faxing q and a

Powerful, Distributed, API Communications

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Expert Q&A: Faxing EditionMay 3rd, 2013

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FROM MINUTES 00:44 through 2:50

Call-in Number: 805-309-5900 Pin 705-705-141 Expert Q&A: Faxing Edition

May 3rd, 2013

Welcome

Our Panelists

Joshua Goldbard

Marketing Ninja, 2600hz

Darren Schreiber

Founder, 2600hz

Some background…

What is a Fax?

• First patented in 1843

• Morse was 1844

• Bell was 1876

• Used to send documents using the most available

infrastructure

• Preceded by flag and smoke signaling

• 1843: Fax Patented• 1924: First Color Fax• 1964: Xerox invents Digital Faxing• 1985: First Computer Fax Board

Very old industry, lots of standardization and lots of weirdness

Major Milestones

IP Faxing is weird…• No transport advantage when compared to other IP transit• Expensive• Slow• Kind of a pain

Why do we do it? Facsimiles are legally binding; ergo regulations. Plus, people are stubborn and used to their “old” technology

Why IP Fax > Analog Faxing• IP Fax is cheaper

• IP Fax can be centralized

• IP is much easier to manipulate and integrate

• Analog has a long setup time (45 days for PRI)

• IP Fax can be geographically fault tolerant

Let’s Get Technical!

Trouble With Faxing• Jitter

• Codec Selection / Compression

• T. 38 Negotiation

• PSTN Equipment Configuration

• Latency

Trouble with Faxing:

SILENCE is DEATH

Faxing is sort of “synchronous”• One side at a time!• One side sends a message while the other side is silent• When the sending side pauses, it means it’s time for the

opposite side to respond

• Fax transmission is made up of tones and silence• Tones represent signaling• “static” sound represents your data image (very fast tones)• Silence represents a hand-off of control to the other side

A standard fax transmissionHi!

I’m ready

Hey! Me Too!

Let’s test out this

line!

Heard you perfectly! Send me a

page!

Here you go!

Done!

OK! What next?

Look Closely• If only one person can “speak” at a time…

How do you tell the other guy you’re done and it’s his turn?

SILENCE IS GOLDEN!

A standard fax transmissionHi!

I’m ready

Hey! Me Too!

Let’s test out this

line!

Heard you perfectly! Send me a

page!

Here you go!

Done!

OK! What next?

This works great on the PSTN• PSTN isn’t perfect• It has cracks, pops, hisses, static• Fax machines were designed for that, so they can remove

those in most cases

• PSTN does have some general guarantees• The audio, even if distorted, almost always makes it• Not really a concept of “cutting out” in PSTN land• So, fax machines assume there will not be cutting out

PSTN call w/ Noise

This will get corrected

Still clear gaps of silence

(end)

But VoIP introduces jitter…• Jitter is a slight pause when audio packets are missing• Usually because the line is too slow/congested and the data

doesn’t get there in time• Or because of packet loss on a misconfigured device• Some other reasons as well, but those are the major ones

VoIP Fax w/ JitterHeard you

perfectly, send me a page!

OK, sending!

Umm, hey, you paused, I thought you were done!

Synchronization is lost…

Dealing with Jitter• First, note that a line which sounds perfectly fine for voice

calls may still have lots of jitter• The human ear tolerates some amounts of jitter so you

don’t notice it• Faxes do not

• You can deal with jitter on VoIP most of the time• Most devices have a jitter buffer. Turn it up (high)• Turn OFF adaptive jitter buffers. Faxes need the timing and

signal to be consistent

Dealing with Jitter• Let’s take a look at how to adjust the jitter buffer settings• It’s so easy!

• Sidebar: Turn off echo cancellation while you’re at it• Since the fax is not listening to itself anyway while it’s

sending, echo almost never matters• Echo cancellation just adds one more “feature” on the

device that might screw up faxing

Making Fax Over IP Work:

T. 38 to the Rescue

T. 38 Overview• Another way of dealing with Jitter is T.38

• T.38 is an adaptation of faxing designed for VoIP• Modifies the transmission mechanism on the IP side• Inserts padding / white-noise on the PSTN side• Intentionally duplicates RTP packets to make sure they get

there

VoIP Fax w/ Jitter + T. 38

Fax would have continued!

T. 38 would have filled this in with

whitespace

T. 38 Overview• T. 38 was mainly designed for converting faxes when running

long-distances between PSTN endpoints• Began being added to endpoint devices directly

• The idea was to get the T.38 conversion to happen as close as possible to the fax machine

T. 38

T. 38Jitter is unlikely here Jitter won’t really

matter here

T. 38• But people always say, T. 38 doesn’t work that well.• Why?

• Different vendors implemented it slightly differently• Sometimes the ATA or device you’re using doesn’t work

with your vendor

• BUT MORE LIKELY• Your vendor sometimes cheats• More on that next…

T. 38• Here’s a secret• When you do a PCMU call, your vendor often has

equipment that just passes the data along with minimal / no processing

• When you do G729 or T. 38 your call must be routed to special equipment on the carrier side to handle that and convert it to PCMU

• That is why some carriers tell you to start fax calls as G. 729

T. 38• This leads to the typical requirement that…• You use a carrier who supports T. 38 (has the equipment)• You start your call as something other than PCMU• You properly setup T. 38 on your side and request it

properly

• Let’s look at a sample request

T. 38• But wait – G729 causes a problem…• Why is this a problem?

FAILBACK!

Making Fax Over IP Work:

CODECS

Voice “Speeds”

Fax Speeds

How do you fit a 14.4kbps

fax over a 8kbps voice

signal?

ANSWER: YOU CAN’T

Lessons from the front lines• What codec is best?

• T.38? Why or why not?

• A cornucopia of telecom equipment

• Fax Servers

• Configuration settings

• NAT Transversal

Let’s take some time to pontificate about

faxing at scale…

Massive Lethal Papercuts

Virtualization in Faxing?

How to avoid

Excessive Finger

Pointing

• Faxing is hard because IP

Introduces unbounded time

Uncertainty

• Variation in time is unexpected

behavior for faxing equipment

• Solution: Reduce Complexity

• (As much as you possibly can)

Recap

QUESTIONS???