This document discusses VoIP basics including: 1. Pulse code modulation and power over Ethernet standards like 802.3af and 802.3at that provide power to IP phones. 2. Common voice codecs like G.711, G.729, G.722 and their bandwidth requirements. Signaling protocols for VoIP calls include H.323, SIP, and MGCP. 3. The typical IP phone boot process which involves getting an IP address via DHCP, retrieving configuration via TFTP, and registering with call servers.

1. VOIP BASICS packetlife.net
Pulse Code Modulation (PCM) Power Over Ethernet (PoE)
14 13.6 13.5 14 14 Cisco Inline Power (ILP)
12
12.3
12.4 12 12 Pre-standard; employs a 340 kHz tone
10 9.2 10 10 to detect devices; power needs
8 9.1 8 8 communicated via CDP
6 6.0 5.9 6 6
4
2.8 2.7
4 4 IEEE 802.3af
2 0.9 1.0
2 2 Detects power requirements of PoE
0 0 0
device by the line resistance present
Sampling Quantization Encoding
IEEE 802.3at
Sampling Uses LLDP to negotiate delivery of up
8000 discrete signal measurements are taken at equal intervals every second to 25 watts in .10 W intervals
Quantization
The level of each sample is rounded to the nearest expressible value IEEE 802.3af Classes
Encoding 0 15.4 W 3 15.4 W
Digital values are encoded as binary numbers for encapsulation
1 4W 4 Reserved
Compression (Optional)
The digital signal is compressed in real time to consume less bandwidth 2 7W
Voice Codecs IP Phone Boot Process
MOS Bandwidth Complexity Free
G.722 SB-ADPCM 4.13 48-64 kbps Medium Yes 3
G.711 PCM 4.1 64 kbps Low Yes
iLBC 4.1 15.2 kbps High Yes 2
5
G.729 CS-ACELP 3.92 8 kbps High No 1 4
G.726 ADPCM 3.85 32 kbps Medium Yes
G.729a CS-ACELP 3.7 8 kbps Medium No TFTP Server Call 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
Signaling Protocols
2. VLANs Learned via CDP or LLDP
ITU-T H.323 Voice and data VLANs communicated via CDP/LLDP
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;
Session Initiation Protocol (SIP) the response includes an IP and DHCP option 150
Text-based, similar in nature to HTTP; defined in RFC 3261; peer-
to-peer call control 4. Configuration Retrieved via TFTP
The phone retrieves its configuration from one of the
Media Gateway Control Protocol (MGCP) TFTP servers specified in the DHCP option
Employs centralized call control; defined in RFC 3661
5. Registration
Skinny Client Control Protocol (SCCP) The phone registers with the call server(s) specified
Cisco-proprietary; limited support on gateways; centralized control in its configuration
Calculating Required Bandwidth Access Switch Port Configuration
G.711/Ethernet Example
interface FastEthernet0/1
Codec Payload
64 Kbps × 20 msec 160 B
(Bitrate × Sample Size) ! Configure data and voice access VLANs
L2 Overhead Ethernet (18) + 802.1Q (4) + 22 B switchport access vlan <VLAN>
switchport voice vlan <VLAN>
L3 Overhead IP (20) + 20 B
! Trust ingress QoS markings
L4 Overhead UDP (8) + RTP (12) + 20 B mls qos trust cos
Packets per Second 1000 msec / 20 msec × 50 pps
! Optionally pre-allocate power for the port
Total Bandwidth 88.8 Kbps power inline static [max <wattage>]
by Jeremy Stretch v1.0