 One can say that layers 1-4 are the responsibility of the communication system engineer. Layers 5, 6, and 7 are the respon- sibility of the data end-user. However, we believe that the telecommunication system engineer should have a working knowledge of all seven layers. Layer Relay open system Open system Open system Application Presentation Session Transport Network Data Link Physical Physical media for OSI Figure 10.23 Only the first three OSI layers are required at an intermediate relay (switching) point. DATA COMMUNICATIONS The transport layer has the ultimate responsibility for providing a reliable end-to- end data-delivery service for higher-layer users. It is defined as an end-system function, located in the equipment using network service or services. In this way its operations are independent of the characteristics of all the networks that are involved. Services that a transport layer provides are as follows: Connection Management. This includes establishing and terminating connections between transport users. It identifies each connection and negotiates values of all needed parameters. Data Transfer. This involves the reliable delivery of transparent data between the users. All data are delivered in sequence with no duplication or missing parts. Flow Control. This is provided on a connection basis to ensure that data are not delivered at a rate faster than the user's resources can accommodate. The TCP (transmission control protocol) was the first working version of a transport protocol and was created by DARPA for DARPANET. ¹ All the features in TCP have been adopted in the ISO version. TCP is often combined with the Internet protocol (IP) and is referred to as TCP/IP. 10.10.3 High-Level Data-Link Control: A Typical Link-Layer Protocol High-level data-link control (HDLC) was developed by the ISO. It has spawned many related or nearly identical protocols. Among these are ANSI ADCCP, CCITT LAPB and LAPD, IEEE Logical Link Control (LLC), and IBM SDLC (Refs. 20-25). The following HDLC definitions include s