iable hence care should be taken to handle frame loss, errors, and retransmission while implementing this protocol. The best-known example of the DLL is Ethernet. Other examples of data link protocols are HDLC (high level data link control) and ADCCP (advanced data communication control protocol) for point-to-point or packet-switched networks and Aloha for local area networks. 10.4 Network Layer This is the third lowest layer (L3) in the ISO/OSI model. While the data link layer deals with devices in a local area, the network layer deals with actually getting data from one computer to another even if it is on a remote network. That is, the network layer is responsible for source to destination packet delivery, while the data link layer is responsible for node to node frame delivery. Network layer achieves end to end delivery using logical addressing. Addressing performed in data link layer is for the local physical device, while the logical address given in network layer must be unique across an entire internetwork. For a network like the Internet, the IP (Internet protocol) address is the logical address and every machine on the Internet needs to identify itself with a unique IP address. The functionalities of the network layer are: Addressing: Every device in the network needs to identify itself with a unique address called the logical address that is assigned in the network layer. Packetizing: The data to be communicated is converted to packets in the network layer and each packet has a network header (see Figure 10.2). The data to be packetized is provided by the higher layers in the OSI model. This activity is also referred to as datagram encapsulation. It can happen that the packet to be sent to the data link layer is bigger than what the DLL can handle, in which case the packets are fragmented at the source before it is sent to the DDL and at the destination the packets are reassembled. Routing: The software in the network layer is supposed to perform routing of packets. Incoming packets from several source