To add VoIP to an existing data network, we must first evaluate the architecture of the network to determine if it will support the addition of voice traffic. If we are able to determine that the network is not capable of handling voice traffic we will need to identify the steps that must be taken to provide this functionality.
When analyzing a customer’s network we need to ask the following questions:
1. Is the customer using half-duplex or full-duplex Ethernet connections?
2. Has the customer enabled or implemented auto-negotiation?
3. Does the customer’s network provide reliable, continuous power?
4. Does the customer use Layer 2 or Layer 3 switches, or is the customer using shared media (such as a hub)?
5. Is the customer implementing the best protocols and applications for their particular situation?
In a VoIP network both sides must support auto-negotiation. Auto-negotiation is the automatic recognition of the functions of the system or device connected to the other end of has, so that devices are able to configure themselves automatically to work with the receiving components. If we do not have full-duplex auto-negotiation at both ends of a connection we will experience call clipping. Some devices, such as hubs, do not support auto-negotiation. If we do not have auto-negotiation enabled we can also have device mismatches occurring.
It is important to build power management into your VoIP network. We need to ensure that redundant power supplies are used for network equipment to ensure a continuous supply of power to the network devices in the event of a power failure. This may include Universal Power Supplies and backup generators. We must also ensure that we have sufficient cooling in our wiring closets so that devices do not overheat. In addition to providing reliability of our backbone equipment, many IP terminals are able to draw power from the network cabling referred to as Power over Ethernet (PoE) or Power over LAN (PoL). Using PoE eliminates the need to plug sets into an AC power source. The devices draw power from unused lines in the Cat5 cable to provide power to the connected device. PoE functionality is included in many modern layer 2 switches, including the Baystack 400 switches from Nortel.
We also need to look at the switching method implemented in the network. Shared media, such as hubs, simply rebroadcast all signals to all ports. This can cause increased network traffic and lead to packet collisions. At a minimum, Layer 2 switches must be used. Layer 3 switches (switches that include routing capabilities) may also be used in VoIP network. Hubs should never be used in a VoIP implementation. A Layer 2 switch operates by identifying the MAC address of the receiving machine and directs the packets accordingly. A Layer 3 switch stores routing information in hardware based on IP information, and can dynamically update the routing paths based on availability of paths. Using a Layer 3 switch can reduce packet latency.
We also need to consider the protocols that are being used on the network. Some protocols require very little bandwidth (such as SNMP and NTP), and are of little concern. Other applications, such as HTTP and SMTP can use a greater amount of bandwidth. We need to take this into consideration when adding VoIP to an existing or new network. Some protocols, such as SQL, use a very large amount of bandwidth. Systems that are running these protocols should be located on their own LAN or VLAN.
When planning a VoIP installation we should follow these steps:
1. Obtain or create diagrams for existing LAN/WAN infrastructure
2. Identify physical and logical locations of important networking equipment
3. Document types and lengths of physical cables and circuits
4. Analyze infrastructure and identify any problematic conditions that might arise in the future – we must then determine if our client’s business and technical goals are realistic