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Voice Feature Guide 01

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Voice QoS

Voice QoS

The voice service requires high real-time performance, low delay, and fast call connection. Therefore, the voice packets should be forwarded with a high priority. The router, however, forwards the packets based on the VLAN priority (complying with 802.1p) and DSCP/ToS set in the packets.

802.1p Priority (Separately Set for Signaling and Media Streams)

Figure 1-166 802.1q frame format

Figure 1-166 shows the Ethernet frame format defined in 802.1q. The four-byte 802.1q header contains the following contents:

  • Tag protocol identifier (TPID): Two-byte tag protocol identifier, with the value of 8100.
  • Tag control information (TCI): Two-byte tag control information. It is a new type of information defined by IEEE, indicating a text added with the 802.1q label. The TCI is divided into the following three fields:
    • VLAN ID: 12-bit, indicating the VLAN ID. Up to 4096 VLANs are supported. All the data packets transmitted from the host that supports 802.1q contain this field, indicating the VLAN to which the data packets belong.
    • Canonical format indicator (cfi): one-bit. It is used in the frame for data exchange between the Ethernet network of the bus type and the FDDI or token ring network.
    • Priority: three-bit, indicating the priority of the frame. Up to eight priorities are supported. It determines the data packet to be transmitted first in case of switch congestion.

The local media IP address and signaling IP address of the MA5600T/MA5603T/MA5608T can be configured in one VLAN or different VLANs according to the networking requirements. The 802.1p priorities (in the range of 0-7) can be set for the media IP address and signaling IP address respectively. By default, the priority for either the media IP address or the signaling IP address is 6.


As defined in the IP protocol, the DSCP and ToS occupy the same field (one-byte) in the IP header. The device on the IP bearer network identifies whether DSCP or ToS is filled in the IP header, and schedules and forwards the packets with the DSCP/ToS field according to the settings to ensure the QoS for different services.

The type of service (ToS) field contains a three-bit precedence subfield (ignored currently), a four-bit ToS sub field, and one reserved bit (it must be set to 0). The four bits in the ToS sub field represent the minimum delay, maximum throughput, maximum reliability, and minimum cost respectively. Only one of the four bits can be set. If all four bits are set to 0, it indicates the common service.

The DSCP identification is based on the IPv4 ToS and the IPv6 traffic class.

As shown in Figure 1-167, the first six bits in the DS field (bits 0-5) are used to differentiate the DS codepoints (DSCPs) and the last two bits (bits 6 and 7) are reserved. The first three bits in the DS field (bits 0-2) are the class selector codepoint (CSCP), which indicates a class of DSCP.

Figure 1-167 DSCP identification format

DSCP is used to select the corresponding per-hop behavior (PHB) on all the nodes of the network. The PHB describes the external visible behaviors when the DS node functions on the data stream aggregation. Currently, IETF defines three types of PHB: expedited forwarding (EF), assured forwarding (AF), and best-effort. For example,
  • BE: DSCP = 000000
  • EF: DSCP = 101110
  • The AF codepoints are as follows:

    Low Discard Priority, j = 1

    Middle Discard Priority, j = 2

    High Discard Priority, j = 3

    AF (i = 4)




    AF (i = 3)




    AF (i = 2)




    AF (i = 1)




    The first three bits (bits 0-2) for one type of AFs are the same. To be specific, the first three bits of AF1 are 001, AF2 010, AF3 011, and AF4 100. Bits 3-4 represent the discard priority, namely, 01, 10, and 11. The larger the value, the higher the discard priority.

The DSCP/ToS value of local media IP packet and signaling IP packet can be configured on the MA5600T/MA5603T/MA5608T respectively. First the configuration policy (DSCP or ToS) is selected, and then the corresponding value is set. By default, DSCP is selected on the MA5600T/MA5603T/MA5608T, with the value of 56 (EF with the highest priority).

Updated: 2019-02-22

Document ID: EDOC1100067358

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