VoIP for Dummies 2005

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Carriers charge a monthly access fee for the dedicated line that connects your location to the carrier’s facilities. This line is called the local loop, or the last mile. The government regulates what the carrier can charge you for dedicated transports. In addition, you are assessed about 7 percent of the total monthly recurring charge for regulatory fees and taxes. These are the only recurring costs you have for the dedicated transport itself. Basically, these are the costs you pay to obtain access to the local loop.

The local loop is one of the biggest costs in any dedicated network. However, if your company runs VoIP, it can eliminate or greatly reduce costs by putting most of their traffic on-net. In many cases, this savings can pay for the local loop access costs for all locations with money left over.

As an example of how much can be saved, I’ll tell you about a large network I designed that had 384 locations nationwide. The client’s total recurring charges for the 384 local loops came to $268,000 per month, and their monthly usage charges were $4,200,000 per month. If they converted to VoIP, they would still have the local loop charges, but they could have saved 35 to 60 percent of their usage charges — a whopping savings of $1,470,000 to $2,520,000!

As you can see, the higher costs of a dedicated transport are easily offset by lower or eliminated usage charges. Because a private, dedicated network can be used to carry all of a company’s internal telephone calls, the company has no monthly usage charges for internal VoIP telephone calls. On-net telephone calling using VoIP reduces or eliminates traditional telephone usage billings.

For calls that must travel off your VoIP network, your company will still have some traditional carrier service bills.

The wireless last mile

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Real-time network management

High-quality VoIP calls

With a dedicated transport, your network can support massive volumes of on-net VoIP calls. Huge networks with multiple locations and hundreds or thousands of callers are best supported with dedicated transports. Dedicated transports enable a QoS that meets or exceeds the quality found in traditional circuit-switched PSTN telephone calls.

Types of Dedication

For the corporate sector, dedicated network transports come in two major flavors: the digital service (or digital signal) CSI and the optical carrier CSI. Chapter 4 covered quite a bit about these CSIs, but you need to know more to make an educated decision about which transport is right for your needs.

The DS transports

The original digital service (DS) series of standards had five levels of dedicated lines. Each DS standard provided a set number of 64 Kbps channels, or DS0s. For example, a DS1 (T1) transport includes 24 channels, and a DS3 (T3) includes 672 channels.

In the old days when DS transports were new and costly, the DS0 was leased as a single 64 Kbps transport line. Today, hardly anyone leases a DS0 transport. If you’re implementing VoIP on your company’s network, you won’t

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want to consider using just a single DS0 channel. Not only is it not enough bandwidth, but it isn’t cost effective.

T1 line

The T in the T1 version of transport represents terrestrial, or over land. The tariffs controlled by the government for setting the pricing of DS transports are based on the total terrestrial mileage between point A and point B.

The T1 transport continues to be the most popular transport on the market, and prices continue to drop. A big reason why the T1 is so popular is that it permits network configuration folks to divide the total available bandwidth into smaller individual channels. This makes the T1 particularly suitable for VoIP networks that run computer data, telephony voice, and even videoconferencing over the same network transport. T1, however, does not provide adequate bandwidth for large multilocation networks with hundreds of users in each location.

If a company has fewer than three hundred employees per location, one option is to multiplex, or bond, multiple T1 lines to act as one big T. For example, if you multiplex (MUX) six T1s, your effective bandwidth would increase to about 10 Mbps. The costs in this scenario should be considered carefully. The T1 lines would cost about $3000 per month. Additional one-time costs include the six DS1 interfaces required to terminate the T1 lines at your location. In addition, there is the cost of the MUX equipment.

T3 line

If you need more bandwidth than what you can obtain from a T1 or a group of T1s, consider the T3 transport. The T3 transport provides a total aggregate bandwidth of 45 Mbps. This breaks down to about 672 DS0 channels. A growing company can also consider upgrading to T3 transports or some mix of T1 and T3 lines. (The latter is more commonplace for larger companies, which use T1 lines for smaller locations and T3 for the larger locations.)

I had the opportunity to design a crosstown network that required a T3. The client was a small college in the middle of downtown Pittsburgh. They wanted to connect their two high-rise dormitory apartment buildings, and they also wanted Internet access on the network. The total distance between the college and the apartment buildings was three miles. The monthly charge for the T3 line was $19,000. A T1 line for the same distance was $450. In addition, the onetime cost of terminating the T3 was $5000, much more than the termination costs of a T1 line. For terminating a T1 line, even if the client had to buy a new router with DS1 interface, the cost range would be $800 to $3000.

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over a dedicated T1 line may still be billed as a regular POTS call. True convergence of the two carrier networks means to merge the packets of a VoIP packet-switched phone call with the circuit-switched signaling of the PSTN.

Figure 7-2 illustrates the convergence of the PSTN with the DS carrier network. The convergence is made possible through the use of a network-to-network interface (NNI). NNIs are typically very expensive and are used by carriers and only the largest customer networks, which have the volume to justify the high monthly cost.

NNI

PSTN

DS

A less expensive option does exist for connecting your VoIP network to the PSTN. Figure 7-3 illustrates how a company can converge their network onto the PSTN using a far more cost-effective PRI transport option.

In this example, on-net VoIP calls are carried over the company’s dedicated network. Outgoing local calls travel on-net as VoIP packets and then travel to the PSTN through the gateway and its attached PRI transport line. Inbound calls from the PSTN travel as circuit-switched calls. The gateway translates inbound calls into VoIP packets.

Inbound calls are paid for by the calling party. Outbound calls over the PRI are billed at whatever rate the carrier charges. This is a good example of VoIP convergence: the packet-switching of the VoIP network operating with the cir- cuit-switching of the PSTN to support both inbound and outbound telephone calls.

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Dedicating channels to applications

As defined in the 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 twenty-four DS0 channels. In a small local area network running VoIP that connects to a larger wide area network running VoIP, the LAN might optimize their dedicated channels by assigning eight channels to support VoIP, eight channels to support their data network, and eight channels to support their videoconferencing system. Figure 7-4 shows an illustration of such a network.

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 is not the only option for channelizing dedicated bandwidth.

Dynamic bandwidth allocation

The dedicated transport can also connect to equipment such as an 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 onboard 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-three 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 twenty-four simultaneous VoIP calls.

Based on historical PSTN standards, one POTS line can fully support the calling pattern needs of six to eight people, on average. It is 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).

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DS

VoIP enables dynamic bandwidth allocation and therefore optimizes bandwidth utilization. This is not the case in traditional networks that run computer data and telephony on separate networks. In traditional networks that use dedicated transports, it is well established that about 60 to 70 percent of dedicated network bandwidth is not 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 is a major reason why companies can add VoIP to their current networks using the dedicated transports already in place.

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Keeping a Switched Line

In fully converged networks, dedicated transports are used for integrated operation of computer data, voice, and video applications. At the same time, there is a role for switched transports in the VoIP network. Note that when it comes down to the local level, the telephone call must ultimately go off-net to reach local telephone numbers connected to the PSTN. Figure 7-5 illustrates an example of a fully converged network with switched transport access to the PSTN.

As you can see, switched transports are used on this network to support local calls. Also supported are fax services into the PSTN. Until the entire world converts to VoIP, any VoIP network, however extensive, needs at least limited access to the PSTN. For the time being, switched transports continue to have a role in dedicated networks, and VoIP networks must make provisions for carrying calls into the PSTN.