Transcript Voice over IP

Voice over IP
IP Networking Overview
• IP assumes delay and packet ordering problems
Voice Delay with IP
One-Way, Mouth-toEar Delay
Description
1-150 ms
Suitable for most delay sensitive applications like
voice.
150-400 ms
Still acceptable, but starts to influence voice
quality. Communication can be annoying if no
actions are taken.
400 ms and greater
Unacceptable for most delay sensitive
applications.
• Delay budget deals with
one-way delay, not a round-trip!
Understanding VoIP
Major VoIP Protocols
VoIP Protocol
Description
H.323
• ITU standard protocol for interactive conferencing.
• Evolved from H.320 ISDN standard.
• Flexible, complex.
Media Gateway Control Protocol (MGCP)
• Emerging standard for Public Switched Telephone Network
(PSTN) gateway control, thin device control.
Session Initiation Protocol (SIP)
• IETF protocol for interactive and non-interactive conferencing.
• Simpler, but less mature, than H.323.
Real-Time Transport Protocol (RTP)
• IETF standard media streaming protocol.
Real-Time Transport Control Protocol
(RTCP)
• Protocol that provides out-of-band control information for an
RTP flow.
Low Latency Queuing (LLQ)
• Priority queuing technique that uses priority
queuing-class-based weighted fair queuing (PQ-CBWFQ).
Signaling from PBX to the
Router
The PBX seizes a trunk line to the router,
and forwards dial digits.
Signaling Between Routers
The Dial Plan Mapper reads the dial digits, and finds
the address of the remote IP peer. The H.323 agent
initiates a Q.931 call to the remote peer.
Signaling from Router to PBX
The remote H.323 agent seizes a PBX trunk,
returns a Q.931 acknowledgment to the origin,
and forwards dial digits to the PBX
VoIP Bandwidth Consumption
Coding
Algorithm
Voice
Bandwidth
(Kbps)
Coded
Frame
Size
(bytes)
Voice
Frames in
VoIP
Packet
Layer 2
Technology
Used
Layer 2
Header
Size
(bytes)
Total
Bandwidth
Required
G.711
64
160
2
40
Ethernet II
26
90.4
G.711
64
160
2
40
MLPPP
7
82.8
G.711
64
160
2
2 (cRTP)
MLPPP
7
67.6
G.729
8
20
2
40
Ethernet II
(Cisco
Default)
26
34.4
G.729
8
20
2
40
MLPPP
7
26.8
G.729
8
20
2
2 (cRTP)
MLPPP
7
11.6
IP+UDP+RT
P Header
Size (bytes)
VoIP QoS Building Blocks
IP Network Issue
Devices
Affected
Solutions
Backbone speed and
scale
• Backbone routers
• High-performance routers
Packet classification
• Edge routers
• IP Precedence
Bandwidth
management and
admission control
• Edge routers
• Voice compression
• Real-Time Transport Protocol (RTP)
header compression
• Gatekeepers
Congestion
management
• Edge routers
• Backbone routers
• Weighted Random Early
Detection (WRED)
Queue management
• Edge routers
• Backbone routers
• Low Latency Queuing (LLQ)
• Other queuing techniques
RTP Header Compression
RTP Headers are
almost always 2
Bytes long when
compressed.
RTP header compression saves bandwidth
by compressing packet headers across WAN
links.
Low Latency Queuing
Configuring IP Precedence
Precedence
Definition
7
Network control
6
Internetwork control
5
Critical
4
Flash-override
3
Flash
2
Immediate
1
Priority
0
Routine
Example: VoIP over
PPP Configuration