Avaya Voice Quality Requirements
This article provides some basic network requirements from Avaya for setting up an H.323 IP with the Avaya Communication Manager solution.
PoE Requirements:

Avaya 9600 Series H. 323 IP desk phones work on the standard Power over Ethernet (PoE) specification of 802.3af. The specification provides for up to 15.4 W of DC power that has a voltage of minimum 44 V DC and a current specification of 350 mA for each device. You should consult your switching vendor to determine what commands will tell you what PoE standards are supported. How many phones can be supported per switch and which ports will supply PoE are key questions to ask when considering how much of a power budget you will need to plan for.

Lan Cabling Requirements

A minimum of Cat 5e or better for all Lan cabling end to end, including all in-house wiring, and all ethernet cables connecting the switches to the patch panels to the endpoints. LAN cabling should not exceed 300 meters from switch port to end device including patch cords. Failure to meet these requirements can result in poor voice quality.

WAN Network QoS Requirements

Avaya highly recommends consideration of the following list of Best Practices when implementing VoIP.

QoS/CoS:
Quality of Service (QoS) for voice packets is obtained only after a Class of Service (CoS) mechanism tags voice packets as having priority over data packets. Networks with periods of congestion can still provide excellent voice quality when using a QoS/CoS policy.
Switched networks are recommended to use IEEE 802.1p/Q. Routed networks should use DSCP (DiffServ Code Points). Mixed networks should use both as a best practice. Port priority can also be used to enhance DiffServ and IEEE 802.1p/Q. Even networks with plentiful bandwidth should implement CoS/QoS to protect voice communications from periods of unusual congestion such as from a computer virus.
Switched Network
A fully switched LAN network is a network that allows full duplex and full endpoint bandwidth for every endpoint that exists on that LAN. Although VoIP systems can work in a shared (hubs or bussed) LAN, Avaya recommends the consistently high results a switched network lends to VoIP.
VLANs: Placing voice packets on a separate VLAN (subnet) from data packets is a generally accepted practice to reduce both broadcast and data traffic from contending for the same bandwidth as voice. Other benefits become available when using VLANs, but there may be a substantial cost for initial administration and maintenance.
Phones should be connected to switch ports configured as either access ports or access ports with configured voice VLANS.

Recommended Transmission Requirements

Network delay One-way between endpoints should be:
80ms (milliseconds) delay or less, which can—but may not—yield toll quality.
80ms to 180ms delay for business communications quality. This is far better than a cell phone quality and in fact, is very well suited for the majority of businesses.
180ms or higher, which may still be quite acceptable depending on user expectations, analog trunks used, codec type, etc.

Network jitter: The measure of the variability of delay between endpoints should be:

An average of less than 20ms, or less than 1/2 the packet payload, for toll quality. This value has some latitude depending on the type of service the jitter buffer has in relationship to other router buffers, packet size used, etc.
Network packet loss: The maximum loss of packets (or frames) between endpoints should be:
1% or less, which can yield toll-quality depending on many factors.
3-5% or less, which should give business communications quality. This quality is much better than cell phone quality.
More than 5% may be acceptable for voice but may interfere with signaling.

DHCP Option 242 for H.323 IP Phones:

Avaya IP Phones support both LLDP and DHCP to enable easier configuration for system administrators.

Phones that are connected to switch ports with a connected PC will need to determine VLAN tagging using DHCP option 242 for the data or native VLAN.

Data VLAN option 242 string command (example VLAN 100)

L2Q=1,L2QVLAN=100
L2Q=1,L2QVLAN=100 is for when you have PCs on one VLAN and voice on another VLAN. In this case, L2Q tells the phone that it needs to enable VLAN tagging and utilize a different VLAN other than the native VLAN. This other VLAN is VLAN 100 as noted by the L2QVLAN=100 setting.

The IP phone will then release its address on the native VLAN and send out an additional DHCP broadcast tagged for VLAN 100. If the phone is on a remote subnet it is very important to ensure that any IP helper-address statements are in place on routers to ensure that the broadcast traffic is routed to the correct DHCP server.

Voice VLAN option 242 string examples

MCIPADD is for the call server
HTTPSRVR is for the file server for the phone to obtain firmware
MCIPADD=10.230.10.20,HTTPSRVR=.10.231.10.100,MCPORT=1719
In our example here the phone will get its MCIPADD setting which will satisfy the phone requirement to have a place to register to. The phone also learns about the fileserver where it will then ask for the upgrade file based on its model number. Depending on the model number of the IP phone the file asked for on the remote server will differ.

Firewall and Port Access requirements

It is recommended to disable Application Layer Gateways also know as Session helpers from Next-Generation Firewalls. They are created by firewall vendors to assist the transmission of voice information but often cause unexpected and difficult to troubleshoot voice problems such as call creation failure and unexpected call drops. Often times these features are not aware of vendors extensions into H.323 and SIP and end up dropping or modifying values in packets that cause the Avaya IP phones to not operate correctly.

Avaya recommends caution when using the following

Network Address Translation (NAT):
Be very careful when using NAT on an internal network between voice equipment and IP Phones. Most implementations using VoIP end-
points behind NAT fail because many H.323 messages (the protocol carrying the voice information) contain multiple instances of the same IP address in a given message. It is unlikely that NAT can find and translate all of them. Avaya Communication Manager will work seamlessly with any static NAT application, even if that NAT is not H.323 aware.

VoIP does not work well with networks that use NAT because most NAT implementations do not support H.323 protocols. The destination IP address is encapsulated in more than one header: the Q.931, H.225, and IP headers. Most NAT implementations change only the address in the IP header resulting in a mismatch that prohibits the control of calls.

If NAT has to be used then a one to one NAT is recommended to ensure that end to end communication is maintained between hosts whenever possible.

Sample IP phone transmission

IP-Phone-Transmission

IP-Phone-Example

IP-Phone-Example2
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