Voip basics

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Voip basics

  1. 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 tone10 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 802.3atSampling Uses LLDP to negotiate delivery of up8000 discrete signal measurements are taken at equal intervals every second to 25 watts in .10 W intervalsQuantizationThe level of each sample is rounded to the nearest expressible value IEEE 802.3af ClassesEncoding 0 15.4 W 3 15.4 WDigital values are encoded as binary numbers for encapsulation 1 4W 4 ReservedCompression (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 LLDPITU-T H.323 Voice and data VLANs communicated via CDP/LLDPOriginally designed for multimedia transmission over ISDN; matureand 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 150Text-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 theMedia Gateway Control Protocol (MGCP) TFTP servers specified in the DHCP optionEmploys centralized call control; defined in RFC 3661 5. RegistrationSkinny Client Control Protocol (SCCP) The phone registers with the call server(s) specifiedCisco-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 cosPackets 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

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