Voice Codecs |
|||||
MOS
|
Bandwidth
|
Complexity | Free | ||
| G.722 SB-ADPCM |
4.13
|
48-64 kbps
|
Medium
|
Yes
|
|
| G.711 PCM |
4.1
|
64 kbps
|
Low
|
Yes
|
|
| iLBC |
4.1
|
15.2 kbps
|
High
|
Yes
|
|
| G.729 CS-ACELP |
3.92
|
8 kbps
|
High
|
No
|
|
| G.726 ADPCM |
3.85
|
32 kbps
|
Medium
|
Yes
|
|
| G.729a CS-ACELP |
3.7
|
8 kbps
|
Medium
|
No
|
|
| G.728 LD-CELP |
3.61
|
16 kbps
|
High
|
No
|
Signaling Protocols |
|
| ITU-T H.323 Originally designed for multimedia transmission over ISDN; mature and widely supported; peer-to-peer call control |
|
| Session Initiation Protocol (SIP) Text-based, similar in nature to HTTP; defined in RFC 3261; peerto- 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 |
Calculating |
Required Bandwidth |
||
| G.711/Ethernet Example | |||
| Codec Payload (Bitrate × Sample Size) |
64 Kbps × 20 msec | 160 B | |
| L2 Overhead | Ethernet (18) + 802.1Q (4) | + | 22 B |
| L3 Overhead | IP (20) | + | 20 B |
| L4 Overhead | UDP (8) + RTP (12) | + | 20 B |
| Packets per Second | 1000 msec / 20 msec | × | 50 pps |
| Total Bandwidth | 88.8 Kbps |
Power Over Ethernet (PoE) |
|
| Cisco Inline Power (ILP) Pre-standard; employs a 340 kHz tone to detect devices; power needs communicated via CDP |
|
| IEEE 802.3af Detects power requirements of PoE device by the line resistance present |
|
| IEEE 802.3at Uses LLDP to negotiate delivery of up to 25 watts in .10 W intervals |
IEEE 802.3af Classes |
|||
0
|
15.4 W | 3 | 15.4 W |
1
|
4 W | 4 | Reserved |
2
|
7 W |
IP Phone Boot Process |
| 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 |
|
| 3. IP Assignment via DHCP The phone sends a DHCP request in the voice VLAN; the response includes an IP and DHCP option 150 |
|
| 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 |
| interface FastEthernet0/1 ! Configure data and voice access VLANs switchport access vlan switchport voice vlan ! Trust ingress QoS markings mls qos trust cos! Optionally pre-allocate power for the port power inline static [max |
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