©Brooks/Cole, 2003 Model and protocol  A model is the specification set by a standards organization as a guideline for designing networks.  A protocol is a set of rules that controls the interaction of different devices in a network or an internetwork.

©Brooks/Cole, 2003 Seven layers of OSI model  Physical layer  Data-link layer  Network layer  Transport layer  Session layer  Presentation layer  Application layer

©Brooks/Cole, 2003 Physical layer  The physical layer is responsible for transmitting a bit stream over a physical medium.  It encodes and decodes bits into groups of bits.  It then transforms a stream of bits into a signal.

©Brooks/Cole, 2003 Data-link layer  The data-link layer organizes bits into logical units called frames.  The data-link layer is responsible only for node-to-node delivery of the frame.  The data-link layer is often responsible for error handling between two adjacent stations.

©Brooks/Cole, 2003 Network layer  The network layer is responsible for delivery of a packet between the original source and final destination.  Using logical addresses (IP addresses) instead of physical addresses.  Example of IP address  (4 Bytes)

©Brooks/Cole, 2003 Transport layer  The transport layer is responsible for source-to-destination delivery of the entire message.  The transport layer is responsible for breaking the entire message into several packets and delivery them to the network layer.  The transport layer is responsible for ensuring that the whole message is transmitted.  If packets arrive out of order, they must be reorganized.

©Brooks/Cole, 2003 Session layer  The session layer is designed to control the dialog between users.  The synchronization points divides a long message into smaller ones and ensure that each section is received and acknowledged by the receiver.  Most network implementations today do not use a separate session layer, their services are usually included in the application layer.

©Brooks/Cole, 2003 Presentation layer  The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems.  It deals with the fact that different systems use different coding methods.  Compress and decompress data  Encrypt and decrypt data  Most implementations do not use a presentation layer today, their services are usually included in other layer.

©Brooks/Cole, 2003 Application layer  The application layer enables the user to access the network.  It defines common applications that can be implemented to make the job of the user simpler.

©Brooks/Cole, 2003 CONNECTINGDEVICESCONNECTINGDEVICES 6.4

Figure 6-8 Repeater  A repeater is an electronic device and operate only in the physical layer of the OSI model.  A repeater can regenerate the signal and send it to the rest of the network.

©Brooks/Cole, 2003 Figure 6-9 Bridge

©Brooks/Cole, 2003 Bridges  In bus topology, a bridge is a traffic controller.  It can divide a long bus into smaller segments so that each segment is independent trafficwise.  The bridge uses a table to decide if the frame needs to be forwarded to another segment.  With a bridge, two or more pairs of stations can communicate at the same time.

©Brooks/Cole, 2003 Figure 6-11 Routers in an internet

©Brooks/Cole, 2003 Routers  Whereas a bridge filters a frame based on the physical address of the frame, a router routes a packet based on the logical address of the packet.  Whereas a bridge may connect two segments of a LAN, a router can connect two independent networks.

©Brooks/Cole, 2003 Gateways  A gateway is a connecting device that acts as a protocol converter.  A gateway is usually a computer installed with the necessary software.  Today the term gateway is used interchangeably with the term router. The distinction between the two terms is disappearing.

©Brooks/Cole, 2003 Protocols  TCP: Transmission control protocol  UDP: User datagram protocol  IP: Internet protocol  FTP: File transfer protocol  SMTP: Simple mail transfer protocol  POP: Post office protocol  HTTP: Hypertext transfer protocol