Điện tử viễn thông c1 6 thoại qua IP (VoiceOverIP) khotailieu

TRƯỜNG ĐẠI HỌC ĐIỆN LỰC
KHOA ĐIỆN TỬ - VIỄN THÔNG

TỔNG ĐÀI &
KỸ THUẬT CHUYỂN MẠCH

Chương 1: Lý thuyết tổng quan và cơ sở
VI. Thoại qua IP (VoiceOverIP)

Giảng viên: Phan Thị Thanh Ngọc

1

EPU - 2012


Multi-media & Data Integration
Benefits
• Productivity increases
– Voice and even multi-media communications become part of
office automation and web environment (“click to talk”).
• Infrastructure savings
– PABX and separate telephone wiring replaced by additional
capacity requirements on data network
– Reduced external telecommunication costs
– Maintenance and management reduced to one single network
• Increased reliability
– PABX single point of failure could be replaced by distributed
software
2

J.Tiberghien - VUB


The Multi-media challenge
Traffic characteristics
Throughput:
Correctness:
Delay:

Data

Voice & Images

Bursty
Essential
Unimportant

Stable
Non-essential
Small & Stable

Conclusion:
Data and multi-media traffic have
totally different requirements
Mixing them on a single network is
technically and economically challenging
3

J.Tiberghien - VUB

Multi-media & Data Integration
Challenges
• Control over delays in IP networks
? Enough network capacity
? GMPLS
? IP V6
? New transport protocols
• Reliability and availability culture in data networks
• Power via data cables for IP phones
• Organizational and cultural changes
• Addressing and Directory management
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J.Tiberghien - VUB


Real Time Voice over the Internet
The initial goal : free international calls

Internet
Sound enabled PC
(full duplex)

Sound enabled PC
(full duplex)

- To communicate, precise appointments need to
be made, ... by phone ?!?
- IP addresses on both sides need to be known

- Voice quality dependent on network delays
- Acceptable if network not overloaded
5

J.Tiberghien - VUB


Internet Telephony
Low cost
POTS
emulation
Local
PSTN

Internet Telephone
Gateways

Local
PSTN

Internet

+ Access through local PSTN
- Quality dependant from network load
6

J.Tiberghien - VUB


Internal Communications

• Internal communications in many organizations :
– voice : PABX
– data : intranet
• If geographically separated sites :
– voice : PABX’s interconnected via
» leased lines
» public telephone network
– data : Intranets interconnected via
» leased lines
» Virtual Private Networks
7

J.Tiberghien - VUB


Traditional Communications
in multi-site organizations
WAN
Intranet

PABX

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PSTN

PABX

J.Tiberghien - VUB

PABX & Intranet Integration
for traditional POTS and data
Transport costs lower
for data than for
telephony !

PSTN

PABX

WAN
Intranet

PABX

PSTN

PABX needs to be
IP compatible
9

J.Tiberghien - VUB


Full voice-data Integration
Infrastructure savings
+
Productivity gains

WAN or LAN
Intranet

What about the 109 pots users ???
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J.Tiberghien - VUB


Full voice-data Integration
Infrastructure savings
+
Productivity gains

WAN or LAN
Intranet

PSTN
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H231 Gateway

J.Tiberghien - VUB


The role of SIP
• Voice or multimedia (= voice, images, streaming music and
video, streaming computer data, …) communication implies a
“connection” between two or several people.
• The host addresses and the port numbers of the appropriate

applications that need to be connected to enable these people
to communicate need to be identified.
• The nature and the characteristics of the communication
between these applications needs to be negotiated
• The connection needs to be established
• The connection needs to be maintained, even when the address
of some partners change (mobility)
• Mobility might imply renegotiation of the communication
characteristics
• When no longer needed, the connection needs to be closed.
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J.Tiberghien - VUB


Addresses & Directories
• Addresses are hard to memorize numbers
– Tel :

+32 (0) 2 673 64 76

– Gsm :

+32 (0) 475 81 27 93

– ip :

134.184.1.25

– ipV6:

64.0.0.0.0.0.0.0.0.0.0.0.134.184.1.25

• Need of directories, but hard to maintain consistency
• Internet solution :
– DNS = distributed directory
– Local addresses maintained by local authority
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J.Tiberghien - VUB


Internet Domains
Root
uk
ibm

be

edu

ulb
tiberghien


org
ieee

mtv

ac

vrt

com

vub
info
tiberghien


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J.Tiberghien - VUB


Domain Name Servers
• In Each domain, there is at least one DNS
– Name & DNS Address of all subdomains
– Name & address of all hosts in the domain
– DNS address of, at least, root of DNS system
– Names & DNS addresses of frequently used domains
• Each host must know the address of one DNS
• Address resolution can be done recursively or by
successive calls to different DNS
• Local DNS is locally maintained
• Caching greatly improves performance of DNS by
keeping the translations of most recently accessed
domains and hosts
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J.Tiberghien - VUB


Names vs. Addresses
• A name identifies a service
– The mail service of a specific person (mailbox)
– The web service of a specific company
• An address specifies a host on which services are available
• Several names can be linked with a single address
–
–
• Several addresses can be linked with a single name
– Redundant web servers
• DNS assumes stable links between names and addresses
– Machines offering services have stable addresses
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J.Tiberghien - VUB


Beyond DNS
• For some applications DNS is insufficient
– Internet telephony = person to person service
– At different times, people are at different locations
– Personal computers seldom have unique address
– One person can be reached through different devices, with
different service levels.
• Session Initiation Protocol (SIP, RFC 3261)
– A SIP server is an agent that handles multi-media services

for one or several people:
» As any server, it has a stable address
» It knows the present address of the people it works for
» It establishes and manages connections between multimedia communicating applications.
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J.Tiberghien - VUB


Session Initiation Protocol


The SIP server can handle both diversity
in terminals and mobility of terminals
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J.Tiberghien - VUB


SIP functionality
•
•
•
•
•
•
•
•
•
•

•
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Call forwarding, unconditional, on busy, …
Call transfer
Caller ID
Call hold
Multiparty conferences
Call return
Follow me
Find me
Call queuing
Do not disturb
……
J.Tiberghien - VUB


SIP in Redirection mode
SIP
Redirection
server

Callers network

Home network
Actual
Data transfer

Session
set-up

• Most efficient data transfer
• Caller has access to actual
called address
20

Mobility
management

Visited network
J.Tiberghien - VUB


SIP in Proxy mode
Callers network

Session
set-up

SIP
Proxy
server

Home network
Actual
Data transfer

• Longer transmission delays
• SIP server can be bottleneck
• Caller has no access to the

actually called address
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Mobility
management

Visited network
J.Tiberghien - VUB


SIP properties
• SIP is user friendly
– People accessed through their URLs
»
»
• SIP is well in line with Internet technology
– Commands in clear text (html)
– Few different commands
– Readable answers to commands
– Details of desired session described with SDP.

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J.Tiberghien - VUB


SIP Requests
• INVITE
Invite user to a session. The session
description is contained in the body of the message,

using SDP. The session description contains the
address where the host wants to receive media streams.
• ACK
Acknowledgment of an INVITE request
• BYE
Sent when a call is to be released
• OPTIONS
Query server about capabilities
• CANCEL
Cancel a pending request
• REGISTER
Register with a SIP server

Each SIP transaction has a unique identifier,
which identifies the session.
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J.Tiberghien - VUB


SIP references
• The initial designers:
www.cs.columbia.edu/sip/
• SIP Forum :
www.sipforum.org
• SIP Charter :
www.ietf.org/html.charters/sip-charter.html

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J.Tiberghien - VUB


The Universal IP Network ?

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J.Tiberghien - VUB