ww.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. TDMA Wireless Telephony (GSM) Key Radio Concepts Type 3, intra-MSC, inter-BSS handover. The call is changing from one controller to an- other within the same switch. The call keeps the same MSC but has a new BSC and BTS. Type 4, inter-MSC, inter-BSS handover. The call has a new MSC, BSC, and BTS. When the mobile telephone is instructed to change to a new GSM carrier and time slot, it tries to synchronize to the new cell. A presynchronized change is called a seam- less handover. In a seamless handover, the mobile telephone sends time-aligned bursts immediately, and there is no pause in the audio signal. This tends to happen when the cell sizes differ by less than 1 km. 7.4.4 GSM short message service A paging capability is built into the GSM standard for short message service (SMS), which is described in Chap. 19. The messages can be up to 160 characters in length and are sent and received by a GSM mobile telephone that is not on a normal voice tele- phone call. In the forward direction, the SMS messages are sent on the CBCH, the control chan- nel, and they can be stored by the system if the mobile telephone is not receiving pages. Just as in the case of a page for a telephone call, the mobile telephone with the correct number picks up the message, and all the other mobile telephones ignore it. In the re- verse direction, GSM SMS uses the SDCCH signaling channel. On the GSM air interface, one can think of SMS as a call that stops at the very first signaling message. At current demand levels, SMS traffic has not been a capacity prob- lem for GSM. On the terrestrial interface, SMS is carried on the Signaling System No. 7 (SS7) net- work described in Chap. 14, where it competes with call signaling and can be a serious capacity issue. The SMS community may find a way to transport its messages over Transmission Control Protocol/Internet Protocol (TCP/IP) link