Chapter 2 Data Communication Models
Figure 2-1 Peer-to-Peer Communication
Figure 2-2 Encapsulation/Decapsulation
Figure 2-3 Service Access Points
Figure 2-4 OSI Model
Figure 2-5 Physical Layer Coordinates the functions required to create a bit link (a physical connection) btw S and R.
Physical Layer Coordinates the functions required to create a bit link (a physical connection) btw S and R. Main responsibilities: • Representation of bits bits signals (digital or analog) • Data rate (transmission rate) defines how fast data should be sent ( # of bits sent per second) • Bit synchronization S and R must be synchronized at the bit level • Characteristics of interfaces defines how fast data should be sent • Transmission medium (TP, Coax, OP, or Air) defines the type and characteristics of the medium • Transmission mode (simplex, half-duplex, full-duplex ) defines the direction of transmission btw two devices
Figure 2-6 Data Link Layer Responsible for hop-to-hop delivery
Data Link Layer Responsible for hop-to-hop delivery Main responsibilities: • Framing marks the beginning of the packet with a specific sequence of bits (different from the idle bits) • Addressing to ensure one-to-one communication from one station to another Each frame should have a source address and a destination address (physical address or MAC address, not the same as IP address) • Medium access control simultaneous TX on a shared medium collision need mechanism to coordinate the TX medium access control (MAC) • Flow control (for fast sender slow receiver) controls the rate the frames are sent on the link (hop-to-hop). can use an ACK mechanism to ensure the frames are received. • Error control detects and retransmits damaged or lost frames
Figure 2-7 Network Layer Responsible for the source-to-destination (end-to-end) delivery of packets, possibly across multiple networks
Network Layer Responsible for the source-to-destination (end-to-end) delivery of packets, possibly across multiple networks Main responsibilities: • Creating a logical end-to-end connection end systems should see a logical connection without worrying about the links and connecting devices • Hiding the details of the lower layer hide the details of Phy.L and D.L from the upper layers (T.L and above) • Addressing D.L handles the addressing problems locally. If a packet passes the network boundary, another type of addressing is needed to distinguish S and D • Routing finds the optimum S-to-D path from possibly many different paths available
Figure 2-8 Transport Layer Responsible for end-to-end error-free delivery of packets (N.L oversees host-to-host delivery of individual packets)
Transport Layer Responsible for end-to-end error-free delivery of packets (N.L oversees host-to-host delivery of individual packets) Main responsibilities: • Service-point addressing several processes running on a computer at the same time. S-to-D means not only host-to-host, but also from a specific process on one computer to a specific process on the other T.L header must include another type of address called a service-point address (or port address) • Segmentation and reassembly Segmentation: a message several numbered transmittable segments, Reassembly: received segments a message, based on the sequence # • Connection control T.L can be either connectionless or connection-oriented.
Transport Layer (continued) A conn-less T.L entity treats each segment as an independent packet and delivers it to T.L entity at the destination. A conn-oriented T.L entity makes a connection with the T.L entity at the destination first before delivering the packets. Connection is terminated after all the data have been transferred. • Flow control Like D.L, T.L is also responsible for flow control. Unlike D.L, flow control at T.L end-to-end across network(s) flow control at D.L hop-to-hop across a single link. • Error Control (making sure the entire msg. is received without error) Like D.L, T.L is responsible for error control. Unlike D.L, flow control at T.L end-to-end across network(s) flow control at D.L hop-to-hop across a single link. Error handling is usually achieved through retransmission.
Session Layer Establishes, maintains, and synchronizes the interaction btw communication systems (S.L is sometimes called the network dialog controller) Main responsibilities: • Half- and full-duplex service S.L can be used with T.L to provide a half-duplex service (two-way dialog, one at a time) S.L can also use two transport connections to provide a full-duplex service. (two-way dialogue, two at the same time) • Synchronization S.L allows two end systems to add check points (synchronization marks) during data transmission. (useful for the transfer of large amount of data in one transport connection) |______|______|______|_____|______|__… • Atomization Several T.L connections single task One of the connections fails the whole task should be cancelled. S.L can be used for this purpose.
Presentation Layer Responsible for data translation, encryption, and compression. (Concerned with the syntax and semantic of the information) Main responsibilities: • Translation S-dependent format common format R-dependent format (ASCII ------------------------------------- EBCDIC) (Big-endian ------------------------------------- Little-endian) • Encryption Original msg. ---------Encrypted msg.--------- Original msg. (Encryption) (Decryption) • Compression Technique to reduce the number of bits to be transmitted. Important for transmitting multimedia data (text, audio, and video)
Application Layer Responsible for providing support protocols for users, human or software, to access the network Main responsibilities: • Support for network management Support-type applications • Support for real application Remote login , Electronic mail , Remote file transfer , World wide web , … • Support for network security Security/Authentication protocols Key exchange/management protocols
Application Layer Responsible for providing support protocols for users, human or software, to access the network Some Application Protocols from TCP/IP Protocol Suite: • Support-type applications SNMP, on top of UDP BGP, on top of TCP ICMP and OSPF, on top of IP • Real application Remote login (TELNET), Electronic mail (SMTP) Remote file transfer (FTP), World wide web (HTTP) (These four are all on top of TCP) • Support for network security Transport Layer Security protocol (TLS) over TCP/UDP Remote authentication Dial-In User Service (RADIUS) over UDP Internet Key Association and Key Management Protocol (ISAKMP)
Figure 2-9 Summary of Layer Functions
Figure 2-10 TCP/IP and the OSI Model
Figure 2-11 LAN Compared with OSI Model
Figure 2-12 IEEE Standards for LANs