Data Communication and Computer Networks Data link layer The Data Link layer of a model typically has the following responsibilities: 1. Creates a frame 2. Creates an error-free logical connection Error control Flow control 3. Makes sure the receiver stays synchronized with the incoming data stream. Data Communication and Computer Networks

Data Communication and Computer Networks Flow Control Ensuring the sending entity does not overwhelm the receiving entity: Preventing buffer overflow. Propagation time. Transmission time. Data Communication and Computer Networks

Data Communication and Computer Networks Error Control Detection errors Lost frames Damaged frames Automatic repeat request Data Communication and Computer Networks

Data Communication and Computer Networks Network layer Layer 3. The network layer is responsible: The delivery of individual packets from the original source to the final destination . Logical addressing: if the packet passes the network boundary we need another addressing system to help (source to destination) connection. Routing : route or switch the packet to final destination. Source-to-destination delivery (End-to-End). Data Communication and Computer Networks

Data Communication and Computer Networks Internet Protocol Provides information about how and where data should be delivered Subprotocol that enables TCP/IP to internetwork To internetwork is to traverse more than one LAN segment and more than one type of network through a router In an internetwork, the individual networks that are joined together are called subnetworks Data Communication and Computer Networks

Internet Protocol cont. IP is an unreliable, connectionless protocol, which means it does not guarantee delivery of data Connectionless Allows protocol to service a request without requesting verified session and without guaranteeing delivery of data Data Communication and Computer Networks

Position of IP in TCP/IP protocol suite Data Communication and Computer Networks

Data Communication and Computer Networks Datagram A packet in the IP layer is called a datagram, a variable-length packet consisting of two parts: header and data. The header is 20 to 60 bytes in length and contains information essential to routing and delivery. Data Communication and Computer Networks

Data Communication and Computer Networks IP datagram Data Communication and Computer Networks

Data Communication and Computer Networks IP datagram fields VER - version numbers, 4 and 6 HLEN - header length in 4-byte words. Value of 5 means 20 byte header DS (differentiated services or Service type ) - Data Communication and Computer Networks

IP datagram fields cont. Precedence bits - never used. Similar to PRI bits in IPv6. TOS (Type of Service) bits - If you want to send a packet with a special type of service, use one of the 5 bit sets. Data Communication and Computer Networks

IP datagram fields cont. If we call these 8 bits Differentiated Services (and not the older Service Type), then the first six bits are called code-points Data Communication and Computer Networks