Snom 4S Media Server Use and care manual

Administrator‘s
Manual
snom 4S
SIP Media Server
Version 2.0

snom technology AG • 3
snom 4S Media Server Version 2.0 Administrator Manual
1. Edition 2002
© 2002 snom technology Aktiengesellschaft. All Rights Reserved.
This document is supplied by snom technology AG for information purposes only to licensed
users of the snom 4S media server and is supplied on an “AS IS” basis, that is, without any
warranties whatsoever, express or implied.
Information in this document is subject to change without notice and does not represent any
commitment on the part of snom technology AG. The software described in this document
is furnished under a license agreement and may be used only in accordance with the terms
of that license agreement. It is against the law to copy or use this software except as
specically allowed in the license. No part of this document may be reproduced, republished
or retransmitted in any form or by any means whatsoever, whether electronically or
mechanically, including, but not limited to, by way of photocopying, recording, information
recording or through retrieval systems, without the express written permission of snom
technology AG.

snom technology AG • 3
“Welcome to the
Media Server!”
When you dial the wrong number with your good old PSTN phone,
you will probably hear something like “The number you have dialled is
temporarily not available”. When you reach the auto attendant of a
company, you will hear something like “Welcome to snom technology!”.
We take messages like these almost for granted.
With most VoIP installations today, you will probably just hear
some beeping of the phone. It is hard to explain why a new technology
can not do what the old technology can do today. Your customers and
your colleagues will expect that a VoIP telephony system will have at least
the same features as your “good old” PBX had.
With a media server you can do exactly this. Together with a
SIP proxy it will provide the functionality that will full the needs of your
telephone users and might also offer some more and helpful features not
known from traditional PBX.
A SIP network consists of several components. Some vendors
group a SIP proxy, registrar, location server and media server to
“SoftSwitch” or iPBX. We chose the term media server to indicate that this
component takes care about everything that is related to audio streams.
This makes sense because handling audio is a real time task and sets
different requirements on the underlying hardware than not-so-critical
requirements of the call control. SIP allows and encourages this distinction
and thus make scalable systems easier to implement. However, practically
all components of a small to medium size installation will run on the same
machine.
Interoperability is the glue of the growing VoIP industry. We have
tried to stick to the SIP standard as well as possible and tested together
with the equipment of other vendors whenever that was possible. We
hope that this will help to build up a ourishing VoIP telephone industry
in which the products of the different vendors work together like the

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products in the computer industry do today. We believe that having a
choice is good for you and therefore good for us.
This manual is the best way to get started with the snom 4S
media server. However, because its hard to keep a manual up-to-date with
all the issues that come up, we provide a list of frequently asked questions
(FAQ) on out web site www.snom.de — together with their answers!
We hope that you enjoy using the media server and it helps you
setting up a great communication environment!
Sincerely,
Dr. Christian Stredicke Nicolas Peter-Pohland

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Table of Contents
“Welcome to the Media Server!”.................................3
How It Works .............................................................7
Signalling and Media ................................................................... 7
Response Times ......................................................................... 8
Multiprocessing.......................................................................................................................................................................................... 8
Hard Disks ........................................................................................................................................................................................................ 9
Privacy ...................................................................................... 9
Installation ..............................................................11
Windows ................................................................................. 11
Installation................................................................................................................................................................................................... 11
Uninstalling in Windows............................................................................................................................................................ 16
Linux ...................................................................................... 16
Manual Starting..................................................................................................................................................................................... 16
Automatic Starting ........................................................................................................................................................................... 17
Command Line Options.............................................................. 18
Setting Up ................................................................21
Licensing ................................................................................. 21
Hostnames................................................................................................................................................................................................... 22
License Key................................................................................................................................................................................................. 22
Fundamental Settings................................................................ 23
SIP Port............................................................................................................................................................................................................. 23
TCP threshold........................................................................................................................................................................................... 24
Log Level ........................................................................................................................................................................................................ 24
Default SIP Domain.................................................................................................................................................................................
24
HTTP port....................................................................................................................................................................................................... 24
HTTP User and Password......................................................................................................................................................... 25
Codecs .................................................................................... 25
Representing Voice.......................................................................................................................................................................... 25
Representing User Input.......................................................................................................................................................... 27

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Accounts ..................................................................29
Registering Accounts ................................................................. 30
Passwords ............................................................................... 30
Music On Hold .......................................................................... 30
How to use Music On Hold..................................................................................................................................................... 31
Conguration of the MOH account............................................................................................................................. 31
Mailbox ................................................................................... 32
Using the Mailbox Account.................................................................................................................................................... 32
Conguring a Mailbox Account........................................................................................................................................ 32
Answer Delay ......................................................................................................................................................................................... 32
Record Length......................................................................................................................................................................................... 33
MWI Destination................................................................................................................................................................................... 33
Register and Pass............................................................................................................................................................................... 33
Conference Server .................................................................... 33
Auto Attendant......................................................................... 34
Error-Announcement ................................................................. 34
Maintenance.............................................................35
Trace ...................................................................................... 35
Logle..................................................................................... 36
Stored Messages ...................................................................... 37

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How It Works
This chapter explains how a media server works in principle and
gives you a feeling on what is important. You will learn the difference
between signalling and media and what is important to operate a media
server.
This is not intended to serve as introduction into VoIP and SIP in
general. You nd great introductions and numerous links in the Internet
(e.g. at http://iptel.org) and in the literature. The snom 4S Proxy/
Registrar manual also gives a brief overview on SIP.
If you are already with the underlying problems with a media
server, go can go straight to the next chapter.
Signalling and Media
In short words, there are two fundamental problems with
telephony: Finding the user you want to talk to and talking.
Finding the user is done with signalling. SIP is one way to nd the
party you want to talk to. This job is characterized by a complexity, for
example when a user redirected his phone or can be reached on several
locations. If the nding process takes a second more or less, this is not
so critical.
When the user has been found, the problem shifts to transporting
the speech as fast as possible between. A second for transporting the
voice from one user to another is unacceptable. It takes a lot of discipline
if you want to establish a conversation like this. When users are talking,
typically every 20 ms a piece of the conversation is transported over the
network. If this stream of packet gets delayed stalls, the other side will
experience a bad audio quality.
While operating systems are very good at scheduling tasks (so
nothing gets forgotten), reacting very fast sometimes gives problems. If
you need to send a packet every 20 ms, you usually don’t have the time
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to get audio data from a network server and even accessing the local hard
disk can degrade the quality.
Response Times
A modern PC is an unbelievable workhorse. An off-the-shelf PC
can process more than a billion operations per second and store easily a
month of a conversation. The bandwidth of a standard Ethernet card is
enough to handle more than a thousand uncompressed media streams.
Set up correctly, it can handle tens and hundreds of parallel calls without
a problem. You would have to spend ten or hundred times more money to
have such a working horse as traditional PBX hardware.
However, in order to operate this workhorse you need to follow
a few rules.
Multiprocessing
All modern operating systems like Windows 32 and Linux support
multiprocessing. Several processes share the same CPU in a way that
the CPU works for a short time on the rst process and then continues
with the next process. The switching is done around a hundred times per
second and typically the user does not recognize this.
However, in a VoIP environments we are talking about sending
packets at a rate of fty packets per second per call and that might result
in a high frequency of process switches.
The operating system knows that the media server has higher
requirements on time critical task switching (this is signalled during the
starting of the media server). However, when there are other tasks on
this computer, the operating system might not be able to give all of them
the CPU attention that they need. So in short words, if you run the media
server and some video games on the same machine don’t get surprised if
your users complain about stuttering voice!
The best approach to solve this problem is to run the media server
on a dedicated the host. Because the proxy usually does not request time
critical priority from the operating system, it is usually not a problem to
run the proxy on that machine as well. However, it is not a good idea to
run the media server on a le server.
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We would like to point out that from a media server perspective,
both Windows and Linux are great operating systems to run the media
server process.
Hard Disks
Hard disks are another miracle of the computer industry. I was
told modern off-the-shelf hard disk can store more information that the
human brain and so it should be good enough to store some mailbox
messages.
The price to pay for this is access time. When information on the
hard disk is scattered, the time to gather information can become too long
to answer requests within twenty milliseconds.
Missing the twenty millisecond answer time does not mean the
end of the call. Phones usually tolerate late packets to a certain degree
and if the packet is missing it will ll the gap with other information. Only
when too many packets get lost, the call will stop.
The general strategy to avoid situations like these is called
caching. Operating systems usually keep les which have already been
opened in the main memory until they are not used and are replaced with
information which is needed more urgently. As long as a le stays cached
there will be no problem.
Needless to say, accessing les (which are also stored on hard
disks) via the network is not an option if a continuous media stream is
wanted.
Privacy
There is another reason to use a dedicated server for the media
server: privacy. Remember that voicemails are normally strictly private,
and therefore it makes sense to separate them from normal le access.
There is nothing better than a physically separated server for this.
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