Internet Technology – Voice over IP
Peter Gradwell
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BT Advert from 1980s
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Welcome to Gradwell
Gradwell provides technology for every line on your business card
Every product we sell solves a problem we had
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The TCP/IP Stack
Where does VoIP fit in?
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Different Types of VoIP
VoIP Comes in Many Flavours
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Voice Protocols
 All systems use some kind of application protocol to communicate
- SIP
- H323
- IAX (Inter Asterisk eXchange)
- Skinny (Cisco Protocol)
 Audio is transported as RTP
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TCP/UDP/Ports
 Traffic is either TCP or UDP
 MTU on Ethernet typically 1500 bytes
 100 bytes of headers, 1400 bytes of data
 Lots of packet overhead
Socket = IP Address + Port
MTU = Max Transfer
Unit/A Packet
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Good book:
 If you're interested in more details
 You can read most of it on amazon
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H.323
 Point to Point Protocol
 H.323 is actually a family of protocols
 H.323 is used a lot on trunk routes, as it's signalling reflects ISDN, but is
becoming obsolete
Modem over IP
Conferencing
Fax
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Inter-Asterisk eXchange
 Asterisk is a major OpenSource phone system
 IAX was invented by Mark Spencer/Digium
 Single UDP stream – port 4569
 Handles the Media inband on the same socket
- Can support multiple RTP streams
- Single signalling link
- Far more bandwidth efficient
- Also much friendlier for firewalls
- Pronounced “eeks”
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SIP – Session Initiation Protocol
SIP is the dominant VoIP protocol
It's a peer to peer protocol, unlike H.323
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SIP Addressing
 SIP is very similar to Email and HTTP
 URI Address format:
- sip:peter@gradwell.com
- sip:100@sip.gradwell.net:5060
- sip:441225800800@sip.gradwell.net
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Types of SIP Message
 INVITE—An INVITE method indicates that the recipient user or service is invited to participate in a
session.
 ACK - An ACK request confirms that the UAC has received the final response to an INVITE request.
- ACK is used only with INVITE requests. ACK is sent end to end for a 200 OK response.
 OPTIONS—A UA uses the OPTIONS request to query a UAS about its capabilities
 BYE—A UA uses BYE to request the termination of a previously established session.
 CANCEL—The CANCEL request enables UACs and network servers to cancel an in-progress
request, such as INVITE. This does not affect completed requests in which the UAS had already sent
final responses.
 REGISTER—A client uses a REGISTER request to register its current location information
corresponding to the AOR of the user with SIP servers.
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SIP Registration
 Phone registers periodically to tell the SIP server where it is
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SIP Call Flow
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A Complete Call
Often we'll draw
SIP call path
diagrams out like
this for debugging
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Debug using wireshark
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You can also do redirects
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Codecs
“Nyquist theorem states that if you sample an analog signal at twice the rate of the
highest frequency of interest, you can accurately reconstruct that signal back into
its analog form”
 Audio is sampled at 8k or 16k times per second
(Hertz) – Pulse Code Modulation
 Codecs trade off quality, bandwidth and CPU to
produce an RTP data packet
 Narrow Band (8k) or Wide Band (16k)
 We waste loads of Bandwidth on packet
overhead
 Audio transmitted as RTP
Codec
G.711
G.729
G.723.1
G.723.1
G.726
G.726
G.728
iLBC
BR
64 Kbps
8 Kbps
6.4 Kbps
5.3 Kbps
32 Kbps
24 Kbps
16 Kbps
15 Kbps
NEB
87.2 Kbps
31.2 Kbps
21.9 Kbps
20.8 Kbps
55.2 Kbps
47.2 Kbps
31.5 Kbps
27.7 Kbps
BR = Bit rate
NEB = Nominal Ethernet
Bandwidth (one direction)
Skype uses iLBC and ISAC
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Jitter & Packet Loss
 “Jitter is the variation of packet interarrival time”
 Jitter is a major problem
- The internet is asymmetrical
- Manage the problem with a Jitter buffer ~ 20ms
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Quality of Service
QoS is needed if a network link is congested
Routers need to enforce traffic prioritisation across
their links
How do they decide?
- Trust the user? - DiffServ, Expedited Forwarding and
Low Delay bits in IP header
- Trust the network – all traffic on port X or from IP
address Y is special
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Shift Change in the handset industry
What about Mobile Handsets?
 SIP over WiFi has a very bad user
experience – Still difficult to setup
 GSM Calls “Just work”
 Costs of calling can be fixed
commercially – e.g. All inclusive call
plans
 Operators are not keen on offloading
voice minutes onto their data networks
 iPhone & Google Voice are changing
things
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Which Mobile App Should I run?
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Google Voice Video
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What is ENUM
 User ENUM
- Allows users to define how calls should be routed to their numbers
- Stores telephone numbers in DNS and routing records
 ENUM = E.164 NUmber Mapping (E164 is an ITU standard)
 Needs a new ‘root’
- .e164.arpa
- Managed by RIPE
 Numbers stored in reverse
- +44 1865 332211 becomes
- 1.1.2.2.3.3.5.6.8.1.4.4.e164.arpa
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What is ENUM
 DNS uses a special form of record
- Name Authority Pointer (NAPTR)
- RFC2915
 NAPTR contains a URI
- Information on how to route call
- Can specify multiple ways to call
- VoIP (SIP/H323)
- Email
- Tel(ephone)
- Fax
 NAPTR records can be very sophisticated
- Can contain time of day and other information
- Can contain regular expressions so say a SIP record relates to the ENUM data
- (Generic ENUM records for a telco)
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What is ENUM
$ORIGIN 2.1.2.1.5.5.5.0.7.7.1.e164.arpa.
IN NAPTR 100 10 "u" "E2U+sip" "!^.*$!sip:information@pbx.example.com!i" .
IN NAPTR 102 10 "u" "E2U+email" "!^.*$!mailto:information@example.com!i"
 This is a record for +1 770 555 1212
 100 / 102 is priority
- Lowest number, highest priority
 10 is a preference level
- Can be ignored here as both records 10
 “u” is a user record i.e. stop processing
 E2U+sip is the way of writing it’s a SIP record
 Regular expression and the final destination
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ENUM call
Company A
Dials +44 987 654321
Company B
PSTN
+44 123 456789
+44 987 654321
SIP:25@compB.co.uk
SIP:01@A.co.uk
Internet
No ENUM
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ENUM call
Company A
Dials +44 987 654321
Company B
PSTN
+44 123 456789
+44 987 654321
SIP:25@compB.co.uk
SIP:01@A.co.uk
+1.2.3.4.5.6.7.8.9.44.164.arpa
ENUM
Internet
D
N
S
+44 987 654321
SIP:25@compB.co.uk
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ENUM in production
 Asterisk supports ENUM “out the box”
 OpenSIPS has an ENUM module
 Various PBX suppliers can support ENUM though may require special
configuration
 Nominet have developed an Android application Enumdroid
- Integrated into dialer
 ENUM in the UK is currently looking for users
- www.nominet.org.uk/enum/
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Enumdroid – real life app
• Developed by Nominet’s Ray Bellis
– published as open source.
– Available on the Android Marketplace
• Dial the number of your contact
• Application performs an ENUM
query and Presents the user the
options available
• Caller make decision of call method
and places the call.
• Presents a number of options
– Has flexibility to turn off GSM if necessary
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Enumdroid – real life app
• Developed by Nominet’s Ray Bellis
– published as open source.
– Available on the Android Marketplace
• Dial the number of your contact
• Application performs an ENUM
query and Presents the user the
options available
• Caller make decision of call method
and places the call.
• Presents a number of options
– Has flexibility to turn off GSM if necessary
www.gradwell.com | 01225 800 800 | info@gradwell.com
Enumdroid – real life app
• Developed by Nominet’s Ray Bellis
– published as open source.
– Available on the Android Marketplace
• Dial the number of your contact
• Application performs an ENUM
query and Presents the user the
options available
• Caller make decision of call method
and places the call.
• Presents a number of options
– Has flexibility to turn off GSM if necessary
www.gradwell.com | 01225 800 800 | info@gradwell.com
Enumdroid – real life app
• Developed by Nominet’s Ray Bellis
– published as open source.
– Available on the Android Marketplace
• Dial the number of your contact
• Application performs an ENUM
query and Presents the user the
options available
• Caller make decision of call method
and places the call.
• Presents a number of options
– Has flexibility to turn off GSM if necessary
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ENUM Conclusions
Story so far
 ENUM allows interoperability between
carriers
 More than just voice
 Most applications can be completed
without “ENUM”, if you control the
network & devices
 It needs a lot of end user applications
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Voice Over IP?
Where are we at with VoIP?
 Protocols and services are well established & here to stay
 Lines (no pun intended) are bluring
 Network convergence is actually happening
 No one wants a desk phone
 Some of the mobile problems will be fixed commercially
 Some problems by changes in end user behaviour and open access to
networks
 The desktop environment is quickly unifying
 Telcos still like building islands of subscribers
 Regulator is looking at mobile number portability
 ENUM gives end-user control, but no applications or drive for it
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Any Questions?
Peter Gradwell
T:
E:
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