Access to Long Distance
There are two basic strategies for accessing long distance services:
- Switched access: This is the model used in residential services. It is also available to commercial services. The subscriber connects their telephone to the LEC's network. When they place an LD call, the local switch has been programmed with the subscriber's Primary Interexchange Carrier (PIC) code. Each IXC has an assigned PIC. Based on that number, the local switch routes the call to the appropriate LD point of presence (POP) within that LATA. For business services, this can be done over a trunk connecting a PBX or a key system to the PSTN. Because part of every LD call is carried over the LEC's network, the IXC is required to pay the LEC an access charge based on the total minutes of usage (MOU) by all LD subscribers in that LATA in a given month. The local switch may also be configured with the International Primary Interexchange Carrier (iPIC) for international calls and the Local Primary Interexchange Carrier (LPIC) for intraLATA toll calls.
- Dedicated access: This model is available in commercial (business) services only, and is typically used when a PBX is in place or a Centrex service is being used. Dedicated access, also known as direct access involves the deployment of a trunk directly from the subscriber's PBX (or the local switch providing Centrex services) and the designated LD POP. In the case of Centrex, the trunk is dedicated to the leasing subscriber. A second trunk connects the PBX to the LEC switch as well. The PBX distinguishes between LD calls and local calls and uses the dedicated LD trunk for LD calls and the trunk to the LEC for local calls. If all of the LD circuits are in use and a new LD call is dialed, the PBX can be programmed to either block the new call, or to overflow the call to the switched access model by using a channel on the trunk to the LEC.
The dedicated access model typically results in lower LD rates because the IXC does not have to pay access charges to the LEC. However, that does not mean it is always preferable. The additional LD trunk has to be paid for, and often this is a more expensive trunk because it has to traverse some part of the LATA to get to the POP. That means it may be billed by the mile for some portion of it. To determine whether or not a dedicated access model is desirable, a bit of calculation is required using the formula <math>((MOU * SR) + LATS) - ((MOU * DR) + LATD + DAT)</math> where:
- MOU is the total minutes of usage in a given month
- SR is the per-minute LD rate for switched access
- LATS is the cost for the LEC access trunks needed to implement a 100% switched access model
- DR is the per-minute LD rate for dedicated access
- LATD is the cost for the LEC access trunks needed for local service in a dedicated LD access model
- DAT is the cost for the IXC access trunks needed for LD service in a dedicated LD access model
Essentially, you take all of the cost for a switched access model (the minutes of usage multiplied by the switched access per-minute rate added to the cost of the trunks) and you subtract from is all of the cost for the dedicated access model (the minutes of usage times the dedicated access LD per-minute rate plus the cost of both the local and LD trunks). If the number is positive, the cost for the switched access model is higher than the cost for the dedicated access model and the latter should be implemented. If the number is negative, the cost for the switched access model is lower than the cost for the dedicated access model and the former should be implemented. If the costs are nearly equal, either model could be used. However, it would be prudent to project forward to see if the variables are expected to change in the future and use that information to guide a selection.
|<mp3>http://podcast.hill-vt.com/podsnacks/2008q2/long_distance.mp3%7Cdownload</mp3> | Long distance|
|<mp3>http://podcast.hill-vt.com/podsnacks/2008q3/switched_vs_dedicated_ld.mp3%7Cdownload</mp3> | Switched vs. dedicated access to LD|