SMS guarantees delivery of the short message. Temporary failures are identified, and the
short message is stored in the network until the destination device becomes available. The
message is sent to the nearest Short Message Service Center (SMSC), which either delivers
it to the addressed mobile device or forwards it to the next SMSC. The messages are stored
and forwarded in the SMSCs until they are received by the addressees. In the delivery
process, the SMSC first sends an SMS request to the Home Location Register (HLR) to locate
the addressed mobile phone. The HLR is a database used for storing and managing
subscriptions and service profiles. If the addressed mobile phone is found, the HLR provides
the routing information. If the HLR cannot locate the addressed phone, then the SMSC
stores the message for a limited time and tries to deliver the message again when the
addressee connects to the network. If the addressee is found, the message is delivered and
the SMSC receives verification that the message has been received by the addressee. The
sender is informed that the SMS message has been received by the addressee.
1.2.2 Enhanced Messaging Service (EMS)
Enhanced Messaging Service (EMS ) evolved from the popular SMS. Service providers
started to offer EMS in early 2000. EMS allows much richer content options than text-based
SMS messages and can include pictures, melodies, sound marks, graphic, animations, fonts,
and formatted text. For example, a person could send this text to his date with an SMS
message "I love you." Using EMS, he could add a short animation of flowers along with a
love melody.
EMS uses the store-and-forward mechanism provided by the SMSCs. Similar to SMS, EMS
uses the control channels. EMS is enabled by SMS concatenation, linking several short
messages together.
EMS supports basic and extended pictures. Basic pictures are black and white small (16x16
pixels) pictures or large (32x32 pixels) pictures. Extended pictures can be, however, black
and white, grayscale, or color. Extended pictures can contain 255x255 pixels and can be
transmitted in a compressed format.
EMS also supports sound. There are a few predefined sounds, including low and high chimes
and chords, Claps, TaDa, and drum sounds, as well as Notify, User-Defined, and Extended
Sounds. Predefined sounds are not transmitted over the air. Only a reference to the sound is
included in the EMS. There are ten different sounds that can be added in the message, and
when the sound reference is being displayed, the referenced sound will be played. The
sender can also download melodies from various web sources. These sounds have to be
formatted according to the iMelody standard. These melodies can take up to 128 bytes.
Animations in EMS provide users with a much stronger alternative for expression than plain
pictures. EMS supports predefined animations that reflect happiness, sadness, flirtatiousness,
gladness, skepticism, and grief. User-defined and extended animations are supported as well.
Similar to sounds and pictures, EMS predefined animations are not sent as animation over
the air. Only a reference to them is included in the EMS message. When the message is
received by the addressee the referenced animation is displayed in a manner that is
specified by the manufacturer. User-defined animations consist of four pictures. There are
two different animation sizes: small (8x8 pixels) animations and large (16x16 pixels)
animations. These animations are sent over the air interface. Extended animations may be
black and white, grayscale, or color. The maximum size of a single animated frame is
255x255 pixels. The repetition of these animations may be controlled by the sender and can
be transmitted in a compressed form.