Network Models Network uses a combination of hardware

Network Models Network uses a combination of hardware and software to send data from one location to another l Performing a task is performed on different layers [Higher ---> Lower] l LAYERED TASKS l Concept of layers in our daily life – Two friends communicate through postal mail

Layered Tasks

Layered Tasks Hierarchy l Task must be done in the order given in the hierarchy – Sender site from up to down – Receiver site from down to up Services l Sender site each layer uses the services of the layer immediately below it Higher Middle Lower Carrier

The OSI Model l l l 1947 established ISO : International Standards Organization ISO model covers network communications is OSI : Open System Interconnection Introduced in the late 1970 s. Open system: a set of protocols that allows any two different systems to communicate regardless of their underlying architecture. Purpose of the OSI : to show to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software. OSI model not a protocol, it is a model for understanding and designing a network architecture that is flexible, robust, and interoperable.

ISO is the organization. OSI is the model

THE MODEL Is a layered framework, for design of computer systems that allows communication across all types of computer systems l Consists of seven layers each defines a part of the process of moving information across network l l Understanding of the fundamentals of the OSI model provides a solid basis for exploring data communications.

Seven layers of the OSI model

Layered Architecture When device A sent message to device B l Message may pass intermediate nodes l Only the first three layers of the model involve l

Layered Architecture At single machine l l Each layer defines functions distinct from other layers Each calls upon the services of the layer below it – Layer 3 uses services from layer 2 – Layer 3 provides services for layer 4 Between Machines l Layer x on one machine communicates with layer x on another machine by protocols. – Communication is governed by an agreed-upon series of rules and conventions called protocols – Process that communicate a given layer on one machine are called Peer-to. Peer process – Communication between machines is therefore peer-to-peer process using the protocols appropriate to a given layer

Peer-to-Peer process On the sending machine l Each layer in sending device – adds its own information to the message it receives from the layer just above it – pass the whole package to the layer below it – At layer 1 the message converted to a form that can be transferred to the receiving machine On receiving machine – Message is unwrapped layer by layer l layer 2 removes the data meant for it, then passes the rest to layer 3 and so on.

Interfaces Between Layers Between any pair of adjacent layers sending and receiving done by interface l Each one defines what information and services must provide for layer above it. l Well-defined interfaces and layer functions provide modularity to a network ( could modified or replace without affect surrounding ) l

Organization of the Layers l l Seven layers could be belonged to three subgroups Network support layers: layers 1, 2 and 3 – deal with physical aspects of moving data from one device to another l l Such as electrical specifications, physical connections, physical addressing, and transport timing and reliability User support layer: layer 5, 6, 7 It allows interoperability among unrelated software systems Layer 4, transport layer – It links the two subgroups – Ensures that what the lower layers transmitted is in a form that the upper layers can use – l l l The upper OSI layers are implemented in software Lower layers implemented in hardware and software except for the physical layer which is mostly hardware.

Organization of the Layers

Organization of the Layers Each layer adds a header to data except layer 2 adds also trailer l When data passes physical layer (layer 1) changed into an electromagnetic signal and transported through physical link l When reaching destination, the signal passes into layer 1 and it transformed back into digital form. l When data reaches the next higher layer l – header and trailers corresponding sending layer are removed – Action appropriate to that layer are taken l In layer 7, message will be in appropriate format ( application )

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

Physical Layer l The physical layer is responsible for transmitting individual bits from one node to the next

Physical Layer The major duties of this layer are: l Physical characteristics of interfaces and medium : – Defines : l characteristic of the interface between the devices and the transmission medium l Type of transmission medium l Representation of bits : Defines the type of encoding (how 0 s and 1 s are changed to signals: electrical or optical) l Data rate ( transmission rate ) : number of bits sent each second (duration of a bit) l Synchronization of bits : synchronized at bit level of sender and receiver

Physical Layer l Line configuration: is concerned with the connection of the devices to the media – Point-to-point configuration – Multipoint configuration l Physical topology: defines how devices are connected to make a network. – Mesh, star, ring, bus, or a hybrid. – Hybrid is a combination of two or more topologies l Transmission mode: defines the direction of transmission between two devices: simplex, halfduplex, or full-duplex.

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

Data Link Layer

Data Link Layer The data link layer is responsible for transmitting frames from one node to the next l Framing : divided bits into manageable data units called frames l Physical addressing : If frames distributed – On same the network, header (physical address of sender and receiver) added to frame – On different networks, receiver address is the address of device that connects networks l Flow control : controls transmission rate between sender and receiver l Error control : Link layer adds reliability to the physical layer by adding trailer to frame : – – l Mechanisms to detect and retransmit damaged or lost frames Mechanisms to prevent duplication of frames Access control : which device has control over at any given time

Hop-to-hop (node-to-node) delivery

Example l l A node with physical address 10 sends a frame to a node with physical address 87 Frame contains physical address in the header Rest of header contains other information needed at this level The trailer contains extra bits needed for error detection

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

Network Layer Responsible for the source-to-destination delivery across multiple networks l Ensures that each packet gets from its point of origin to its final destination l No need for network layer if systems on the same networks l

Network Layer l Duties of the network layer: – Logical addressing : physical address locally but for universal – A header added to the packet include logical address of sender and receiver – Routing : internetwork connected by devices ( routers or gateways ) which route or switch the packets to their final destination, network layer provide this mechanism

Source-to-destination delivery

Example l Send from A to P ( network address ) or from 10 to 95 ( physical address ) l Different networks l Two addresses required l Network address is universal address l Network address = logical address Logical address remain the same from source to destination ( A and P ) l Physical address changes l

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

Transport Layer Responsible for process-to-process delivery of the entire message Compare with Network layer l l – – Delivery of individual packets Does not recognize any relationship between those packets Ensures that whole message l – – Intact: not changed or broken In order ( manage error control + flow control)

Transport Layer

Transport Layer l Service-point addressing: transport layer gets the entire message to the correct process on that computer (from specific process [ running program ] on one computer to a specific process [ running program ] on the other. – – – l Segmentation and reassembling: the message is reassembled – – l Correctly upon sequence number Identify and replace packets that lost in the transmission Connection control : either – – l l Transport layer adds header called a service—point address (port address) Network layer gets each packet to the correct computer Transport layer gets the entire message to the correct process on that computer connectionless : each packets treat independent and delivers it to destination connection-oriented : make connection first then delivers packets then terminated Flow control: not on single link (data link) But end to end. Error control: not on single link? But process-to-process – – Sending transport layer ensures message arrives at transport layer in receiving without error (damage, loss, or duplication) error correction is achieved by retransmission.

Reliable process-to-process delivery of a message

The session layer is responsible for dialog control and synchronization.

Session Layer l l The session layer is the network dialog controller. IT establishes, maintains, and synchronizes the interaction among communicating systems. Responsibilities: l Dialog control: – – Allows two system to enter into a dialog Allows the communication between two processes to take place in: l l l Half-duplex or Full-duplex Synchronization: – Allows process to add checkpoints (synchronization points) to a stream of data – Example : add checkpoint every 100 pages for sending 2000 pages – If crash happens during the transmission of page 523 retransmission began at page 501.

Session Layer

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

Presentation Layer Concern with the syntax and semantic of the information exchanged between two systems

Presentation Layer Responsibilities l Translation : Information should change to bits stream before transmit – Working with different encoding methods ( different computers) – Presentation layer is responsible for interoperability between these different encoding methods. – At sender l Change information ( sender-dependent format to common format) – At receiving l Change information (common format to receiver-dependent format) l Encryption – Encryption at sending and Decryption at receiving l Compression : – Data compression reduces the number of bits to be transmitted – Compression needs in transmitted multimedia

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

Application layer Enables the user (human or software) accessing the network l It provides user interfaces and services ( electronic mail, remote file access and transfer, World Wide Web) l

Application layer l Mail services : the basis for e-mail forwarding and storage l File transfer and access: lets user to access file in a remote host – Change or read – Retrieve files – Manage or control files Remote log-in: log into a remote computer and access the resources l Accessing the WWW l

Summary of Layers

TCP/IP PROTOCOL SUITE Invented before OSI The layers in the TCP/IP protocol suite do not exactly match those in the OSI model. l The original TCP/IP protocol suite was defined as having four layers: l l – – l host-to-network equivalent to internet, equivalent to transport, equivalent to application. equivalent to physical + data link network part of duties of session + presentation + application However, when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five layers: – – – physical, data link, network, transport, application.

TCP/IP PROTOCOL SUITE l The first four layers provide functions that correspond to the first four layers of the OSI model: Physical standers – Network interfaces – Internetworking – Transport functions – l The three topmost layers functions in the OSI model are represented in TCP/IP by application layer

TCP/IP and OSI model

TCP/IP PROTOCOL SUITE l l l l TCP/IP is a hierarchical protocol made up of interactive modules Each layer in TCP/IP provides specific function The modules are not necessarily interdependent, but in OSI module specifies which functions belong to each of its layers. Each layer has independent protocols TCP/IP is a hierarchical protocol : each upper-level supported by one or more lower-level protocols depending on the needs of the system. AT transport layer, TCP/IP defines three protocols – Transmission Control Protocol (TCP) – User Datagram Protocol (UDP) – Stream Control Transmission Protocol (SCTP) At network layer, the main protocol defined by TCP/IP is the Internetworking Protocol (IP)

TCP/IP PROTOCOL SUITE Physical and Data Link Layers – TCP/IP does not define any specific protocol but it supports all the standard protocols. – A network in TCP/IP internetwork can be local or a wide-area network Network Layer – At Internetwork layer, TCP/IP supports the Internetworking Protocol. IP uses four supporting protocols: l l ARP RARP ICMP IGMP

TCP/IP PROTOCOL SUITE Internetworking Protocol (IP) – It is the transmission mechanism used by the TCP/IP protocols. – It is unreliable and connectionless protocol (a best-effort delivery service). l A best effort: IP provides no error checking or tracking. l IP assumes the unreliability of the underlying layers and does its best to get a transmission through to its destination, but with no guarantees. – IP transport data in packets separately called datagrams. – Datatgrams can travel along different routes and can arrive out of sequence or be duplicated. – IP does not keep track of the routes and has no facility for reordering datagrams when they arrive their destination. – IP provides bare-bones transmission service that free the user to add only those facilities necessary for a given application and thereby allows for maximum efficiency. (covers the weakness in its functionality)

TCP/IP PROTOCOL SUITE Address Resolution Protocol – Associated a logical address with a physical address. – On a typical physical network, LAN, each device is identified by a physical or station address, usually imprinted on the network interface card (NIC). – ARP is used to find the physical address of the node when its Internet address is known. Reverse Address Resolution Protocol – RARP allows a host to discover its Internet address when it knows only its physical address. – RARP is used when a computer is connected to a network for the first time or when a diskless computer is booted.

TCP/IP PROTOCOL SUITE Internet Control Message protocol ICMP is a mechanism used by a host and gateways to send notification of datagram problems back to the sender. – ICMP sends query and error reporting messages. – Internet Group Message Protocol – IGMP is used to facilitate the simultaneous transmission of a message to a group of recipients. Transport Layer Traditionally the transport layer represented in TCP/IP by TCP and UDP. IP is host-to-host protocol, deliver a packet from one physical device to another. UDP and TCP are transport level protocols responsible for delivery of a message from a process to another process. – SCTP, has been devised to meet the needs of some newer applications. – – –

TCP/IP PROTOCOL SUITE User Datagram Protocol – – UDP is the simple of the two standard TCP/IP transport protocol. UDP is process-to-process adds: l l l port addresses Checksum error control Length information to the data from the upper layer Transmission Control Protocol – – – TCP provides full transport-layer services to applications. TCP is a reliable stream (connection-oriented) transport protocol. At sending end of each transmission: l l – TCP divides a stream of data into smaller units called segments Each segment includes a sequence number for recording after receipt An acknowledgment number for the segments received Segments carried across the internet inside of IP datagrams. At receiving end l TCP collects each datagram as it comes in and reorders the transmission based on sequence numbers

TCP/IP PROTOCOL SUITE Stream Control Transmission Protocol – SCTP provides support for newer applications as voice over the Internet. – SCTP is a transport layer protocol that combines the best features of UDP and TCP. Application Layer – Application layer in TCP/IP is equivalent to the combined session, presentation, and applications layers in the OSI model. – Many protocols are defined at this layer.

ADDRESSING Four levels of addresses are used in an internet employing the TCP/IP protocols:

ADDRESSING Each address is related to a specific layer in the TCP/IP architecture.

ADDRESSING Physical Addresses – It is known as the link address. – It is the address of a node as defined by its LAN and WAN. – It is included in the frame used by the data link layer. – It is the lowest-level address. – It has authority over the network (LAN or WAN). – The size and format of these addresses vary depending on the network. l The Ethernet uses a 6 -byte (48 bit) physical address that is imprinted on NIC. l Local. Talk (Apple) has 1 -byte dynamic address.

ADDRESSING 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.

ADDRESSING As we will see in Chapter 13, 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: 07: 01: 02: 01: 2 C: 4 B A 6 -byte (12 hexadecimal digits) physical address.

ADDRESSING Logical Addresses – It is necessary for universal communications. – Physical addresses are not adequate in an internetwork environment where different networks can have different address format. – A universal addressing system is needed in which each host can be identified uniquely, regardless of the underlying physical newtwork. – A logical address in the Internet is currently a 32 -bit address. – No two publicly addressed and visible hosts on the Internet can have the same IP address.

IP Addresses 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.

Port Addresses The end objective of Internet communication is a process communicating with another process. l In TCP/IP architecture, the label assigned to a process is called a port address. l A port address in TCP/IP is 16 bits in length. l

Port 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

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

Port Addresses As we will see in Chapter 23, 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.

Specific Addresses E-mail address Universal Resource Locator (URL)