VOIP_Basics.pdf

(76 KB) Pobierz

VOIP B ASICS

packetlife.net

Pulse Code Modulation (PCM)

Power Over Ethernet (PoE)

Cisco Inline Power (ILP)

Pre-standard; employs a 340 kHz tone

to detect devices; power needs

communicated via CDP

13.6

13.5

14

12

10

8

6

4

2

0

14

12

10

8

6

4

2

0

14

12

10

8

6

4

2

0

12.4

9.2

12.3

9.1

6.0

5.9

IEEE 802.3af

Detects power requirements of PoE

device by the line resistance present

2.8

2.7

0.9

1.0

Sampling

Quantization

Encoding

IEEE 802.3at

Uses LLDP to negotiate delivery of up

to 25 watts in .10 W intervals

Sampling

8000 discrete signal measurements are taken at equal intervals every second

Quantization

The level of each sample is rounded to the nearest expressible value

Encoding

Digital values are encoded as binary numbers for encapsulation

Compression (Optional)

The digital signal is compressed in real time to consume less bandwidth

IEEE 802.3af Classes

0 15.4 W

3 15.4 W

1 4 W

4 Reserved

2 7 W

Voice Codecs

IP Phone Boot Process

MOS

Bandwidth

Complexity Free

3

G.722 SB-ADPCM 4.13

48-64 kbps Medium

Yes

G.711 PCM 4.1

64 kbps

Low

Yes

2

iLBC 4.1

15.2 kbps

High

Yes

5

4

G.729 CS-ACELP 3.92

8 kbps

High

No

1

G.726 ADPCM 3.85

32 kbps

Medium

Yes

G.729a CS-ACELP 3.7

8 kbps

Medium

No

Call Server

TFTP Server

G.728 LD-CELP 3.61

16 kbps

High

No

1. Power Over Ethernet (Optional)

Power is supplied via IEEE 802.3af/at or Cisco ILP

2. VLANs Learned via CDP or LLDP

Voice and data VLANs communicated via CDP/LLDP

Signaling Protocols

ITU-T H.323

Originally designed for multimedia transmission over ISDN; mature

and widely supported; peer-to-peer call control

3. IP Assignment via DHCP

The phone sends a DHCP request in the voice VLAN;

the response includes an IP and DHCP option 150

Session Initiation Protocol (SIP)

Text-based, similar in nature to HTTP; defined in RFC 3261; peer-

to-peer call control

Media Gateway Control Protocol (MGCP)

Employs centralized call control; defined in RFC 3661

Skinny Client Control Protocol (SCCP)

Cisco-proprietary; limited support on gateways; centralized control

4. Configuration Retrieved via TFTP

The phone retrieves its configuration from one of the

TFTP servers specified in the DHCP option

5. Registration

The phone registers with the call server(s) specified

in its configuration

Calculating Required Bandwidth

Access Switch Port Configuration

G.711/Ethernet Example

interface FastEthernet0/1

Codec Payload

(Bitrate ċ Sample Size)

64 Kbps ċ 20 msec

160 B

! Configure data and voice access VLANs

switchport access vlan <VLAN>

switchport voice vlan <VLAN>

L2 Overhead Ethernet (18) + 802.1Q (4)

+

22 B

L3 Overhead IP (20)

+

20 B

! Trust ingress QoS markings

mls qos trust cos

L4 Overhead UDP (8) + RTP (12)

+

20 B

Packets per Second 1000 msec / 20 msec

ċ

50 pps

! Optionally pre-allocate power for the port

power inline static [max <wattage>]

Total Bandwidth

88.8 Kbps

by Jeremy Stretch

v1.0

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