# Prioritization and QoS

In order to ensure that an IP network with limited bandwidth resources can also supply the necessary bandwidth for call connections, the voice packets should be separated and prioritised over other data packets.

## QoS on layer 2 with VLAN

If several IP hardphones are used locally, then it is recommended to separate the call data from the other data in the IP network and set up a VLAN. Set this up by using a VLAN-compatible switch and connect the communication server and IP hardphones to the ports configured for this VLAN:

Table 1. VLAN configuration
AIN element | VLAN configuration
Switch | Configure VLAN with the following ports: Access port for communication server (node) Trunk port for IP hardphones
Communication server | QoS settings – Layer 2, setting: Frame type = Standard. Note: With this setting, the integrated CoS and VLAN functions on the communication server are switched off. This is available due to compatibility with older systems and is not normally used.
IP hardphones | Assign the intended VLAN to the telephone. This setting can either be made locally on the phone or through the DHCP options. If necessary, the PC port on the phone can also be assigned to a (different) VLAN.

As IP softphones are connected to the IP network via the Ethernet interface of a computer, they cannot be integrated in the VLAN.

## QoS on layer 3 with DiffServ (Differentiated Services)

The DiffServ method is used for the classification and prioritisation of data in the IP network, and is particularly recommended for WAN links. In doing so, it interprets the value of the first six bits of the ToS field as the DSCP class. In theory, it can differentiate between up to 64 classes. The standardised values are listed in the Internet standard documents rfc-2597 and rfc-2598.

From R2.1 onwards, the data for signalling, language and video can be classified individually. For FoIP (T.38), the DSCP class of the language applies.

In an AIN, the DSCP classes are defined in the master. The master then transmits the values automatically to the satellites and the IP system phones and Mitel SIP phones.

Prioritization takes place in the routers or in Layer 3 switches. The routers or layer 3 switches used must therefore support DiffServ (in general) and the selected DSCP classes (in particular), and must be configured accordingly.

Table 2. Recommended DiffServ settings
Parameter | Parameter value 1
DSCP class signalling | 40
DSCP class language | 46
DSCP class video | 34

## QoS on layer 3 with ToS

With the ToS method (RFC 791, page 11 and RFC 1349), the same six bits in the ToS/DSCP field are interpreted as in the DiffServ method (RFC 2474).

The ToS method interprets the first three bits (priority) in order to specify the priority level. With bit three to five, the transmission can be optimised according to one of the following criteria: High throughput, high reliability or low latency. The routers used must thus support ToS prioritisation and be configured accordingly. Non-prioritised data packets are given the standard priority by the router.

Using the following table, DiffServ classes can be converted to ToS values and vice versa. Example: DiffServ class 46 corresponds to ToS prioritisation Critical and the ToS service type High Throughput and Low Latency.

Table 3. Conversion table DiffServ/ToS
ToS service type (right) ToS prioritisation (bottom) | Normal Service | High Reliability | High Throughput | Low Latency | High Throughput / Low Latency
Best Routine | 0 | 1 | 2 | 4 | 6
Priority | 8 | 9 | 10 | 12 | 14
Immediate | 16 | 17 | 18 | 20 | 22
Flash | 24 | 25 | 26 | 28 | 30
Flash Override | 32 | 33 | 34 | 36 | 38
Critical | 40 | 41 | 42 | 44 | 46
Internetwork Control | 48 | 49 | 50 | 52 | 54
Network Control | 56 | 57 | 58 | 60 | 62

Parent topic: DHCP server
1 All here correspond to the standard values.
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