PDF of this tutorialExternal Short Messaging Entities
An ESME is a device that may receive or send short messages. The short message entity (SME) may be located in the fixed network, a mobile device, or another service center.
- VMS—The VMS is responsible for receiving, storing, and playing voice messages intended for a subscriber that was busy or not available to take a voice call. It is also responsible for sending voice-mail notifications for those subscribers to the SMSC.
- Web—The growth of the Internet has also affected the world of SMS. Therefore, it is almost mandatory to support interconnections to the World Wide Web for the submission of messages and notifications. The increasing number of Internet users has a positive impact on the SMS traffic increment experienced in the last few years.
- E-Mail—Probably the most demanded application of SMS is the ability to deliver e-mail notifications and to support two-way e-mail, using an SMS–compliant terminal. The SMSC must support interconnection to e-mail servers acting as message input/output mechanisms.
- Others—There are several other mechanisms to submit short messages to the SMSC that include, but are not limited to, paging networks, specialized software for PC–based messaging and operator bureaus.
SMSC
SMSC is a combination of hardware and software responsible for the relaying and storing and forwarding of a short message between an SME and mobile device.
The SMSC must have high reliability, subscriber capacity, and message throughput. In addition, the system should be easily scalable to accommodate growing demand for SMS in the network.
Normally, an IN–based solution will allow for a lower entry cost compared to point solutions because it can support other applications on a single hardware platform and share resources, thereby spreading the deployment cost over several services and applications.
Another factor to be considered is the ease of operation and maintenance of the application, as well as the flexibility to activate new services and upgrade to new software releases.
Signal Transfer Point
The STP is a network element normally available on IN deployments that allows IS–41 interconnections over signaling system 7 (SS7) links with multiple network elements.
HLR
The HLR is a database used for permanent storage and management of subscriptions and service profiles. Upon interrogation by the SMSC, the HLR provides the routing information for the indicated subscriber. Also, if the destination station was not available when the message delivery was attempted, the HLR informs the SMSC that the station is now recognized by the mobile network to be accessible, and thus the message can be delivered.
Visitor Location Register (VLR)
The visitor location register is a database that contains temporary information about subscribers homed in one HLR who are roaming into another HLR. This information is needed by the MSC to service visiting subscribers.
MSC
The MSC performs the switching functions of the system and controls calls to and from other telephone and data systems. The MSC will deliver the short message to the specific mobile subscriber through the proper base station.
Air Interface
The air interface is defined in each one of the different wireless technologies (GSM, TDMA, and CDMA). These standards specify how the voice or data signals are transferred from the MSC to the handset and back, as well as the utilization of transmission frequencies, considering the available bandwidth and the system’s capacity constraints.
The Base Station System
All functions related to the transmission of electromagnetic radio signals between the MSC and the mobile devices are performed in the base station (BS). The BS consists of base station controllers (BSCs) and the base transceiver stations (BTSs), also known as cell sites or simply “cells.” The BSC may control one or more BTSs and is in charge of the proper resource assignment when a subscriber moves from one sector of one BTS to another, regardless of whether the next sector lies within the same BTS or in a different one.
The Mobile Device
The mobile device is the wireless terminal capable of receiving and originating short messages. Commonly, these devices have been digital cellular phones, but more recently the application of SMS has been extended to other terminals such as POS, handheld computers, and personal digital assistants (PDAs). The wireless network signaling infrastructure is based on SS7. SMS makes use of the mobile application part (MAP), which defines the methods and mechanisms of communication in wireless networks and employs the services of the SS7 transactional capabilities application part (TCAP). An SMS service layer makes use of the MAP signaling capabilities and enables the transfer of short messages between the peer entities.
The capabilities of the terminal vary depending on the wireless technology supported by the terminal. Some functionality, although defined in the SMS specification for a given wireless technology, may not be fully supported in the terminal, which may represent a limitation in the services that the carrier can provide. This trend, however, is disappearing as service providers’ merger and acquisition activity demands uniform functionality across all the constituents of the parent companies. Also, some manufacturers may include additional functionality, not considered in the specification, attempting to offer a more attractive product for service providers as well as end users. This will be the case more often as service provider continue to incorporate SMS into their revenue-generating and customer-loyalty strategies.
