1: Review Of Semester 1v2 in Sem 2v Provide an overview of encapsulation. Networking evolves to support current and future applications. By dividing and organizing the networking tasks into separate layers/functions, new applications can be handled without problems. The OSI reference model organizes network functions into seven categories, called layers. The task of most network managers is to configure the three lowest layers. Peer-to-peer functions use encapsulation and de- encapsulation as the interface for the layers.

Describe three needs that drive enterprise network improvements. The enterprise is a corporation, agency, school, or other organization that ties together its data, communication, computing, and file servers. Developments on the enterprise network include:  interconnected LANs that provide access to computers or file servers in other locations  higher bandwidth on the LANs to satisfy the needs of the end users  technologies that can be relayed for WAN service

Why Layered Network Model

Each layer uses its own layer protocol to communicate with its peer layer in another system. Each layer's protocol exchanges information, called protocol data units (PDUs), with its peer layers. For example, in TCP/IP the transport layer of TCP communicates with the peer TCP function by using segments. Each layer uses the services of the layer below it in order to communicate with its peer layer.

Step 1 A computer converts an message into alphanumeric characters that can be used by the internetworking system. This is the data. Step 2 The message data then changes to segments for transport on the internetwork system. The transport function ensures that the message hosts at both ends of the system can reliably communicate. Step 3 The data then forms a packet, or datagram, that also contains a network header that includes a source and destination logical address. The address helps the network devices send the packet across the network along a chosen path. Step 4 Each network device puts the packet into a frame. The frame enables the device to connect to the next directly-connected network device on the link. Step 5 The frame changes to a pattern of 1s and 0s for transmission on the medium (usually a wire). Some clocking function enables the devices to distinguish bits as they travel across the medium.

The physical layer provides access to the network media. The data link layer provides support for communication over several types of data links, such as Ethernet/IEEE media. Addressing schemes such as Media Access Control (MAC) and Internet Protocol (IP) provide a very structured method for finding and delivering data to computers or to other hosts on a network.

 Bridges that connect LAN segments and help filter traffic  Hubs that concentrate LAN connections and allow use of twisted-pair copper media  Ethernet switches that offer full-duplex, dedicated bandwidth to segments or desktops  Routers that offer many services, including internetworking and broadcast control

Ethernet—The first of the major LAN technologies, it runs the largest number of LANs. Token Ring—From IBM, it followed Ethernet and is now widely used in a large number of IBM networks. FDDI—Also using tokens, it is now a popular campus LAN. We will be studying the Ethernet IEEE LAN standards.

The 10Base-5 and 10Base-2 standards provide access for several stations to the same LAN segment. Stations are attached to the segment by a cable that runs from an attachment unit interface (AUI) in the station to a transceiver that is directly attached to the Ethernet coaxial cable.  10Base-2 (thin Ethernet) - allows coaxial cable network segments up to 185 m. long  10Base-5 (thick Ethernet) - allows coaxial cable network segments up to 500 m. long  10Base-T - carries Ethernet frames on inexpensive twisted-pair wiring

The Ethernet and data links prepare data for transport across the physical link that joins two devices. For example, as this graphic shows, three devices can be directly attached to each other over the Ethernet LAN.

The purpose of this target indicator is to show the listening and transmitting parts of CSMA/CD. The host listens for silence on the LAN (CS, Carrier Sense). Every host on the LAN is free to transmit when it hears silence (MA, Multiple Access). One node’s transmission traverses the entire network and is received and examined by every node.

Broadcasting is a powerful tool that can send a single frame to many stations at the same time. Broadcasting uses a data link destination address of all 1s (FFFF.FFFF.FFFF in hexadecimal). When improperly used, broadcasting can seriously affect the performance of stations by unnecessarily interrupting them. Broadcasts should, therefore, be used only when the MAC address of the destination is unknown, or when the destination is all stations

When a station wishes to transmit a signal, it checks the network to determine whether another station is currently transmitting. If the network is not being used, the station proceeds with the transmission. While sending a signal, the station monitors the network to ensure that no other station is transmitting at that time. If two stations transmit at the same time if this should occur, they would cause a collision. All stations stop sending frames for a randomly selected time period.

Two important types of addresses are data link layer addresses and network layer addresses. Data link layer addresses, also called physical hardware addresses or MAC addresses, are typically unique for each network connection. In fact, for most LANs, data link layer addresses are located on the NIC (network interface card).

One way in which the sender can ascertain that MAC address that it needs is to use an ARP (Address Resolution Protocol). Router will provide its own MAC address if the host and destination are on different subnets.

In a TCP/IP environment, end stations communicate with servers or other end stations. This can occur because each node using the TCP/IP protocol suite has a unique 32-bit logical address. This address is known as the IP address.

Subnets improve the efficiency of network addressing. Adding subnets does not change how the outside world sees the network, but within the organization, there is additional structure.

From an addressing standpoint, subnets are an extension of a network number. Network administrators determine the size of subnets based on the expansion needs of their organizations. Network devices use subnet masks to identify which part of the address is for the network and which part represents host addressing.

A host number of 0 is reserved for the wire (or subnet) address, and a host value of all 1s is reserved because it selects all hosts—that is, it is a broadcast. The 3 bits in the example are enough for the required five hosts per wire (actually, giving you host numbers 1 - 6).

6 host addresses and 30 useable subnets. BitsSubnet MaskSubnetsHosts

The presentation layer (Layer 6) formats and converts network application data into text, data encryption, graphics, video, audio, or whatever format is necessary for the receiving device to understand it. The session layer (Layer 5) establishes, manages, and terminates communication interactions between applications. NFS, SQL and X Windows System all operate at this layer The transport layer (Layer 4) is responsible for transporting and regulating the flow of information from source to destination, and for doing it reliably and accurately. Its functions include:  connection synchronization  flow control  error recovery  reliability through windowing The application layer (Layer 7) supports the communicating component of an application. It does not provide services to any other OSI layer. However, it does provide services to application processes lying outside the scope of the OSI model (e.g. spreadsheet programs, Telnet, WWW, etc.)

In the context of the OSI reference model, the application layer (Layer 7) supports the communicating component of an application.

The presentation layer (Layer 6) of the OSI reference model is responsible for presenting data in a form that a receiving device can understand. It serves as the translator - sometimes between different formats - for devices that need to communicate over a network, by providing code formatting and and conversion. Another function of Layer 6 is the encryption of data Layer 6 converts and translates the two different formats.  PICT -  TIFF -  JPEG -  MIDI -  MPEG -  QuickTime -

The session layer (Layer 5) establishes, manages, and terminates sessions between applications. It coordinates the service requests and responses that occur when applications establish communications between different hosts.

As the transport layer sends its data segments, it also ensures the integrity of the data. This transport is a connection-oriented relationship between communicating end systems. Some of the reasons for accomplishing reliable transport are as follows:  It ensures that senders receive acknowledgement of delivered segments.  It provides for retransmission of any segments that are not acknowledged.  It puts segments back into their correct sequence at the destination device.  It provides congestion avoidance and control.

One reason for using a multi-layer model such as the OSI reference model is so that multiple applications can share the same transport connection. Transport functionality is accomplished segment by segment. This means that different data segments from different applications, being sent to the same destination or to many destinations, are sent on a first-come, first-served basis.

In concept, one device places a call to another device that the other device must accept. Protocol software modules in the two operating systems communicate by sending messages across the network to verify that the transfer is authorized and that both sides are ready.

While data transfer is in progress, congestion can occur for two different reasons. First, a high-speed computer might generate traffic faster than a network can transfer it. Second, if many computers send datagrams simultaneously to a single destination, that destination can experience congestion.

Reliable connection-oriented data transfer means that data packets arrive in the same order in which they are sent. Protocols fail if any data packets are lost, damaged, duplicated, or received in the wrong order. In order to ensure transfer reliability, receiving devices must acknowledge receipt of each and every data segment.

Reliable delivery guarantees that a stream of data that is sent from one device will be delivered through a data link to another device without duplication or data loss. Positive acknowledgment with retransmission is one process that guarantees reliable delivery of data streams. It requires a recipient to send an acknowledgment message to the sender whenever it receives data.

Each of the upper-level layers performs its own functions. However, their functions depend on lower-layer services. All four upper layers - application (Layer 7), presentation (Layer 6), session (Layer 5), and transport (Layer 4) - can encapsulate data in end-to-end segments The End