ingly, the layer 3 func- tionality is called MTP-3. TABLE 14.1 Seven-Layer OSI Reference Model 7. Application layer 6. Presentation layer 5. Session layer 4. Transport layer 3. Network layer 2. Data link layer 1. Physical layer Downloaded from Digital Engineering Library @ McGraw-Hil (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hil Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Signaling with SS7 Signaling with SS7 14.3.1 Message transfer part (MTP), level 2 The layer 2 functionality of the MTP is to frame the message, to check the frame for er- rors, and to provide sequence numbering, which can be used both to ensure that all frames are received in sequence and to provide flow control to avoid congestion. Fram- ing is accomplished by separating messages with a flag, which is an 8-bit pattern 01111110 that is never repeated within the message and therefore can be used reli- ably to detect the start of a frame. 14.3.2 Message transfer part (MTP), level 3 The layer 3 functionality of the MTP includes carrying the network addresses of the sending and receiving nodes and information useful for routing and congestion control. The MTP can be loosely compared to the functionality of the IP in that it creates the addressed envelope needed by upper layers of the protocol to get the message to the destination. Once at the destination, it is the job of the upper layers to get the message to the correct application. Standards efforts are presently under way to define how to do the work of SS7 over the IP network. Now that we have an envelope to get messages across the SS7 network, we can de- fine how to get the information in the messages to the correct destination. 14.3.3 Signaling connection control part (SCCP) The SCCP of the SS7 protocol suite provides a way to address the individual applica- tions within a node of the SS7 network. Such an application might be the conversion of 1-800 numbers into PSTN numbers. It also provides the ability to provide both