Packet Guide to Voice Over IP: A system administrator's guide to VoIP technologies

299 Pages·2013·23.58 MB·English

by  Bruce Hartpence

Packet Guide to Voice over IP
Bruce Hartpence
Beijing • Cambridge • Farnham • Köln • Sebastopol • Tokyo
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Preface
A short while ago, as network engineers made plans for the future, one of the
considerations was the eventuality of Voice over the Internet Protocol, or VoIP.
For several years, VoIP was always “on the horizon” or “around the corner,” as
many believed that it was coming but were unsure about the timing. The
question was whether network designers and educational programs should
become early adopters, building in capacity and knowledge now or whether they
should make it part of the next deployment cycle. Pulling the trigger early might
put you at risk of making the wrong decision in terms of vendor or protocol.
Adopting late might put you behind the competition or make you rush to deploy
a system that is not well understood by the local staff.
Voice over the Internet Protocol, a.k.a. Voice over IP, or VoIP, is a huge topic.
Those trying to really understand how VoIP systems operate and the issues
associated with their deployment must delve into protocols and architecture
requirements such as power over Ethernet, or PoE. New security issues arise
because voice is now packetized on the data network and accessible via
ubiquitous wireless links. Quality-of-service issues associated with mixing data
and voice on the same network cause headaches as network administrators are
inundated with real-time data. Interconnecting IP voice connections with the
public switched telephone network (PSTN) and unified communications (UC)
brings additional concerns and increasing workloads to the beleaguered staff.
This book provides an explanation of VoIP from the perspective of operating
networks and the packets caught on those networks. Since the topologies were
built for the purpose of developing content for the book, the issues and
supporting structures necessary for VoIP are also explored. Thus, readers will get
a firsthand under-the-hood view of the protocols and architectures used by VoIP-
based systems as we track connections from the time VoIP phones boot, through
calls and during subsequent connection teardown. Like the previous Packet
Guide books (O’Reilly’s Packet Guide to Core Network Protocols and the
Packet Guide to Routing and Switching), the tool of choice for viewing the
packets will be Wireshark, which is still available for free out at wireshark.org.
The author built and configured everything seen in this book.
Most basic packetized voice networks start of with some very similar
components; Chapter 1 will begin with these. Components include not only
VoIP-specific items such as gateways and phones but also requirements such as
Dynamic Host Configuration Protocol (DHCP) and Trivial File Transfer
Protocol (TFTP).
The files and website support the lab activities in this book. Simple networking
experiences can be accomplished on almost any topology. However, it is not
always possible to obtain the resources necessary to build and study voice
networks. So, for the lab activities in this book, I have posted capture files
posted on the companion website. For additional background, a YouTube
channel provides another resource.
With the exception of those for Skinny, all of the references used for this book
are standards from the ITU-T (International Telecommunications Union-
Telecom), the IEEE (Institute of Electrical and Electronics Engineers), Request
for Comments (RFC) from the Internet Engineering Task Force, or material
obtained from operating networks.
Audience
I had several folks in mind when I wrote the Packet Guide books: instructors,
students, professionals, and those seeking information to boost their skill set.
While the first two books covered topics that are part of almost every single
network and this one focuses on a particular area, the goal and the audience have
not changed. My goal in writing these is to provide the background to
understand the issues but also take an in-depth look at the protocols and
operations that are part of a VoIP architecture. A student who reads this book and
completes the exercises will be conversant in this important area and will have
obtained valuable practical knowledge. A professional looking to brush up or
change jobs will gain the necessary leg up or at least knock the rust off. In either
case, I hope you enjoy the read.
Contents of This Book
Chapter 1, Introduction to Voice over the Internet Protocol
This chapter provides the foundation for the book. It includes the
requirements for a basic VoIP topology and describes the issues associated
with deploying packetized voice and video. Readers will also come to
understand critical topics such as codecs and power over Ethernet.
Chapter 2, Traditional Telephony
Every data network must eventually connect to the rest of the world via the
Internet. For VoIP, this usually means connecting to the global telephony
network, the uses of which continue to include traditional connectivity. This
chapter will familiarize the reader with traditional telephony concepts that
will typically be a part of their lives as VoIP administrators including local
loop, tip and ring, T carriers, and the necessary protocol conversations.
Chapter 3, Session Initiation Protocol
Most VoIP pundits agree that the Session Initialization Protocol, or SIP, is
taking over the VoIP world, and I am no different. As a result, SIP will be the
first “signaling protocol” that we will discuss in this book and will form the
basis for comparisons made throughout the other chapters. As an Internet
Engineering Taskforce request for comments, SIP enjoys wide industry
support and shares many characteristics with other common web protocols
such as the Hypertext Transfer Protocol, making it easy to understand and
read.
Chapter 4, The Real-Time Transport Protocol and the Real-Time Control
Protocol
VoIP protocols are broken into two categories: signaling and transport. The
Real-Time Transport Protocol (RTP) and its sidekick, the Real-Time Control
Protocol (RTCP), fall into the latter category. Almost every voice or video
stream created via signaling protocols such as SIP or H.323 are carried by
RTP. RTCP provides information about the stream. This chapter will cover
the operation and fields for both protocols. It will also provide some practical
information for their deployment.
Chapter 5, Codecs
At the center of all voice and video streams is the need to convert analog
data to digital for transmission across the network. A codec or coder/decoder
is the tool used for this purpose. The proper choice of codec can make the
difference between a successful rollout and one that leaves the users
questioning your ability. This chapter will spend time on both voice and
video codecs, their operation, and the decision process used in making the
correct choice.
Chapter 6, H.323 ITU-T Recommendation for Packet-Based Multimedia
Communications Systems
H.323 became the de facto standard for Internet Telephony mostly because it
was the early standard developed for video conferencing. Actually a protocol
suite containing subprotocols, H.323 saw wide deployment, which is the
reason for its inclusion here. Even though it is slowly being supplanted by
SIP, it is still quite common for practitioners to run into H.323, requiring
them to manage integration or conversion.
Chapter 7, Skinny Client Control Protocol
A Skinny is a proprietary signaling protocol from Cisco, and normally this
would exclude it from a book about standard network protocols. However,
there are millions of Cisco VoIP phones installed in networks around the
world. Even though Cisco is transitioning away from Skinny in favor of SIP,
network administrators should have a good handle on Skinny operation and
its idiosyncrasies. This chapter will cover the operation, messages, and
requirements of a basic Cisco topology.
Conventions Used in This Book
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TIP
This icon signifies a tip, suggestion, or general note.
CAUTION
This icon indicates a warning or caution.
Using Code Examples
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Acknowledgments
Good editors make good books. Thanks to the folks at O’Reilly, especially Andy
Oram and Maria Gulick, for helping me through the rough spots; someday I may
have a handle on XML and Docbook. And thanks for your patience as we
struggled with some other challenges. May the electronic revolution go easy on
you.
To Rachel B. Steely and Kristiana Burtness—gratitude for the copyediting from
the comma-challenged.
Good books also need good reviewers, and I would like to thank Mark Indelicato
and Chris Ward for helping out with some tough chapters. I would especially
like to thank Jason Burns for reading every page and providing invaluable input.
Your knowledge is impressive and willingness to help very much appreciated.
Dedication
Many thanks to my wife and family for continuing to put up with my writing
bug. Big hugs.
And here’s to the telecom folks trying to understand data and the data folks
trying to understand telecom—may you meet in the middle.