e quickly discarded. Since a transmitter using a TCP expects to receive an acknowledgement for a correctly received packet, all discarded packets will auto- matically be retransmitted. A UDP communication will simply disregard the lost packet. Example 8.1: Communication Establishment A data communication using the internet goes through the following processes. An application in device A (e.g. a client) initiates a data transmission to another device B (e.g. a server) by generating a request to the transport layer. A transport layer protocol such as TCP attempts to establish a logical connection with the server by sending a connection request to the IP layer, indicating the application ports of both server and client. The IP layer adds the IP address of the client and server devices with further control information, forwarding the packet to the link layer which will forward the message to the next node in the local network. Each node is Packetised Data Communications Host A Host B Port or App Y service access point (SAP) App X App X App Y Logical connection (TCP connection) Global network address Network access protocol Logical connection Network access protocol (e.g. virtual citcuit) Physical Router Physical NAP 1 NAP 2 Network Network Physical Physical Figure 8.13 Internet packet flow identified by its static MAC address, a 48-bit address which uniquely identifies a device (a computer, a printer, a switch, etc.) A MAC layer packet is usually referred to as a 'frame', and carries the data and headers of all higher layers. Routers forward the message towards its des- tination from node to node, based on a locally determined path. The destination switch then receives the link layer packet, extracts the IP layer packet, then transport layer packet and finally the application data. Through this process an application-to-application connectivity, and layer-to-layer logical (and not physical) connectivity is established over which data flows. This is illustrated in Figure 8.13. Telecommunications Ecosystem and the Inte