d to the same network and they share the transmission capacity of a broadcast channel. The MAC sublayer cares about the functions dependent on the network hardware. The most popular LAN technology is carrier sense multiple access with collision detection (CSMA/CD), or the "Ethernet," which is available at many data rates (see Section 6.5). If we upgrade our network to a higher data rate LAN, we change only the MAC sublayer. The LLC considers most of the data integ- rity aspects, such as retransmission and acknowledgments, and it remains the same. In the case of a simpler point-to-point link there is no need for a sepa- rate MAC layer and one data link layer protocol specification may cover the whole data link layer. In a LAN each computer has its own MAC address (hardware address). This address is used to identify the source and destination of each frame in the broadcast channel. With the help of this address, computers can have a point-to-point connection via a broadcast channel that is shared by many other point-to-point connections. Note that this hardware or MAC address is used only inside a LAN, it is not transmitted to other networks (see Section 6.5.4). Some examples of data link layer protocols are as follows: Data Communications IS 3309: HDLC; variants are used in most modern networks, such as GSM and ISDN; Q.921, LAP-D, ISDN layer 2: HDLC-based data link layer protocol; IEEE 802.X = IS 8802-X: MAC layer "Ethernet"-type LANs and WLANs; IEEE 802.2 = IS 8802-2: LLC of LANs (when OSI stack is in use); for a complete LAN data link layer both 8802-2 (LLC) and 8802-X (MAC) are needed. 6.3.3.4 Network Layer The layers below the network layer are only interested in the point-to-point connections between two nodes. The network layer has some knowledge about the structure of the network and, together with the network layers of the other nodes it services, packets are routed through the network to the destination. Each node has its own (network layer) global address. A key issue is to determine how packets are rout