The VoIP phones themselves take the analogue speech input through the mouthpiece or headset and convert it into one of a number of recognized digital formats. The device that does this is referred to as a CODEC (Coder / Decoder).
Each VoIP device such as an IP phone will have a unique MAC Address configured by the vendor before shipping. The first 3 bytes of a MAC Address is known as the OUI (Organizationally Unique Identifier), and each vendor will have purchased an allocation from the IEEE. Because the OUI is deterministic, the Ethernet switch to which the device is attached can be configured to recognize voice traffic by means of this OUI and so ensure that frames with this OUI are added to the appropriate Voice VLAN. See below for a list of common OUIs associated with some vendor VoIP equipment:
00:03:6B - Cisco
00:E0:BB - 3Com
00:01:E3 - Siemens
00:E0:75 - Polycom
00:60:B9 - NEC/Philips
Taking Cisco as an example, any access port can be configured to carry VoIP and VoIP control traffic. The voice traffic needs to be prioritized if it is to be given precedence over less critical traffic. In order to accommodate this, most Cisco IP phones can mark the packetized voice traffic at either layer 2 using CoS (Class of Service), layer 3 using IP precedence or DiffServ (Differentiated Services) or both. Most Cisco IP phones have an integrated 3 port 10/100 Ethernet switch. One port connects the VoIP phones to the switch, a second port is an internal port designed to carry the VoIP traffic and a final port is an access port, allowing a device such as a workstation to be ‘Piggybacked’ on the back of the IP phone.
Using the Voice VLAN feature a single access port on the switch can configured with 2 VLANs, one for the voice traffic and another for traffic passing through the IP phone from an attached workstation. Most Cisco switches are able to process traffic tagged at Layer 2 with IEEE 802.1p (CoS), Layer 3 IP precedence or Layer 3 DiffServ.
The access ports on the switch can be configured to send CDP (Cisco Discovery Protocol) packets to the IP phone instructing the phone to either configure the phone’s access port in trusted mode or untrusted mode.
In trusted mode, the traffic from the piggybacked device passes through the IP phone to the access layer switch without any CoS or IP precedence changes.
In untrusted mode the phone will reconfigure the CoS, IP Precedence or DiffServ values according to what is required of the traffic from the attached device. If no configuration is made on the phone then the CoS value for this traffic is 0.
In summary, a switch with Layer 2 access ports can identify traffic coming from an IP phone and place that traffic in a predetermined Voice VLAN and any pass-through traffic into a data VLAN. The switch can then act upon any Class of Service or Quality of service markings within those VLANs
Voice VLANs are discussed on a number of our Training Courses.