Layer. The Data Link Layer The Data Link Layer is responsible for ensuring bit-level integrity of the data being transmitted. In short, its job is to make the layers above believe that the world is an error-free and perfect place. When a packet is handed down to the Data Link Layer from the Network Layer, it wraps the packet in a frame. In fact, the Data Link Layer is sometimes called the frame layer. The frame built by the Data Link Layer is made up of several fields, shown graphically in Figure 2-35, that give the network devices the ability to ensure bit-level integrity and proper delivery of the packet, which is now encased in a frame, from switch to switch. Please note that this is different from the Network Layer, which concerns itself with routing packets to the final destination. Even the addressing is unique: Packets contain the address of the ultimate destination, used by the network to route the packet properly; frames contain the address of the next link in the network chain (the next switch), used by the network to move the packet along, switch by switch. Frame Figure 2-35 Data Link Layer frame Address Control PKT 5 Packet Downloaded from Digital Engineering Library @ McGraw-Hill (www.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. Protocols Chapter 2 As the diagram illustrates, the beginning and end fields of the frame are called flags. These fields, made up of a unique series of bits (0111110), can only occur at the beginning and end of the frame-they are never allowed to occur within the bitstream inside the frame through a process that we will describe momentarily. These flags are used to sig- nal to a receiving device that a new frame is arriving or that it has reached the end of the current frame, which is why their unique bit pat- tern can never be allowed to occur naturally within the data itself-it could indicate to the receiver (falsely) that this is the end of the current frame. 