To understand nongeographic or toll-free number portability, we first need to understand what a subscriber means in regard to a nongeographic/toll-free number. Like a regular fixed or mobile telephone number subscriber, there is also a subscriber associated with every
valid/activated toll-free number; these subscribers are usually companies, organizations, or even a person. Not to confuse them with regular telephone subscribers, we will refer to these types of subscribers as entities. An entity can choose who is going to be the provider for de- livery of calls to its premise. In a number portability environment, it is further desirable that while maintaining the same toll-free number, an entity might want to have multiple providers handle its call delivery based on percentage traffic allocation and/or time of the day.
Our illustration here for nongeographic number portability is based on the North Amer- ican environment. A similar concept can be deployed in other countries. First recall from the earlier discussion about 1-800 number routing that the 800-number SCP is involved in looking up a routable number. Thus, first we need to understand the process of an entity requesting change in number portability, which is eventually communicated to the 800-number SCP.
That is, there are two distinct phases in regard to number portability: the first is the process in regard to requesting a change and how this is communicated to the 800-number SCP, and the second is the actual routing when a regular subscriber dials an 1-800 number.
13.10.1 800-Number Management Architecture
An independent administration for 800-number maintenance, to be referred to as an 800- Service Management System (SMS/800) Administration Center is responsible for maintaining the most up-to-date record in regard to 800-number translation. A responsible organization, or RespOrg in short, that has been certified is allowed access to the SMS/800 database located at the SMS/800 administration center. A RespOrg can request a change for an entity for chang- ing service provider or allocation. Once this information is recorded at the SMS/800 database
F I G U R E 13.13 Service Management System/800-number architecture: process and communication to SCPs.
and tested, a copy of the current information in the SMS/800 is downloaded to Service Con- trol Points (SCPs). There are currently approximately 20 mated pairs of 800-number SCPs across North America. The basic architecture is shown in Figure 13.13.
13.10.2 Message and Call Routing
A LEC accesses the closest 800-number SCP pair using the SS7 network. Thus, when a regular subscriber dials a 1-800 number, the originating TDM switch (SSP) generates a TCAP query that is routed to the 800-number SCP using the SS7 network. The 800-number SCP looks up the entry applicable at that instant for the dialed 800-number and returns the CIC number along with a routable number or the dialed toll-free number. If the CIC number happens to be for the LEC itself, then the originating switch identifies the SSP for the switch (either tandem or terminating switch) to which the ISUP IAM message is to be sent that contains the TCIC code. If the CIC number happens to belong to an IXC, then the originating switch identifies IXC’s PoP SSP and determines the trunkgroup ID and TCIC to be used for this call; an ISUP IAM message is then generated that includes the TCIC information; the called number field is the routable number obtained from the 800-number SCP and this message is sent to the PoP SSP. Note that the ISUP IAM message can include the dialed 800-number as the called number if the 800-number SCP returns this number; in this case, the carrier that receives this message does further translation internally for ISUP IAM message routing and call routing;
this phase is completely transparent to the originating TDM switch.
For illustration, we first note that the basis architecture for call routing remains the same as shown earlier in Figure 13.9. Thus, we concentrate here on the message routing part of the call setup phase from the point of view of the originating TDM switch as shown in Fig- ure 13.14. At the 800-number SCP database, we show the conceptual view of information ac-
F I G U R E 13.14 800-number portability, lookup, and ISUP message routing: 1—call initiated by user; 2, 3—TCAP messages; 4—IAM message.
cessed in regard to the dialed 800-number, which can be mapped to multiple routable numbers with percentage allocation; the allocation may be based either on different service providers or multiple numbers for the same service provider. It should be noted that in actuality more than two routable numbers can be listed and the time granularity for a routable entry is not fixed; here, for brevity, we show a maximum of two entries for each 800-number and change in entries on an hourly basis. This is time-of-the-day routing. There is another important point to note; it is possible that at a particular time instant, calls arriving at central offices in differ- ent geographic locations that are destined for a particular 800-number are routed to different routable numbers or different providers. In other words, the entry need not be unique nation- wide at a particular point of time.