Scenario
With the continual development of technology, IP phone is being used more and more widely. It converts analog signals into digital signals which are transmitted over the IP network to the receiver. Then, upon the receipt of data packets, the receiver converts digital signals back to the analog signals. Combined other voice devices, IP phone offers high capacity and low cost voice communications solution for users.
Voice VLAN is specially designed for voice streams. By creating a voice VLAN and adding the ports connecting voice devices to the voice VLAN, users can centrally transmit voice streams in the voice VLAN, and configure QoS specific for voice streams to improve the priority of voice stream transmission and ensure voice quality.
Basic network
Following figure displays the basic network of Voice VLAN:
There are usually two types of connection (see Figure above) between IP phones and Voice VLAN:
1) IP phones separately access to Voice VLAN with only voice streams transmitted. This type of connection is usually applied to IP phones arrangement in meeting rooms or occasions where PC is not necessary for data processing.
2) PC and IP phones form a daisy chain to access the network with voice and data streams transmitted. In this case, voice streams and data streams are transmitted in voice VLAN and data VLAN respectively. Generally, this type of connection is applied when office clerks need to conduct both data communication with PC and voice communication with IP phones.
Voice VLAN-enabled device determines whether the packet is the voice stream to the specific voice device by matching the source MAC address of incoming packet with the OUI (Organizationally Unique Identifier) of the voice device. If so, the packet is partitioned into the voice VLAN for transmission.
Auto mode and manual mode of Voice VLAN
A port can work in the auto mode or manual mode of voice VLAN with different join methods.
1) Auto mode：When a subscriber runs an IP phone and sends protocol packet through a voice VLAN-enabled equipment, the equipment identifies the source MAC address of protocol packet and matches the MAC address with the OUI address set on the switch. If succeed, the equipment will automatically add the input port of the voice message to Voice VLAN and issue the policy to modify the priority of voice message as the one voice stream of voice VLAN configured on the equipment. Meanwhile, the subscriber may set Voice VLAN aging time on the equipment. When no voice message is received from the input port within the aging time, the system will delete the port from Voice VLAN. Adding or deleting a port to or from voice VLAN is automatically executed by the system. Port aging mechanism may prevent the speech equipment port out of use for a long time from remaining in Voice VLAN.
2) Manual mode：Users manually add the IP phone connected port to the voice VLAN on voice VLAN supported
Equipment. In the course IP phone communication, the equipment identifies the source MAC Address of data packet and matches the MAC addresswith the OUI address of the configured voice VLAN. If succeed, the equipment will automatically add the input port of the voice message to Voice VLAN and issue the policy to modify the priority of voice message as the one voice stream of voice VLAN configured on the equipment. In manual mode, adding or deleting a port to or from voice VLAN is done by administrators manually. Auto mode is better for PC--IP phone framework that transmit voice data and pc data at the same time.
Manual mode is better for only IP phone framework that transmit voice data only.
The way Voice VLAN and IP phone works
Generally speaking, there are two kinds of IP phones by the way to obtain IP address and voice VLAN message.
1) Automatically acquire IP address and Voice VLAN numbers. This kind of IP phones sends tagged or untagged voice streams.
2) Manually configure IP addresses and Voice VLAN numbers. This kind of IP phones can only send tagged voice streams.
IP phone working principle
The same to other network device, IP phone need an IP address to work in network. There're 2 ways to aquire IP address:
1) DHCP automatically
2) Configuration manually
When request IP address from DHCP Server automatically, IP phone request for Voice VLAN info at the same time. IP phone begin to send voice stream with Voice VLAN tag once DHCP Server return Voice VLAN info and send voice stream without Voice VLAN tag if DHCP doesn't return Voice VLAN info.
If user set IP phone IP address and Voice VLAN info manually, IP phone may send voice stream with tagged or untagged based on the manual configuration.
Voice VLAN safe mode
Safe mode is available for separate transmission of voice streams and data streams. When safe mode is enabled, Voice VLAN only permits voice stream transmission. Only the streams whose source MAC address matches the OUI address of voice VLAN are allowed to transmit in voice VLAN, others are dropped. When safe mode is disabled, the source MAC address of streams will not be checked, and all streams will be permitted to transmit within Voice VLAN.
Voice VLAN Working Principles
By transmitting data streams and voice streams in the data VLAN and the voice VLAN respectively, the voice VLAN-supported equipment avoids mutual influence between them. Meanwhile, the equipment issues priority policy to improve the priority of voice streams and guarantee session quality. The basic working principle is described as below:
Step 1: The user creates on the equipment one VLAN dedicated to transmitting voice packets, i.e., Voice VLAN, and enables Voice VLAN function on the port that connects with IP phone.
Step 2: As a key step, the port that connects with IP phone joins Voice VLAN in different ways by the working mode of Voice VLAN:
Under auto mode, after receiving untagged message from the port, the equipment will match its source MAC address with legal OUI address.If the source MAC is OUI address, the message will be considered to be voice message.And the equipment will automatically join the port in Voice VLAN, and learn this MAC address on the port at the same time.
Under manual mode, subscribers should manuallyconfigure the port which connects with IP phones to join Voice VLAN.
Step 3: Whether under auto mode or manual mode, the equipment will issue a policy and improve the priority of packets whose source MAC address atching the OUI address of voice VLAN. The CoS is set to 6 and DSCP is set to 46 for matched voice packets. Following these steps, the port that connects with IP phone joins Voice VLAN, and voice packets will be centrally transmitted in Voice VLAN and forwarded out with high priority.
If IP phone supports LLDP, user doesn't need to configure OUI manually .Switch could recognize and identify IP phone by the LLDP packets sending from IP phone.
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