Wrapping of data with necessary protocol (adding header or trailer or both) Why Data Encapsulation P ITo add control information(in form of header or trailer or both) n. The data after encapsulated is called ders are added to the data at layers 6, 5, 4, 3, and 2. Trailers are usually added only at layer2. The control information falls into the three categories: D Address: The address of the sender and/or receiver may be D Error-detecting code: some sort of frame check sequence 16005000181010 D Protocol control: additional information is included to implement the protocol functions such as: priority, quality service and security Functions of the Physical Layer Functions of the physical layer Physlca-01010111101 Line configuration-the connection of devices to the medium (point-to-point or multipoint configuration) 111 topology- the configuration for the devices to b orm a network(mesh topology, star topology, ring bus topology card). It also defines the type of transmission medium(e. g. Representation of bits- encoding the bit (Os The physical layer also defines th between two de electrical or optical signals. This layer also defines the type of ncoding(e. g. ASK or FSK on rate in term of number of bit sent Example for the physical layer protocol Synchronization of bits- the sender and the receiver clocks stAt ust be synchronized Functions of the data link le Functions of the Data Link Layer very, it eceiver is less than the rate produced in the yer will impose a flow control mechanism to prevent overwhelming network layer). It also take care of the following functions: mit damaged Framing- The data link layer divides the stream of bits received lost frames and also prevent duplication of frames. Error control is aye ta define sei si ah a ddress nf the s endes s beae ard esse to determine whid nd/or receiver(desti device has contr he link at any given time.4 哈工大计算机学院 李全龙 Network Application Development 1: Introduction 19 Data Encapsulation Application Presentation Session Transport Network Data link Physical 7 6 5 4 3 2 1 PC 1 PC 2 User data AH PH SH AH PH SH TH NH DT AH PH SH TH NH DH 10101010101011111110000000101010 A-PDU S-PDU P-PDU T-PDU N-PDU Application Presentation Session Transport Network Data link Physical 7 6 5 4 3 2 1 User data AH PH SH AH PH SH TH AH PH SH TH NH DT AH PH SH TH NH DH 10101010101011111110000000101010 A-PDU S-PDU P-PDU T-PDU N-PDU Network Wrapping of data with necessary protocol information before network transit (adding header or trailer or both) 哈工大计算机学院 李全龙 Network Application Development 1: Introduction 20 Why Data Encapsulation ? ‡To add control information (in form of header or trailer or both) to the data being encapsulated in order to ensure accurate and secure communication. The data after encapsulated is called protocol data unit (PDU) The control information falls into the three categories: ‡ Address: The address of the sender and/or receiver may be indicated. ‡ Error-detecting code: some sort of frame check sequence ‡ Protocol control: additional information is included to implement the protocol functions such as: priority, quality of service and security ‡Note: Headers are added to the data at layers 6,5,4,3, and 2. Trailers are usually added only at layer2. 哈工大计算机学院 李全龙 Network Application Development 1: Introduction 21 Functions of the Physical Layer 10101010111101 DL - PDU From data link layer Physical layer 10101010111101 DL - PDU To data link layer Physical layer • Physical characteristics of interfaces and media - interface between the devices and the transmission medium (e.g. network card). It also defines the type of transmission medium (e.g. wire) • Representation of bits - encoding the bit (0s and 1s ) into electrical or optical signals. This layer also defines the type of encoding (e.g. ASK or FSK) • Data rate - the transmission rate in term of number of bit sent each second (e.g. 56 Kb/s) • Synchronization of bits - the sender and the receiver clocks must be synchronized 哈工大计算机学院 李全龙 Network Application Development 1: Introduction 22 Functions of the Physical Layer • Line configuration - the connection of devices to the medium (point-to-point or multipoint configuration) • Physical topology - the configuration for the devices to be connected to form a network (mesh topology, star topology, ring topology or bus topology) • Transmission mode - The physical layer also defines the direction of transmission between two devices: Simplex, half￾duplex, or full-duplex Example for the physical layer protocol: • RS-232 哈工大计算机学院 李全龙 Network Application Development 1: Introduction 23 Functions of the Data Link Layer 10101010111101 DT DH N - PDU Data link layer Frame To physical layer From network layer 10101010111101 DT DH N - PDU Data link layer Frame From physical layer To network layer The data link layer is responsible for node-to-node delivery, it makes the physical layer appear error free to the upper layer (network layer). It also take care of the following functions: • Framing - The data link layer divides the stream of bits received from the network layer into manageable data units called frames • Physical addressing - the data link layer adds a header to the frame to define the physical address of the sender (source address) and/or receiver (destination address) of the frame 哈工大计算机学院 李全龙 Network Application Development 1: Introduction 24 Functions of the Data Link Layer 20 25 32 48 62 DT Data 20 62 Trailer Source address destination address • Flow control - If the rate at which the data are absorbed by the receiver is less than the rate produced in the sender, the data link layer will impose a flow control mechanism to prevent overwhelming the receiver • Error control - mechanism that can detect and retransmit damaged or lost frames and also prevent duplication of frames. Error control is normally achieved by the trailer at the end of the frame • Access control - When one or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time