Computer Communication & Networks Lecture # 02 Nadeem Majeed Choudhary

Layering & Protocol Stacks

Reference Models OSI reference model TCP/IP

OSI Reference model Open System Interconnection 7 layers 1. Crate a layer when different abstraction is needed 2. Each layer performs a well define function 3. Functions of the layers chosen taking internationally standardized protocols 4. Number of layers – large enough to avoid complexity

Seven layers of the OSI model

Exchange using OSI Model

Peer-to-peer Processes Layer x on one machine communicates with layer x on another machine - called Peer-to-Peer Processes. Interfaces between Layers Each interface defines what information and services a layer must provide for the layer above it. Well defined interfaces and layer functions provide modularity to a network Organizations of the layers  Network support layers : Layers 1, 2, 3  User support layer : Layer 5, 6, 7 It allows interoperability among unrelated software systems  Transport layer (Layer 4) : links the two subgroups

The interaction between layers in the OSI model

OSI Layers

Physical layer Transporting bits from one end node to the next - type of the transmission media (twisted-pair, coax, optical fiber, air) - bit representation (voltage levels of logical values) - data rate (speed) - synchronization of bits (time synchronization) - deals with the optical, mechanical and electrical features - transmission modes ( half duplex, full duplex) physical connection

The physical layer is responsible for movements of individual bits from one hop (node) to the next. Note

Protocols: - Modems - Optical Cables, Connectors Network Devices: - Hubs, Repeaters, and Amplifier.

Data Link layer Transporting frames from one end node to the next one logical connection - framing - physical addressing - flow control - error control - access control

Data Link layer - hop-to-hop delivery-

Data Link layer - example-

The data link layer is responsible for moving frames from one hop (node) to the next. Note

Data Link layer Network Devices: -Bridge, Switch, ISDN Router, Intelligent Hub, NIC, Advanced Cable Tester

Data Link layer Protocols: Logical Link Control error correction and flow control manages link control and defines SAPs Media Access Control controls the type of media being used: CSMA/CD (Ethernet) Token Bus (ARCnet) Token Ring Demand Priority

Network Layer The network layer is responsible for the delivery of individual packets from the source host to the destination host.

20 Network layer End-to-End packet delivery  From the original source to a destination Needed when 2 devices are attached to different networks  What is the network definition here? Main duties: 1. Logical addressing 2. Routing 3. Switching 4. Congestion control and QoS Not a message

21 Source to destination delivery Data Link Network layer

22 Network layer - example - Network layer addresses Data Link layer addresses

23 The network layer is responsible for the delivery of individual packets from the source host to the destination host. Note

Network Layer Router works as the post office and network layer stamps the letters (data) for the specific destinations. Protocols: These protocols work on the network layer IP, ICMP, ARP, RIP, OSI, IPX and OSPF. Network Devices: Network devices including Router, Frame Relay device and ATM switch devices work on the network layer.

25 Transport layer Process-to-Process delivery of the entire message  From the original source to a destination Needed when several processes (running programs) active at the same time Main tasks:  Port addressing  Segmentation and reassembly  Congestion control  Flow control  Error control

Transport Layer  The transport layer is responsible for the delivery of a message from one process to another.

27 Transport Layer

28 Transport layer -an example of a reliable delivery -

29 The transport layer is responsible for the delivery of a message from one process to another. Note

Transport Layer Protocols: These protocols work on the transport layer TCP, UDP, SPX, NETBIOS, ATP and NWLINK. Network Devices: The Brouter, Gateway and Cable tester work on the transport layer.

31 The session layer is responsible for dialog control and synchronization. Note

Session Layer The session layer is responsible for dialog control and synchronization.

Presentation Layer

Presentation layer is a best layer for cryptography.

35 The presentation layer is responsible for translation, compression, and encryption. Note

Application Layer The application layer is responsible for providing services to the user.

Application Layer Services provided by Application layer: - File transfer, Access - Mail services

38 Application layer Enables user to access the network Provides services to a user   Remote file access and transfer (Telnet, FTP)  Access to WWW (HTTP)

39 The application layer is responsible for providing services to the user. Note

Application Layer Protocols: FTP, DNS, SNMP, SMTP, FINGER, TELNET, TFTP, BOOTP and SMB protocol are operated on the application layer. Network Devices: Gateway network device is operated on the application layer.

41 OSI Reference Model A convenient aid for remembering the OSI layer names is to use the first letter of each word in the phrase: All People Seem To Need Data Processing

42 Summary of layers and protocols Low-level protocols define the electrical and physical standards to be observed, bit- and byte-ordering and the transmission and error detection and correction of the bit stream High-level protocols deal with the data formatting, including the syntax of messages, the terminal to computer dialogue, character sets, sequencing of messages

43 TCP/IP Protocol

44 TCP/IP Vs OSI Model

45 Four Level of Addresses

46 Relationship of Layers & Addresses in TCP/IP

47 The physical addresses will change from hop to hop, but the logical addresses usually remain the same. Note

In Figure 2.19 a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link (bus topology LAN). As the figure shows, the computer with physical address 10 is the sender, and the computer with physical address 87 is the receiver. Example 2.1

Figure 2.19 Physical addresses

most local-area networks use a 48-bit (6-byte) physical address written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon, as shown below: Example :01:02:01:2C:4B A 6-byte (12 hexadecimal digits) physical address.

Figure 2.20 shows a part of an internet with two routers connecting three LANs. Each device (computer or router) has a pair of addresses (logical and physical) for each connection. In this case, each computer is connected to only one link and therefore has only one pair of addresses. Each router, however, is connected to three networks (only two are shown in the figure). So each router has three pairs of addresses, one for each connection. Example 2.3

Figure 2.20 IP addresses

Figure 2.21 shows two computers communicating via the Internet. The sending computer is running three processes at this time with port addresses a, b, and c. The receiving computer is running two processes at this time with port addresses j and k. Process a in the sending computer needs to communicate with process j in the receiving computer. Note that although physical addresses change from hop to hop, logical and port addresses remain the same from the source to destination. Example 2.4

Figure 2.21 Port addresses

Example 2.5 A port address is a 16-bit address represented by one decimal number as shown. 753 A 16-bit port address represented as one single number.

56 Readings Chapter 2 (B. A Forouzan)  Section 2.2,2.3, 2.4, 2.5

Q & A