Voice over IP Fundamentals

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26 Οκτ 2013 (πριν από 3 χρόνια και 9 μήνες)

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Voice over IP
Fundamentals

M.
Arvai

NEC Senior Technical Eng.

1

Agenda


Switching Telephony Network Fundamentals

o
Overview

o
Enterprise Telephony

o
Telephony Signaling

o
PBX Features




VoIP and IP Telephony Technology

o
Data Network Basics

o
VoIP Basics

o
VoIP Compression

o
VoIP Signaling

o
Networking


2

Telephony Network

3

Privately Owned Switches

4

Basic Call Setup

5

Digital versus Analog Connections

6

E1 Channel Associated Signaling

E1 framing and signaling, 30 of the 32 available channels, or
time slots, are used for voice and data



7

Q.Sig


The QSIG (Q Signaling) protocol is based on the
standard
and provides signaling for private
integrated services network exchange
devices.



QSIG is implemented on PRI interfaces only. By
using QSIG PRI signaling, a
Router can
route
incoming voice calls from a PINX across a WAN
to a peer
Router,
which can then transport the
signaling and voice packets to a second PINX

8

VoIP Advantages


Flexibility

The
sophisticated functionality of IP networks
allows organizations to be flexible in the types of
applications and services they provide to their customers
and users.



Advanced
features

o
Advanced call routing

o
Unified messaging

o
Integrated information systems

o
Long
-
distance toll bypass

o
Encryption

o
Customer relationship


9

VoIP Protocols


H.323

An ITU standard protocol for interactive
conferencing. H.323 was originally designed for
multimedia in a connectionless environment,
such as a LAN.



Media Gateway Control Protocol (MGCP)
defines
a protocol to control VoIP gateways that are
connected to external call
-
control devices,
referred to as call agents

10

VoIP Protocols


Session Initiation Protocol (SIP)
A detailed
protocol that specifies the commands and
responses to set up and tear down calls. SIP also
details features such as security, proxy, and
transport (TCP or User Datagram Protocol [UDP])
services.
SIP
defines end
-
to
-
end call signaling
between
devices.It

also adopts a modified form
of the URL
-
addressing scheme used within e
-
mail
that is based on Simple Mail Transfer Protocol
(SMTP).

11

VoIP Protocols


Real
-
Time Transport Protocol (RTP)
An IETF
standard media
-
streaming protocol. RTP carries
the voice payload across the network. RTP
provides sequence numbers and time stamps for
the orderly processing of voice
packets



RTP Control Protocol (RTCP)
Provides out
-
of
-
band
control information for an RTP flow

12

Issues and Solutions for VoIP


Latency

o
Increase bandwidth.

o
Choose a different CODEC type.

o
Fragment data packets.

o
Prioritize voice packets.


Bandwidth

o
Calculate bandwidth requirements, including voice,
payload, overhead, and data
.


Jitter

o
Use
dejitter

buffers
.



13

VoIP Network Architectures


Centralized Network Architectures



IP Net.

14

VoIP Network Architectures


Distributed
Network Architectures



IP Net.

15

Comparing Network Architectures


Configuration


o
The
centralized call control model provides superior
control of the configuration and maintenance of the
dial plan and endpoint database. It simplifies the
introduction of new features and supplementary
services. The centralized call control model also
provides a convenient location for the collection and
dissemination of call detail records (CDRs).

o
The distributed model requires distributed administration
of the configuration and management of endpoints.
This approach complicates the administration of a dial
plan. Distributed call control simplifies the deployment
of additional endpoints while making new features and
supplementary services difficult to implement
.

16

Comparing Network Architectures


Security

Centralized call control requires that endpoints
be known to a central authority. This approach avoids or
reduces security
concerns. The
autonomy of endpoints in
the distributed model elevates security concerns
.



Efficiency

Centralized call control fails to take full
advantage of call routing intelligence that resides in the
endpoints. It also consumes bandwidth through the
interaction of the call agent and its endpoints.


17

Comparing Network Architectures


Reliability

o
The
centralized model has two points of vulnerability:
single point of failure and contention. It places high
demands on the availability of the underlying data
network, possibly requiring a fault
-
tolerant WAN design.

o
The distributed call control model minimizes the
dependence on shared common control components
and network resources. This approach reduces
exposure to single points of failure and contention for
network resources.



18

Building Scalable Dial Plans


Numbering plan
:
A
numbering plan identifies each
VoIP endpoint and application in the network with a
unique telephone number.


Dial plan
: is
a key element of an IP telephony system and
an integral part of all call
-
processing agents.
Primary
functions of a dial plan include
:

o
Endpoint addressing

o
Path selection

o
Calling privileges

o
Digit manipulation

o
Call coverage

o
Overlapping number processing



19

Building Scalable Dial Plans

Hierarchical Numbering
Plans
A hierarchical design has
the following advantages
:


o
Simplified provisioning Refers to the ability to easily add new
groups and modify existing
groups

o
Simplified
routing Keeps local calls local and uses a
specialized number, such as an area code, for long
-
distance
calls

o
Summarization Establishes groups of numbers in a specific
geographical area or functional group

o
Scalability Provides additional high
-
level number groups

o
Management Controls number groups from a single point in
the overall network



20

Building Scalable Dial Plans

The challenges faced with numbering plan
integration include the following
:



Varying number lengths


Specialized
services


Voice
mail


Necessity
of prefixes or area codes


International
dialing
consideration



21

Signaling and Call Control


In
the traditional telephony network, a voice call
consists of two
paths:

o
An
audio path carrying the
voice

o
A
signaling path carrying administrative information
such as call setup, teardown messages, call status, and
call
-
progress signals
.


VoIP call leg requires two paths:

o
A protocol stack that includes RTP, which provides the
audio call leg

o
One or more call control models that provide the
signaling path


22

Signaling and Call Control

Call Control
Models


o
H.323

describes the architecture to support multimedia
communications over networks without quality of
service (
QoS
) guarantees
.


o
Session initiation protocol (SIP)
is
an Internet Engineering
Task Force (IETF) RFC 3261 call control model for
creating, modifying, and terminating multimedia
sessions or calls.


23

Signaling and Call Control

Call
Setup

24

Optimizing Voice Quality

Factors that Affect Voice
Quality


o
Fidelity
The bandwidth of the transmission medium
almost always limits the total bandwidth of the spoken
voice
.


o
Echo

A result of electrical impedance mismatches in
the transmission path.


o
Jitter
Variation in the arrival of coded speech packets
at the far end of a VoIP network
.


o
Packet drops
The discarding of voice packets
.




25

Optimizing Voice Quality

Factors that Affect Voice
Quality


o
Delay

The time between the spoken voice and the
arrival of the electronically delivered voice at the far
end
.


o
Sidetone

The purposeful design of the telephone that
allows the speaker to hear the spoken audio in the
earpiece
.


o
Background noise
The low
-
volume audio that is heard
from the far
-
end connection.




26

Thank You

Questions ?


a
rvahi

@ nec
-
unified.ir



27