An Instant Message Service Center (IMSC) is a network element in the mobile telephone network which delivers instant messages.
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Operation
When a user sends an instant message (IM message) to another user, the phone sends the message to the IMSC. The IMSC stores the message and delivers it to the destination user when they are available. The IMSC usually has a configurable time limit for how long it will store the message.
Companies
Few companies who make many of the IMSCs in use in the GSM world are Miyowa, Followap and OZ. Other players include Acision, Colibria, Ericsson, Nokia, Comverse Technology, Now Wireless, Jinny Software, Miyowa, Feelingk and few others.
References
Instant message service center Wikipedia(Text) CC BY-SA