Managing VoIP Bandwidth

By Timothy V. Kelly from VoIP For Dummies

Dedicated transports are channelizable, meaning that their total bandwidth can be divided by channel and assigned to support specific applications (computer data, voice or telephony, and videoconferencing). Each dedicated transport is standardized to have a preset number of digital service bandwidth channels.

Dedicating channels to applications

As defined in the digital service carrier services infrastructure (DS CSI), each transport channel is called a DS0 and provides 64 Kbps of bandwidth. A DS0 is the smallest unit of bandwidth in the dedicated carrier services network. All dedicated transports are based on some multiple of DS0s.

For any dedicated transport, the transport line can be programmed to deliver its total aggregate bandwidth capacity (all channels). Or, the channels can be divided and assigned to specific applications. For example, a T1 line has a total aggregate bandwidth capacity of 24 DS0 channels. In a small local area network (LAN) running VoIP that connects to a larger wide area network (WAN) running VoIP, the LAN might optimize its dedicated channels by assigning eight channels to support VoIP, eight channels to support the data network, and eight channels to support the videoconferencing system. Figure 1 shows an illustration of such a network.

Figure 1: Allocating channels to specific network purposes.

Sometimes this type of configuration is referred to as a fixed-channel solution. The T1 transport enters the company's premises and is terminated on a multiplexer. From there, individual cables connect to their respective application's terminating equipment. But fixed-channel isn't the only option for channelizing dedicated bandwidth.

Dynamic bandwidth allocation

The dedicated transport can also connect to equipment such as an Internet protocol-private branch exchange (IP-PBX) that assigns bandwidth channels on demand, a process known as dynamic bandwidth allocation.

Here is how it works: An IP-PBX supports an on-board T1 interface module. A dedicated T1 line is used to physically connect the location through this T1 module on the IP-PBX. At the carrier's facility, the same T1 line connects this location's IP-PBX to the customer's private, dedicated WAN running VoIP. (A multiway gateway, router, or level 3 switch could also be used in place of the IP-PBX.)

Whenever anyone on the LAN side makes an on-net call, the IP-PBX assigns it a single DS0 channel from the channel pool. When the call ends, the system returns the DS0 channel to the pool. The VoIP system at this location is capable of bringing up and maintaining 24 simultaneous VoIP calls.

Based on historical public switched telephone network (PSTN) standards, one plain old telephone service (POTS) line can fully support the calling pattern needs of six to eight people, on average. It's no different in the VoIP world. A VoIP channel can support six to eight people, on average. So a T1 transport used for VoIP can support 144 to 192 persons (6 x 24 to 8 x 24).

VoIP enables dynamic bandwidth allocation and therefore optimizes bandwidth utilization. This isn't the case in traditional networks that run computer data and telephony on separate networks. In traditional networks that use dedicated transports, it's well established that about 60 to 70 percent of dedicated network bandwidth isn't even used.

Dynamic bandwidth allocation ensures that your network gets the bandwidth it needs when it needs it, and it enables a company to manage its bandwidth to minimize cost and increase productivity. It's a major reason companies can add VoIP to their current networks using the dedicated transports already in place.