz)Analog (~200–~3600Hz)

PCM Layer—PCM Layer—8-bit Words 8000/sec8-bit Words 8000/sec

A-lawA-law(NA)(NA)

• -law-law(World)(World)

DS0 Layer—DS0 Layer—Sync SerialSync SerialDigital ChannelDigital Channel

CAS (RBS)CAS (RBS)ChannelChannel56—63Kbps56—63Kbps

64Kbps64KbpsClearClearChannelChannel

PSTN LinkPSTN Link(Lines and (Lines and Trunks)Trunks)

Loopstart Analog Local LinesLoopstart Analog Local LinesE&M Analog TrunksE&M Analog Trunks

CAS CAS ChannelizedChannelizedT1sT1s

PRI, BRIPRI, BRIR2 E1R2 E1

Clear-Channel Trunks Clear-Channel Trunks in SS7 Networksin SS7 Networks

PhysicalPhysical 2-Wire Copper, 4-Wire Copper2-Wire Copper, 4-Wire Copper SONET, HDSL, 2B1Q, etc.SONET, HDSL, 2B1Q, etc.

DCEDCEAsyncAsyncFramingFraming

Modem Protocol Chart—Modem Protocol Chart—Transmission PlaneTransmission Plane


Modem OperationsModem Operations

BufferBuffer

CompressorCompressor

Modulator/Modulator/Demodulator Demodulator

PacketizerPacketizer

BufferBuffer

CompressorCompressor

Modulator/Modulator/Demodulator Demodulator

PacketizerPacketizer

RTSCTS

TxD RxD

Flow Control

CompressDecompress

ChecksumRetransmit CP

DSPModulation


DSPDSP

UUAARRTT

Binary Data from User’s Application

Digitally Encoded Analog

Samples

Analog Signal

Anatomy of an Analog ModemAnatomy of an Analog Modem

SerialSerialPortPort

RJ11RJ11JackJack

A/DA/D

D/AD/A

++

RXRX

TXTX


Modem StandardsModem Standards

Modem StandardModem Standard Key FeaturesKey Features

Bell 103/V.21/V.23 FSK Modulation

Bell 212A DPSK Modulation

V.22 Data Scrambler, International Standard

V.22bis QAM Modulation, Channel Equalization

V.32 Trellis Coded Modulation, Echo Canceller

V.32bis Multiple Bit Rates, Deeper Trellis

V.32terbo Higher Bit Rate

V.34 Pre Emphasis, Multiple Baud Rates, Multiple Carrier Frequencies, Shell Mapping, Precoding, Constellation Shaping, Power Control, Line Probing, Non-Linear Encoding, 4D-trellis

V.34+ Higher Bit Rate

V.90/KFlex/X2 PCM Modulation, Spectral Shaping, DigitalImpairment Probing, Dynamic Constellations


Modem Standards (Cont.)Modem Standards (Cont.)

MODEMMODEM DUPLEXDUPLEX RATE (kbps)RATE (kbps) MODULATION MODULATION V.21/B103F/D FDM 0.3 FSK

V.23/B202F/D FDM 0.6, 1./0.075 FSK

V.22/B212F/D FDM 1.2 DPSK

V.26bis F/D FDM 2.4 DPSK

V.22bis F/D FDM 2.4, 1.2 QAM

V.27ter H/D 4.8, 2.4 QAM

V.29 H/D 9.6, 7.2, 4.8 QAM

V.17 H/D 14.4, 12.0, 9.6, 7.2 QAM

V.32 F/D EC 9.6, 4.8 TCM/QAM

V.32bis F/D EC 14.4, 12.0, 9.6, 7.2, 4.8 TCM/QAM

V.32terbo F/D EC 4.8 to 19.2 TCM/QAM

V.34+ F/D EC 2.4 to 33.6 FSK/DPSK/TCM/QAM

V.90/KflexF/D EC 28 to 56 PCM-PAM


• Bit stream is divided into N-bits per symbol sequence

Each bit sequence specifies an amplitude and phase change of Each bit sequence specifies an amplitude and phase change of a sinusoidal carriera sinusoidal carrier

. . . : 1 1 0 0 : 1 0 1 0 : 1 1 1 1

(4 Bits Per Symbol)QAM SignalQAM Signal

2 Phase Bits= 4 Phases

2 Amplitude Bits= 4 Amplitudes

ModulateModulateCosine CarrierCosine Carrier

A[j]*cos(fc*t + ph[k])A[j]*cos(fc*t + ph[k])

j = 0, 1 (4 Discrete Amplitudes)k = 2, 3 (4 Quadrature Phase Changes)

0

90

180

270

QAM (V.34) ModulationQAM (V.34) Modulation


V.22 bis2400 bps QAM

V.274800 bps DPSK

V.32 bis9600 bps QAM with Trellis

Some Standards IllustratedSome Standards Illustrated




The Voiceband (Analog) CircuitThe Voiceband (Analog) Circuit


-60

-50

-40

-30

-20

-10

0 1 2 3 4 5

Frequency (KHz)

Lev

el (

dB

m)

The Old World CircuitThe Old World Circuit

• A passbandbetween 2.4 and4 Khz wide

• Some amount ofnear-end andfar-end echo

• SNR between 25and 40 dB

• Some amount ofattenuation,hopefully < 20dB,increasing at higherfrequencies (slope)

What an Analog Modem Has to Work with:


Symbol Rate

2400

2743

2800

3000

3200

3429

Carrier Frequency Bandwidth Requirements Maximum Bit Rate

1600 Hz1800 Hz1646 Hz1829 Hz

1680 Hz1867 Hz1800 Hz2000 Hz1829 Hz1920 Hz1959 Hz

400-2800 Hz600-3000 Hz274-3018 Hz457-3200 Hz

280-3080 Hz467-3267 Hz300-3300 Hz500-3500 Hz229-3429 Hz320-3520 Hz244-3674 Hz

21600 bps 21600 bps 26400 bps 26400 bps

26400 bps 26400 bps 28800 bps 28800 bps 31200 bps 28800 bps 33600 bps

Note that a Classical 300-3400Hz Voiceband Yields at Best 28800 bps

V.34 Speeds as a Function of V.34 Speeds as a Function of BandwidthBandwidth


Example Chart of SNR Example Chart of SNR PerformancePerformance


POTS (Analog) circuit

Analog DigitalDigital

An Analog Modem CallAn Analog Modem Call

• Traditionally, a 3KHz analog channel (300 to 3300Hz)

• Signal/noise ratio of 25dB (?)

• Suitable for V.34 analog modem—maybe 19200 bps?

• If you’re lucky, you get 3.5KHz and 38dB SNR, andreach 33600 bps

This is very near the theoretical information capacity (Shannon) limit for such a channel




The Digital PSTN: PCM and V.90The Digital PSTN: PCM and V.90


= Sample = Sample

8 kHz (8,000 Samples/Sec)8 kHz (8,000 Samples/Sec)

Voice Bandwidth = Voice Bandwidth = 300 Hz to 3400 Hz300 Hz to 3400 Hz

Sampling StageAnalog Audio Source

t

v

Pulse Code Modulation—Pulse Code Modulation—Nyquist TheoremNyquist Theorem


Quantizing Stage

Quantization Noise

10010011 11011001

A—Law (Europe)A—Law (Europe)

µ—Law (USA–Japan)µ—Law (USA–Japan)

Note: PCM Note: PCM Quantization Quantization

Imposes a Imposes a ~38dB SNR ~38dB SNR Noise FloorNoise Floor

Pulse Code Modulation—Pulse Code Modulation—Analog to Digital ConversionAnalog to Digital Conversion


DS0Trunks

DS0Trunks

Analog2 Wire

Analog2 Wire

Telco Network

Switch Switch

POTS Call Thru the Modern PSTNPOTS Call Thru the Modern PSTN

Analog Digital Analog

SignalingNetwork

Switch


PSTN

DTE DTE

TATA

3.1KHz Analog

Digital Digital—64Kb DS0 Digital

Analog Call Through the Analog Call Through the Digital NetworkDigital Network

• Analog modems modulate a digital signal (up to 33.6Kb) in a 3.1KHz analog channel which is transmitted through the network in a 64Kb DS0, silly, isn’t it?

• How to take advantage of the digital nature of the PSTN to achieve faster speeds between the DTEs?


Digital—64Kb DS0

PSTN

DTE DTE

TATA

Using the Digital Network: ISDNUsing the Digital Network: ISDN

• How to take advantage of the digital nature of the PSTN to get faster speeds between the DTEs?

• The best way: ISDN—run pure digital end-to-end, and use The best way: ISDN—run pure digital end-to-end, and use the full 64Kb DS0the full 64Kb DS0 (but costs more)


Analog V.34 Signal (up to 33.6Kb)Analog V.34 Signal (up to 33.6Kb)

PSTN

DTENAS

DigitalV.90 Modem

AnalogV.90 Modem

Digital Modems in the PSTNDigital Modems in the PSTN

• The cheap, complicated way: PCM modemsThe cheap, complicated way: PCM modemsAnalog (client) PCM modem transmits an analog signalAnalog (client) PCM modem transmits an analog signal

Analog modem receives a digital (PCM) signalAnalog modem receives a digital (PCM) signal

AnalogLocalLoop Digital—64Kb DS0

Digital PCM Signal (up to 56Kb)Digital PCM Signal (up to 56Kb)

How to Take Advantage of the Digital Nature of the PSTN to Get Faster Speeds between the DTEs?


PCM Modems Use a Technique Called Pulse AmplitudePCM Modems Use a Technique Called Pulse AmplitudeModulation (PAM); Information Is Carried in theModulation (PAM); Information Is Carried in the

Amplitude of Each PCM Digital SampleAmplitude of Each PCM Digital Sample

PCM—Pulse Amplitude PCM—Pulse Amplitude ModulationModulation

• PCM modems do not modulate a carrier

• Information is carried as amplitude pulses encoded in the mu/A-law digitally encoded samples (PCM data)

• This is possible only because the transmit modem is directly connected to the digital network


DSPDSP

~~UUAARRTT

Binary Data From

Internet

Digitally Encoded Analog Samples

E1/T1 Digital Signal

Anatomy of an Analog ModemAnatomy of an Analog Modem

T1/E1T1/E1FramerFramer

RXRX

TXTX

RouterRouterInterfaceInterface

TelcoTelcoInterfaceInterface

(and Modem Management Commands)

TDMTDMMuxMux


PCM ModulationsPCM Modulations

• X2—pre-standard PCM modulation from 3COM (USR)

• K56Plus—pre-standard modulation from Rockwell (now Conexant)

• K56Flex—Rockwell and Lucent’s update to K56Plus

• V.90V.90—ITU-T PCM standard


X2X2

• 3COM (USR)’s pre-standard PCM modulation

• Never supported by Cisco

• Supports 32–64k in 1333 bps steps

• Uses V.8 to communicate capabilities


K56PlusK56Plus

• Rockwell’s original pre-standard

• Uses V.8 for capabilities

• 32000 to 60000 in 2000 bps steps

• Supported by Microcom 56k modems, notnot by MICA


• Rockwell and Lucent defacto “standard” (V1.0 is Rockwell only)

• Uses V.8bis for capabilities

• 32000–6000 in 2000 bps steps

• Microcom 56K supports V1.0 and above; MICA supports V1.1

• Detects 0, 3 and 6dB Lucent digital pads; 0 and 6 dB Nortel pads

K56FlexK56Flex


The V.90 StandardThe V.90 Standard

• Ratified by the ITU-T in 9/98

• V.8 for capabilities; V.8bis is optional

• 28 to 56k in 1333 bps steps

• 3200/3000 Hz uplink support required (31200 bps); 3429 Hz optional

• Fractional digital pads detected via DIL (Digital Impairment Learning)



What V.90 WantsWhat V.90 Wants


PSTNDTE

NAS

Client Client DTEDTE

ClientClientDCEDCE

HouseHouseWiringWiring

LocalLocalLoopLoop DS0DS0

CircuitCircuitthroughthrough

PSTNPSTN

DigitalDigitalLineLine NN

AASS

DigitalDigitalModemModem

What V.90 Wants…Piece by PieceWhat V.90 Wants…Piece by Piece


What V.90 Wants: Digital ModemsWhat V.90 Wants: Digital Modems

• MICA modems running good portware (2.6.2.0)

Turn on digital pad compensation (S52=1) for higher speeds

• Microcom modems running good firmware (5.1.20)

&F (digital pad compensation is onby default)


What V.90 Wants: NASWhat V.90 Wants: NAS

• NAS model and Cisco IOS® version basically irrelevant to modem connections (NAS just passes DS0 through)

• Minimum Cisco IOS versions for V.90:

MICA: 12.0(1+)* 11.3(5+){T,AA,NA} 11.2(16+)P (5300/3600)

Microcom: 11.3(5+){T,AA,NA} 12.0(1+)* 11.2(14+)P (5200)


• Should have very low BER to switch

• PRI is better than RBS CAS CT1 (RBS damages 1333 bps per DS0)

If CAS (RBS or R2), be sure NO signaling noise during call setup!

• Pure digital straight into digital switch: NONO channel bank!

What V.90 Wants: Digital LineWhat V.90 Wants: Digital Line(T1, E1, BRI)(T1, E1, BRI)


PhoneSwitch

Channel Bank

Trunk Side Service

Line Side Service

No Channel Banks!No Channel Banks!

What V.90 Does What V.90 Does NotNot Want: Want:Digital LineDigital Line

PhoneSwitch


What V.90 Does What V.90 Does NotNot Want: Want: Channel BanksChannel Banks

• Since V.90 requires a digital path from the digital modem to the last D/A on the client’s POTS line, a channel bank on the NAS access line will destroy V.90

• MICA is notnot supported in a channel bank application (because it doesn’t want to see near-end echo)

• Microcom is limited to mediocre V.34 in a channel bank topology


What V.90 Wants: PSTN CircuitWhat V.90 Wants: PSTN Circuit

• Very few slips or BERs (fairly rare nowadays)

• No analog trunks (rare unless the NAS is connected to a PBX)

• Each RBS link destroys 1333 bps (unless they happen to align)

• No sub-64k coding (e.g. 32k ADPCM will limit you to 16800 bps V.34)


Must Have Exactly One D/A Must Have Exactly One D/A Conversion in the Circuit!Conversion in the Circuit!

Which Digital Modems Can Get PCM to Which Analog Modems?

NASa

Client A Client C

NASb

What V.90 Does Not Want in PSTN What V.90 Does Not Want in PSTN Circuit: Multiple D/ACircuit: Multiple D/A

• NASa to Client A?

• NASa to Client B?

• NASa to Client C?

• NASb to Client B?

• NASb to Client C?

• NASb to Client A?

Client BChannel

BankChannel Bank

1AESS

1AESS1AESSChannel

Bank

5ESS

Channel Bank


CodecCodec

64k PCM Stream

-13dBm

6dB PadPCM

-19 dBm-19dBmAnalog

PSTN Circuit Digression:PSTN Circuit Digression:Digital PadsDigital Pads

Reduce Signal Amplitude—Get “Fixed Loss” across All Circuits

LookupLookupTableTable

Digital Pads AreNormally—but Not

Necessarily—in the Line Direction

in the Subscriber’s LE


Digital Pads and PCM Digital Pads and PCM ModulationsModulations

• K56Flex can only cope with 3 and 6 dB digital pads (and has trouble with Nortel 3dB), K56Flex always does pad compensation (PCM level boost)

• V.90 can learn any pad value, pad compensation is optional for V.90


V.90 Digital Pad CompensationV.90 Digital Pad Compensation

CodecCodec

64k PCM Stream

6dB PadPCM

-13dBmAnalog

We Can Boost the PCM Amplitude if We Know There’s a Pad

LookupLookupTableTable

“I Can Built a PCM

Constellation that

Compensates for theCircuit

Padding”

“OK, well I Hear a 6dB Pad,So Boost Your PCM Signal”

-7dBm-7dBm

-13 dBm-13 dBm


V.90 with and without Pad V.90 with and without Pad CompensationCompensation

On This Circuit Path with a 6dB Pad, Turning on Pad Compensation Earned an Extra 4kbps


What V.90 Wants: PSTN CircuitWhat V.90 Wants: PSTN Circuit

• Basically, all V.90 really wants from the PSTN is a 64k clear channel data circuit, for the price of a POTS call!

• But, unlike 64k ISDN, V.90 can tolerate digital impairments, such as pads, a-law-to-u-law conversion, and robbed bits

• In the future: bidirectional V.90-like PCM? (“V.91”)


PSTN

PSTN

< 3 Miles Good Old Twisted Pair Copper

or

DigitalDarrier

< 3 Miles UTP

“Integrated” SLC(Subscriber Line Concentrator)

What V.90 Wants: Local LoopWhat V.90 Wants: Local Loop


Load Coils (Every 18kft on Long Loops)

ChannelBank

What V.90 Does What V.90 Does NotNot Want: Want:Local LoopLocal Loop

PSTN

PSTN

DigitalCarrier“Non-Integrated” AKA

“Universal”SLC

or


LocalLoop

Each Line on a Separate Unshielded Twisted

Pair, or Only One Line Per Quad Cable

Walljack

Short CordRun Straightto the Modem

Note: V.90 does Not RequireRequire Any Particular House Wiring(in Fact, My Wiring Looks a Lot Like the Following Slide—

Yet I Get Good V.90 Nonetheless)ButBut: Poor House Wiring CAN Make the Difference

between Good V.90 and Mediocre V.34

What V.90 Wants: House WiringWhat V.90 Wants: House Wiring


11LocalLoop

Walljack

Splitter

44

1.1. Crosstalk from Two Lines in the Same Quad Cable2.2. Corroded or Shoddy Connectors3.3. Bridge-Tapped Line Running Parallel to

Fluorescent Light4.4. Flat Silver Satin Parallel to Power Cable5.5. Extraneous Equipment with Dubious Electrical

Characteristics on the Same Line

NIU

What V.90 (or V.34 for that Matter) What V.90 (or V.34 for that Matter) Does Not Want: House WiringDoes Not Want: House Wiring

2233

55



Hooking Things upHooking Things upRight and Proving ItRight and Proving It


Order the Right Digital LineOrder the Right Digital Line

• Get a digital path into a digital switch not into a channel bank

MICA won’t work right when connected into a channel bank (no near-end echo cancellers

Microcom can work, but only V.34—no PCMconnects possible

• Use PRI vs. RBS if you can

PRI is 64Kb clear channel, gives good DNIS, ANI, crisp channel cut-through

RBS is 62-2/3Kb (each additional RBS trunk adds another 1333Kb of degradation, down to 56K), restricted functionality, sloppy cut-through


Order the Right Order the Right Digital Line, (Cont.)Digital Line, (Cont.)

• Order the right signaling

If PRI, it usually just works, (We are user-side PRI, not network-side—some PBXes can only beuser-side)

If RBS, we recommend e&m-fgb (wink-start)

With e&m-fgb, MICA (not Microcom) canhandle incoming DNIS (not ANI), in MF or DTMF (not pulse)

With e&m-fgd, MICA can handle incoming DNIS and ANI, but there are some restrictions

If R2…get to know your switch guys real well, and you’d better get an E2 analyzer ready


Hook It Up Right—Hook It Up Right—Prove the Digital LineProve the Digital Line

• Get the line buildout right, use “cablelength long” or “cablelength short” (DSX1) if needed, (no “cablelength short” on AS5200)

• Make sure your T1/E1 is (almost) perfect

• Extended BERT with various patternsis nice

• T1 errors can show up on the switch side but not on the NAS side

In some Cisco IOS versions, NAS “show controller” can fail to show errors that it really receives


debug serial intshow contr t1 [/e1] ! show ABCD stateconf term service timestamp debug date msec service internalmodem-mgmt csm debug-rbs ! see ABCD changes debug modem csm ! see MICA DNIS/ANI decode

Hook It Up Right—Hook It Up Right—Prove the SignalingProve the Signaling

• Make a handset call and be sure to hear the good music (watch out for noise due to RBS/R2 mixups)


Hook It Up Right—Hook It Up Right—Prove the DS0sProve the DS0s

• If fortunate enough to be doing PRI, then place a sync data call thru and verify that data moves flawlessly thru the B channel

This proves the DS0 path from the NAS to the LE (and hence proves the digital carrier—T1/E1—as well)


Hook It Up Right—Prove Voice Hook It Up Right—Prove Voice Path Thru the LEPath Thru the LE

• The modem equivalent of a ping to the next-hop router

• Place a reverse telnet modem call out to the LE and back in again—should get a good 33600 (clear-channel DS0s), 31200 (RBS) or 26400 (channel bank) connection


$ telnet 172.16.24.116 2055 ! NAS IP, 2000+lineTrying... Connected.access-3 line 26 MICA V.90 modemsatdt 5703932CONNECT 33600 /V.42/V.42bisaccess-3 line 52 MICA V.90 modems router>term len 0router>show modem log ! move some datarouter>show modem op 1/1 ! µcom: “modem at” Param #9 TX,RX Bit Rate: 33600, 33600 Param #11 TX,RX Symbol Rate: 3429, 3429 Param #21 Signal Noise Ratio: 41 dB Param #26 Far End Echo Level: -68 dBm ! <-55

Reverse Telnet TestReverse Telnet Test


Hook It Up Right—Hook It Up Right—26/3600 NM-AM26/3600 NM-AM

• In general, things are much simpler when hooking up an analog modem such as the analog modem network module (NM-AM) for the 2600/3600

• HoweverHowever, I have one important point for hooking up an NM-AM system

Be absolutely sure to use a properly grounded power source; otherwise your modem calls will suffer from a horrible buzz


Getting Things Getting Things BasicallyBasically Working Working

• At this point, you have proven that the NAS and its connection to the PSTN are healthy:

T1/E1 is proven to be (nearly) flawless

You can make a strong V.34 modem call thru the T1/E1, to the local switch, and back into yourself

You are running known good Cisco IOS and firmware/portware

• Now, let the users dial in and see howthey do


• Perfection is unattainable in this world

Expect a CSR of 95% (V.34) or 92% (V.90) , maybe 10% premature drops

• Use:

show modem summary

show modem connect-speeds

show modem call-stats

To get the overall picture

First, Look at the Big PictureFirst, Look at the Big Picture


• Now that everything’s perfect on the NAS and in the digital line to the switch…it’s time to gather data; for each problematic call, get the following info if possible:

show modem op (MICA) / AT@E1 (Microcom) while connected

show modem log for the session of interest (after disconnect)

ANI (caller’s number)

Time of day

Client modem hardware / firmware revision

Interesting info from client (after disconnect)—ATI6, ATI11, AT&V, AT&V1, etc.

Data to Gather for Trend AnalysisData to Gather for Trend Analysis


Incoming calls Outgoing calls Busied Failed No SuccUsage Succ Fail Avail Succ Fail Avail Out Dial Ans Pct. 0% 6297 185 64 0 0 0 0 0 0 97%

show modem summaryshow modem summary

• No ans: call came into modem but modem did not go offhook (CPU too busy?)

• Fail: modem went offhook but modems failed to train up

• Succ: modems trained up; Cisco IOS saw “DSR” go high (still doesn’t mean that PPP negotiated successfully, etc.)


compress retrain lostCarr rmtLink trainup hostDrop wdogTimr inacTout Mdm # % # % # % # % # % # % # % # % Total 9 41 271 3277 7 2114 0 0

• rmtLink is good—it means that EC was in effect, and the client DTE decided to hang up

• hostDrop (aka dtrDrop) is usually good—it means that the host DTE (Cisco IOS) decided to hang up

Idle timeout

Circuit clear from the telco

PPP LCP termreq from the client

(It’s hard to figure out dtrDrop reasons without AAA accounting)

• Other reasons are bad—should be < 10% of total

show modem call-statsshow modem call-stats


as1-icg>show modem connect 33600 0 Mdm 26400 28000 28800 29333 30667 31200 32000 33333 33600 TotCntTot 614 0 1053 0 0 1682 0 0 822 6304

• Look for a healthy distribution of V.34 speeds

A peak at 26.4 (CT1s in the boonies) up to 31.2 (if using ISDN) is normal

• Look for a smattering of K56Flex, V.90 speeds

No PCM connects? Network topology problem

• Few connects at impaired V.34 speeds(21.6, 16.8–19.2 is a special case)

show modem connect-speedsshow modem connect-speeds


0 10000 20000 30000 40000 50000 60000

Speed (bps)

N c

on

ne

cti

on

s

Rx

Tx

DCE Speed DistributionDCE Speed Distribution


*May 31 18:11:09.558: %CALLRECORD-3-MICA_TERSE_CALL_REC: DS0 slot/contr/chan=2/0/18, slot/port=1/29, call_id=378, userid=cisco, ip=0.0.0.0, calling=5205554099,called=4085553932, std=V.90, prot=LAP-M, comp=V.42bis both, init-rx/tx b-rate=26400/41333, finl-rx/tx brate=28800/41333, rbs=0, d-pad=6.0 dB, retr=1, sq=4, snr=29, rx/tx chars=93501/94046, bad=5, rx/tx ec=1612/732, bad=0, time=337, finl-state=Steady, disc(radius)=Lost Carrier/Lost Carrier, disc(modem)=A220 Rx (line to host) data flushing - not OK/EC condition - locally detected/received DISC frame -- normal LAPM termination

modem call-record terse modem call-record terse (11.3AA/12.0T)(11.3AA/12.0T)


•show modem operational-status dumps the parameters pertaining to the current (last) connection

http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/12cgcr/dial_r/drprt1/drmodmgt.htm#xtocid10451279

• modem at-mode AT@E1 is the analogous feature for Microcom

http://www.cisco.com/univercd/cc/td/doc/product/access/acs_serv/5300/mod_info/atcmnds1.htm#23729

show modem operational-show modem operational-statusstatus (MICA) (MICA)


It’s very important to gather client-side performance data, to find client-specific trends:

• Client hardware model, firmware version: ATI3I7

• Client-reported disconnect reasons:

ATI6 or AT&V1

PC’s modemlog.txt, ppplog.txt

Client-Side Performance DataClient-Side Performance Data


Analyze the Performance DataAnalyze the Performance Data

• Now that you have collected and understood the performance data for your modem system, it’s time to look at any remaining patterns/ components that may have roomfor improvement


• Are problems associated with particular NAS modems? (show modem)

• Are problems associated with specific DS0s on your digital line? (show controller t1 call-counters)

Bad Modems/Channels?Bad Modems/Channels?


• Use show modem, show modem call-stats and look for any modems with abnormally high rates of trainup failure or bad disconnects

• (MICA) If adjacent pairs of modems look hosed, then probably a hung/dead DSP problem. Use copy flash modem to the affected HMM to recover. If not using portware 2.6.2.0, please upgrade ASAP

• Verify that all modems are correctly configured. Use modem autoconfigure type with debug confmodem to ensure correct settings. May need to reverse telnet to fix up modems that are badly misconfigured

Problems with ParticularProblems with ParticularServer ModemsServer Modems


• Bad DS0s are rare but possible. (Always the telco’s fault, never ours :-)

• Use show controller t1 call-counters

• Look for any DS0s with abnormally high TotalCalls and abnormally low TotalDuration

• Busy out DS0s (isdn service dsl, ds0 busyout) to target specific suspected bad ones

TimeSlot Type TotalCalls TotalDuration1 pri 873 1w6d2 pri 753 2w2d

3 pri 4444 00:05:22

Problems with Particular DS0sProblems with Particular DS0s


Trend: Bad Circuit PathsTrend: Bad Circuit Paths

• If:Long distance calls have problems but local do not (or vice versa)

Calls at certain times of day have problems

Calls from specific remote exchanges have problems

• Then:You might be getting bad circuit paths through the PSTN


Trend: LD Bad, Local GoodTrend: LD Bad, Local Good

• If long distance calls are bad but local calls are good, then:

Double-check to make sure that the digital line connects into a digital switch notnot a channel bank

Check with telcos to examine the circuit paths used for long distance


Trend: Some Calling Areas Trend: Some Calling Areas Have ProblemsHave Problems

• If calls from specific geographical regions/exchanges tend to have problems:

Learn the network topology from the telco

If multiple A/D conversions (non-integrated SLCs, analog switches) are used to serve an area, then PCM modem connects will be impossible, and V.34 may be somewhat degraded


Trend: Particular Client Modems Trend: Particular Client Modems Have Particular ProblemsHave Particular Problems

• So now the great majority of callsconnect in the desired modulationwith the desired speed

• …but there are still a fewfew particular client modems running some particular firmware on some particular POTS lines calling via some particular circuits which are failing to behave as desired…

• So you can now adjust to living with imperfection, or else move in forhand-to-hand combat



Living with ImperfectionLiving with Imperfection


An Individual Modem Connection Will Manifest Various Symptoms of

Suboptimality:

Symptoms of a Suboptimal Symptoms of a Suboptimal ConnectionConnection

• Failure to train up

• Don’t train up in the desired modulation

• Don’t train up in the desired speed

• Train up, but no EC

• Inadequate performance (throughput)

• Premature disconnection


There are Three Fundamental Causes ofImperfect Modem Connections, the First

Two Causes are Just Corollaries of Each other:

The Underlying CausesThe Underlying Causes

• Poor circuit quality (relative to the desired modulation/speed)

• Excessive modulation/speed (relative to the circuit quality)


The Third Underlying Impediment to Perfect Modem Connections is What One Can Call Modem QualityModem Quality. There are Several Aspects

to Modem Quality:

The Underlying Causes (Cont.)The Underlying Causes (Cont.)

• The ability of the modem optimally to resolve the data from the analog signal received

• The ability of the modem to select the optimal receive modulation/speed for the given circuit

• The ability of the modem to interoperate nicely with the vast and evershifting range of peer modems (of various quality) encountered inthe field


Addressing the Underlying Addressing the Underlying Causes—Circuit QualityCauses—Circuit Quality

• Circuit quality problems?Circuit quality problems?

• Reengineer the circuit

• This may prove difficult, as it may require pulling new cable, replacing equipment, involving multiple providers, paying for new carrier facilities, etc

• To locate the problematic portion of the circuit, experiment with various call paths


• If all modems were of infinite quality, then, controlling for circuit quality, they would always select the optimal modulation/speed

• If you hold the peer modem and circuit constant, you can vary the modem to find another of higher quality (for that particular circuit and peer)

• If you have quality issues with a modem, you may contact the vendor for improved firmware, etc. See http://56k.com for links to client firmware updates (http://808hi.com/56k/x2-lucent.htmfor LT winmodem)

Addressing the Underlying Addressing the Underlying Causes—Modem QualityCauses—Modem Quality


Testing against Known Good Testing against Known Good Modems on a Known Good CircuitModems on a Known Good Circuit

• See if the client modem/circuit is capable of good V.90 connections by dialing a knowngood NAS with digital modems, on a known good circuit

• Cisco’s test AS5300 (MICA and Microcom modems) at +1 408 570 3930 / 3932 (http://www.cisco.com/warp/public/471/83.html)


Addressing the Underlying Addressing the Underlying Causes—Excessive SpeedCauses—Excessive Speed

• GivenGiven that circuits are in fact imperfect, so that they cannot always attain the ideal speed, and givengiven that some modems are imperfect, so that they will not always choose the optimal speed for the imperfect circuit…

• The remaining option is to helphelp the modems choose a better (slower) speed for the imperfect circuit


Detuning ModemsDetuning Modems

• Given a pair of modems and a circuit, they usually can (and sometimes will) select a carrier speed that yields an unstable connection

• A nominally higher DCE speed may produce worse performance than a nominally lower one, due to retrains, EC retransmits and lost carrier

• If you cap the modems to restrict their range of modulations and/or DCE speeds, they may be able to train up (where they had been unable to do so before), or may achieve more stable connections


Detuning Modems (Cont.)Detuning Modems (Cont.)

• You can cap the DCE modulations/speeds at the server side or the client side

• Server side caps will improve stability across the board, without requiring configuration effort on the client side, the downside is that this must be a lowest-common-denominator setting, and will therefore reduce performance for some {modem, modem, circuit} tuples that couldcould handle a higher speed

• Client side caps can be used to achieve optimal performance on a link-by-link basis, the downside: greater configuration effort is required (often by unsophisticated users)


Detuning Modems (Cont.)Detuning Modems (Cont.)

• Normally, the DTE speed will not directly affect the stability of a modem connection

• A properly implemented DTE will be able to handle the full speed (e.g. 115200 bps), and will use flow control as needed if it can’t keep up.

• However, it has been reported that that some Windows systems can prematurely disconnect unless the DTE speed is reduced and/or the async serial buffer sizes are reduced.


Server Side Caps (Cont.)Server Side Caps (Cont.)

• Consider (carefully!) configuring the server modems so as to be less aggressive andmore persistent:

MICA:

S19 (EC retransmission limit), S34 (Fall-forward timer), S38 (Lost carrier timer), S34 (fall-forward timer), S40 (retrain threshold)

S54=26 (rate-cap overaggressive clients, enable Signal Quality Checking) (2.6.2.0)

Increase S32 (SQ threshold) to 3 (more conservative Rx speeds across the board) (pre-2.6.2.0)

Microcom: Increase S10 (Lost carrier timer), disablerate renegotiates (:t110=0), increase EQM trip threshold (:t34)


Client Side CapsClient Side Caps

• This is the same idea as server side caps:At a gross level, you can successively disable modulations till you find one that works

At a fine level, you can keep stepping down through the DCE speeds supported by the top modulation, till you find a stable one

Remember: always test using a terminal program (e.g., Hyperterminal)…PPP just gets in the way

• Rockwell (ITU V.25terter): use the +MS command to control the modulations and DCE speeds

Example: AT+MS=12,1,300,36000

Allow V.90 and lower modulations; min speed 300; max speed 36000


Client Side Caps (Cont.)Client Side Caps (Cont.)

• TI (USR/3COM): use &U to set minimum DCE speed, &N for maximum

Example: AT&U4&N11

Min speed 4800; max speed 21600

• PCTEL: use S37 with N0 for max DCE speedExample: ATN0S37=12

Max speed 44k if V.90, 56k if K56Flex, 31200 if V.34

• Mwave: use S28 to cap DCE speedExample: ATS28=15

Max speed 16800

• Lucent and Hayes: check docs



Hand-to-Hand Combat Hand-to-Hand Combat with Individual with Individual ConnectionsConnections


So You Really Want to Wrestle So You Really Want to Wrestle with a Modem Problemwith a Modem Problem

• If you really want to get to the bottom of an individual modem problem, you’ll want to:

Get your hands to the AT prompt at the client modem, while it’s attached to the POTS line of interest

Be prepared to gather a .wav file of the trainup music


DataV.8 bis

V.8 (Phase I)

Line Probe(Phase II)

Half DuplexEcho Cancel and

Equalizer Training(Phase III)

Full DuplexTraining

Digital ImpairmentLearning (DIL)

V.90 TrainingV.90 Training


OffHook

V.8 BisTone

2100 Hz ABT

Rest of V.8 Training

K56Flex V.90 + V.34

Phase Reversalsand 15 Hz Modulation

Phase Reversals 2100 HzTone

V.32

V.22 bis

V.90V.90

K56FlexK56Flex

V.34V.34

V.32V.32

V.22 bisV.22 bis

FSKFSK

CompleteTraining

V.42 V.42 (LAP-M)(LAP-M)

MNP-3/4MNP-3/4

Async.Async.modemode

MNP-2/4MNP-2/4

Optional

Optional

Optional

Rest of V.8 Bis.

Negotiation Overview Negotiation Overview




K56Flex TrainingK56Flex Training




Listening to the DILsListening to the DILs

• The DIL (Digital Impairment Learning sequence) is the musical score (PCM sequence) that the V.90 analog modem tells the digital modem to play back to it, so that it can discern anydigital impairments in the circuit; (such as multiple D/A conversions, a-law/μlaw,robbed bits, digital pads)

• If you don’t hear the DIL, then the modemsdid not negotiate V.90 in V.8/V.8bis (i.e., a modem compatibility issue; if you DO hearthe DIL, but then a retrain in V.34, then the analog modem decided, on the basis of theDIL playback, that V.90 was infeasible


Pctel 7.55 USR (3COM) LT winmodem5.28

Rockwell (Conexant) 2.2

Some V.90 DILsSome V.90 DILs


Symptom: Trainup FailureSymptom: Trainup Failure

• Is the symptom a failure to train?

Does the music have noise in it? If so, then clean up the circuit

Does the client give up quickly, without running V.34 training? E.g., perhaps it flips out when it hears V.8bis Cre; in which case, try disabling V.8bis (hence K56Flex) on the server (if acceptable), or get new client firmware, or swap it out


Symptom: Trainup Failure (Cont.)Symptom: Trainup Failure (Cont.)

• Does the client try training in an advanced modulation (V.90, K56Flex, V.34), but eventually give up? Then try capping the client at successively lower modulations/DCE speeds

• (Or improve the circuit, or improve the modem code, of course)


Symptom: Not the Desired SpeedSymptom: Not the Desired Speed

• Does the client train up in the desired modulation, but at a slower than yearned-for speed?

• If so, then simply coercing the modems to connect at a faster nominal DCE rate would nearly always be a grave mistake

• The best solution to this problem will normally be to improve the circuit; improving the modem code will rarely yield results here (the exception being a modulation with much recent churn, such as V.90)


Symptom: Trainup, but No ECSymptom: Trainup, but No EC

• Does the client train up in the desired modulation, fail to negotiate error control?

• If so, then the probableprobable cause is high BER in the selected modulation, resulting in data damage or retrain during the sensitive EC negotiation phase, in this case, the useful approach will be to detune the client for a slower modulation/DCE speed

• Another possibility is modem bugs in EC negotiation or late modulation trainup; (seen with some popular client firmware), in which case, pursue improved client firmware


Symptom: Inadequate ThroughputSymptom: Inadequate Throughput

• Is inadequate throughput observed thru themodem link (given the chosed modulation/speed)?

• If so, then first rule out the possibility of higher-layer protocol deficiencies, (e.g., do a simple ping thru the link)

• Monitor the EC retransmits (block errors) on the link, if they are too high (> 5% of frames), then modem detuning is in order, similarly, if the slowness is due to excessive retraining, then detuning should help


Symptom: Premature DisconnectionSymptom: Premature Disconnection

• Does the modem link terminate before the user wanted it to?

• If so, then first ascertain whether the disconnection was initiated by one of the DTEs, if so, then fix the DTE application

• If neither DTE initiated the disconnect, then (as always), modem detuning, circuit improvement, and/or a firmware upgrade, should yield good results


Symptom: Premature Symptom: Premature Disconnection (Cont.)Disconnection (Cont.)

• Monitor the DTE links to see who initiates the disconnect

• Using the old RS-232/AT interface model, a DTE-initiated disconnect will show as a DTR drop or as “+++ATH<CR>” on the Tx lead, a DCE-initiated disconnect will show as a DCD drop (which Cisco IOS calls “DSR”)

• The idealideal is for all disconnects to be voluntary—i.e., initiated by one or the other of the DCEs

DTE DTE

DCE DCE



• “Debug modem” will tell the truth, however, if the drop was initiated by a PSTN circuit clear, this will show as a DTR drop (because, on a system with digital modems, the DTE handles the PSTN interface), so also check the PSTN signaling (“debug isdn q931”, “modem-mgmt csm debug-rbs”)

• Use “modem call-record terse” to get all the good info in one nice package, if the disconnect was a pure Cisco IOS-side decision, then this will give the Cisco IOS explanation

Tracking Whether DCE or DTE Initiated the Drop - from the Cisco IOS Side



• Turn on the TAPI modem log (control panel -> modems -> properties -> connection -> advanced, or some such)

• After the disconnect, see whether the disconnect was initiated by the PC or by the modem; (don’t take TAPI’s interpretation of events very seriously, it tends to lie)

• If it was the PC that initiated the disconnect, next contact the friendly folks at Microsoft who will be eager to help solve the problem

Tracking Whether DCE or DTE Initiated the Drop - from the PC Side:



• “modem call-record terse” on Cisco IOS showed:

disc(radius)=User Request/Received Terminate disc(modem)=A220 received DISC frame -- normal LAPM termination

• PC’s modem log showed:

Hanging up the modem.

Hardware hangup by lowering DTR.

DTE-Initiated Example: the PC User (Me) Clicked the DUN Disconnect Button



• “modem call-record terse” on Cisco IOS showed:disc(radius)= (n/a)/(n/adisc(local)=9 DTR Drop

Alas this doesn’t tell the REAL reason why Cisco IOS dropped the call—that the caller line went loop-open, (note to self: DDTS this)

• PC’s modem log showed:Remote modem hung up.Recv: <cr><lf>NO CARRIER<cr><lf>.

Alas this shows that TAPI has no way of knowing what’s reallyreallygoing on—it’s default behavior when it sees a network-side disconnect is to blame the remote modem!

• If you’d been listening to the voiceband at this time, then you would have known what happened!

DCE-initiated example: the PC user (me) unplugged the phone cord while in the midst of transferring data


For Further InsightFor Further Insight

• Gilbert Held, The Complete Modem Reference, 3rd Ed.The Complete Modem Reference, 3rd Ed.

• The cisco-nas mailing list (email

[email protected]; archives athttp://cisco-nas.datasys.net)

• John A. C. Bingham, The Theory and Practice of The Theory and Practice of Modem DesignModem Design

• John G. Proakis, Digital CommunicationsDigital Communications


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