Week 2

structured as a hierarchy of layers each layer offers certain services to

the higher layers, while hiding from the higher layers the details of how those services are implemented: hierarchical modularity

a particular layer in one network node communicates with the corresponding layer in another network node by using an agreed protocol for that layer this communication may be actual or virtual.

Layering reduces the complexity of building scalable networks through encapsulation and modularity

Network Model

Application

Process-to-Process channels

Host-to-Host Connectivity

Hop-to-Hop Connectivity

Physical Connectivity

In the late 1970’s ISO(International Standard Organization) formulated a Reference OSI(Open Systems Interconnection) Model:◦ To provide a common basis for the coordination of

standards development◦ To allow existing and evolving standards to be

placed into perspective with one another◦ has become the stand model for classifying

communications functions The Model is concerned with the structuring of

Communication to provide a reliable, open communication service which is independent of any specific manufacturer’s equipment or conventions

ISO - OSI ModelISO - OSI Model

As the message travels from A and B, it may pass through many intermediate nodes.

These intermediate nodes usually involves only the first three layers of the OSI model.

Within a single machine, each layer calls upon the services of the layer just below it.

Between machines, layer x on one machine communicates with layer x on another machine.

Communication between machines is peer-to-peer process using the protocols appropriate to a given layer.

Interface between each pair of adjacent layers allow passing of data through layers from A to B and then from B to A

Each interface defines the information and Services a layer must provide for layer above it.

Physical Address◦ Also known as link address, is the address of a node as defined by its LAN or

WAN.◦ It is included in the frame used by the data link layer. ◦ It is the lowest level address◦ Ethernet uses a 6 byte (48-bit) physical address known as MAC address.

Logical Address◦ Used for universal communications that are independent of underlying

physical networks.◦ Used to identify each host uniquely, regardless of underlying physical

network also known as IP Address◦ Logical address in the internet are 32-bit and 64-bit addresses also called

IPv4 and IPv6 address. Port Numbers

◦ It is 16-bit in length used to identify process on a host. Physical Address will change from hop-to-hop, but logical and port addresses usually remain same

Specific Address (host name)◦ Is user friendly address. E.g URL or email

Define the characteristics of the interface between the devices and the transmission media

Encode bits into signals and decode signals to get bits

Define transmission rate, which must be the same for both sender and receiver

Synchronize clocks

Framing: divide the data stream into manageable data units called “frames”

Physical addressing: insert the physical address of the next node into frame’s header

Flow control: prevent overflow at receiver

Error control: make sure that frames are correctly received

Access control: make sure that there is no link access conflict

A node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link. At the data link level this frame contains physical addresses in the header. These are the only addresses needed. The rest of the header contains other information needed at this level. The trailer usually contains extra bits needed for error detection

Logical addressing: e.g., IP addresses Routing: how to get to the destination?

We want to send data from a node with network address A and physical address 10, located on one LAN, to a node with a network address P and physical address 95, located on another LAN. Because the two devices are located on different networks, we cannot use physical addresses only; the physical addresses only have local jurisdiction. What we need here are universal addresses that can pass through the LAN boundaries. The network (logical) addresses have this characteristic.

Port addressing: A process is associated with a “port” Segmentation and reassembly: Application data are divided into segments and each segment has sequence no. Connection control: connection-less or connection-oriented? End to end reliability Flow control ( here its is performed end to end rather than across single link.) Service point addressing Error control (here its is performed end to end rather than across single link.) error correction is achieved through re-transmission.

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

Conceptual only and never seriously implementedSession: dialog controllerPresentation: handle syntax, semantics of the information exchanged between the 2 systems, designed for encryption, decryption, and compression

Dialog controller. The session layer allows the two systems to enter into a dialog. It allows the communication between two processes to take place either in half duplex or full duplex.

Synchronization. Session layer allows a process to add checkpoints or synchronization points into a stream of data. E.g. check file of 2000 pages after every 50 pages and is acknowledged independently. If system crashes due to some reason at page# 1000, then retransmission will take place and page 1000 and onward will be sent.

Translation ( Common Format) Encryption: To ensure privacy, encryption

is done. Compression: Reduces the no of bits to

be transmitted. E.g. data, video or audio file being compressed.

Internet (TCP/IP) Internet (TCP/IP) ArchitectureArchitecture

UDP TCP

Data Link

Physical

Applications

The Hourglass Model

Narrow Waist

FTP HTTP TFTPDNS

TCP UDP

IP

NET1 NET2 NETn…

… …

FTP HTTP TFTP

TCP UDP

IP

NET1 NET2 NETn…

TCP/UDPIP

Port No.

Network

Protocol Field

Type Field