each to achieve a certain objective. DS1 is a format as are SDH and SONET. In this section we will familiarize the reader with basic protocol functions. This is followed by a discussion of the Open System Interconnection (OSI), which has facilitated a large family of protocols. A brief discussion of HDLC (high-level data-link control) is provided. This particular protocol was selected because it spawned SO many other link layer protocols. Some specific higher layer protocols are described in Chapter 11. 10.10.1 Basic Protocol Functions There are a number of basic protocol functions. Typical among these are: Segmentation and reassembly (SAR) Encapsulation Connection control Ordered delivery Flow control Error control A short description of each follows. Segmentation and reassembly. Segmentation refers to breaking up the data message or file into blocks, packets, or frames with some bounded size. Which term we use depends on the semantics of the system. There is a new data segment called a cell, used in asynchronous transfer mode (ATM) and other digital systems. Reassembly is the reverse of segmentation, because it involves putting the blocks, frames, or packets back into their original order. The device that carries out segmentation and reassembly in a packet network is called a PAD (packet assembler-disassembler). Encapsulation. Encapsulation is the adding of header and control information in front of the text or info field and parity information, which is generally carried behind the text or info fields. Connection control. There are three stages of connection control: 1. Connection establishment 2. Data transfer 3. Connection termination Some of the more sophisticated protocols also provide connection interrupt and recovery capabilities to cope with errors and other sorts of interruptions. Ordered delivery. Packets, frames, or blocks are often assigned sequence numbers to ensure ordered delivery of the data at the destination. In a large network with many nodes and possible routes to a destination, especially w