Objective Questions

1. B) Star & Hybrid 2. A) LAN 3. C) Open System Interconnection4. B) TCP/IP 5. C) Twisted Pair 6. D) Half-duplex & Multipoint 7. B) Greater than LAN but less than WAN 8. C) WAN 9. C) internet 10.D) OSI 11. C) full Duplex12.C) Repeater13.D) RJ-4514. A) impulse noise15.A) Framing16.C) 3 1 4 217.B) Layer – 1 (Physical)18.C) Presentation layer19.C) Network layer20.B) ii iv i iii21.A) Half duplex protocol22.B) Piggy backing23.C) Application24. The Network 25.Data Link 26.Transport27.Session 28.Presentation 29.A) device that allows wireless devices to connect to a wired network30.B) CSMA/CA31.D) all of the mentioned32.A) personal area network33.D) Overlay network34.B) Message35.A) End to end36.A) port37.d) None of the mentioned38.d) Channel coding39.a) bit-by-bit delivery40.B) physical layer

Answers to Subjective Questions

1. Data Communication is the exchange of data (in the form of 0s and 1s) between two devices via some form of transmission medium (such as a wire cable).The five components are :

Message - It is the information to be communicated. Popular forms of information include text, pictures, audio, video etc. Text is converted to binary, number doesnt converted, image is converted to pixels, etc.

Sender - It is the device which sends the data messages. It can be a computer, workstation, telephone handset etc.

Receiver - It is the device which receives the data messages. It can be a computer, workstation, telephone handset etc.

Transmission Medium - It is the physical path by which a message travels from sender to receiver. Some examples include twisted-pair wire, coaxial cable, radio waves etc.

Protocol - It is a set of rules that governs the data communications. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating.

2. Typical block diagram of a communication system is

Transmitter :Transmitter is the first component in this block diagram. Using this system we can generate the messages which is to be sent through this system.Encoder:Encoder is the second element in the communication system. It performs the encoding of the given data, which means that this system converts the messages in the form of  symbols for transmission purpose. In this system, a sequence of characters are created in a special format for an effective transmission. This encoding system is used for security purpose.Noisy Channel:This is the third block in the block diagram of communication system. Noisy channel is nothing but the medium through which the message is transmitted. Messages are conveyed through this channel. Different channels have different strengths and weaknesses. Each channel has its own frequency and different applications have different operating frequencies.Decoder:Decoder is used to decode the encoded message and retrieve the actual message. Decoding must be done correctly . If this part is not performed well then the message which is received might not be correctThis encoding and decoding will be very help full in military and mobile communications.

Receiver:This is the final block in block diagram of communication system. This can be said as the target to which the information need to be delivered.

3. In computer networks, bandwidth is used as a synonym for data transfer rate, the amount of data that can be carried from one point to another in a given time period (usually a second). Network bandwidth is usually expressed in bits per second (bps); modern networks typically have speeds measured in the millions of bits per second (megabits per second, or Mbps) or billions of bits per second (gigabits per second, or Gbps).Analog bandwidth on the other hand refers to the range of frequencies on which an amplifier or filter works.

4. Different types of communications include serial and parallel communication -Serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels.Serial communication is used for all long-haul communication and most computer networks, where the cost of cable and synchronization difficulties make parallel communication impractical. Serial computer buses are becoming more common even at shorter distances, as improved signal integrity and transmission speeds in newer serial technologies have begun to outweigh the parallel bus's advantage of simplicity (no need for serializer and deserializer, or SerDes) and to outstrip its disadvantages (clock skew, interconnect density). The migration from PCI to PCI Express is an example.parallel communication is a method of conveying multiple binary digits (bits) simultaneously. It contrasts with serial communication, which conveys only a single bit at a time; this distinction is one way of characterizing a communications link.

The basic difference between a parallel and a serial communication channel is the number of electrical conductors used at the physical layer to convey bits. Parallel communication implies more than one such conductor. For example, an 8-bit parallel channel will convey eight bits (or a byte) simultaneously, whereas a serial channel would convey those same bits sequentially, one at a time. If both channels operated at the same clock speed, the parallel channel would be eight times faster. A parallel channel may have additional conductors for other signals, such as a clock signal to pace the flow of data, a signal to control the direction of data flow, and handshaking signals.

Parallel communication is and always has been widely used within integrated circuits, in peripheral buses, and in memory devices such as RAM. Computer system buses, on the other hand, have evolved over time: parallel communication was commonly used in earlier system buses, whereas serial communications are prevalent in modern computers.

5. Noise refers to any unwanted signal introduced in the original piece of speech or signal.Noise can be classified into following types – 1) External Noise2) Internal Noise

External Noise

External noise is defined as the type of Noise which is general externally due to communication system. External Noise are analyzed qualitatively. Now, External Noise may be classified asa) Atmospheric Noise : Atmospheric Noise is also known as static noise which is the natural

source of disturbance caused by lightning, discharge in thunderstorm and the natural disturbances occurring in the nature.

b) Industrial Noise : Sources of Industrial noise are auto-mobiles, aircraft, ignition of electric motors and switching gear. The main cause of Industrial noise is High voltage wires. These noises is generally produced by the discharge present in the operations.c) Extraterrestrial Noise : Extraterrestrial Noise exist on the basis of their originating source. They are subdivided into –i) Solar Noiseii) Cosmic Noise

Internal Noise

Internal Noise are the type of Noise which are generated internally or within the Communication System or in the receiver. They may be treated qualitatively and can also be reduced or minimized by the proper designing of the system. Internal Noises are classified as :

1) Shot Noise : These Noise are generally arises in the active devices due to the random behaviour of Charge particles or carries. In case of electron tube, shot Noise is produces due to the  random emission of electron form cathodes.

2) Partition Noise : When a circuit is to divide in between two or more paths then the noise

generated is known as Partition noise. The reason for the generation is random fluctuation

in the division.3) Low- Frequency Noise : They are also known as FLICKER NOISE. These type of noise

are generally observed at a frequency range below few kHz.  Power spectral density of these noise increases with the decrease in frequency. That why the name is given Low- Frequency Noise.

4) High- Frequency Noise : These noises are also known TRANSIT- TIME Noise. They are observed in the semi-conductor devices when the transit time of a charge carrier while crossing a junction is compared with the time period of that signal.

5) Thermal Noise : Thermal Noise are random and often referred as White Noise or Johnson Noise. Thermal noise are generally observed in the resistor or the sensitive resistive components of a complex impedance due to the random and rapid movement of molecules or atoms or electrons.

6. Baud rate - The baud rate is the rate at which information is transferred in a communication channel. In the serial port context, "9600 baud" means that the serial port is capable of transferring a maximum of 9600 bits per second.

Bit rate - In telecommunications and computing, bit rate (sometimes written bitrate or as a variable R) is the number of bits that are conveyed or processed per unit of time.

Transmit (TX) - Also known as Data Out or TXO. The TX line on any device is there to transmit data. This should be hooked up to the RX line of the device with which you would like to communicate.

Receive (RX) - Also known as Data In or RXI. The RX line on any device is there to receive data. This should be hooked up to the TX line of the device with which you would like to communicate.

7. The Open Systems Interconnect (OSI) model has seven layers. This article describes and

explains them, beginning with the 'lowest' in the hierarchy (the physical) and proceeding to the

'highest' (the application). The layers are stacked this way:

Application

Presentation

Session

Transport

Network

Data Link

Physical

Physical layer

The physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers. It provides:

Data encoding: modifies the simple digital signal pattern (1s and 0s) used by the PC to better accommodate the characteristics of the physical medium, and to aid in bit and frame synchronization. It determines:

What signal state represents a binary 1

How the receiving station knows when a "bit-time" starts

How the receiving station delimits a frame

Physical medium attachment, accommodating various possibilities in the medium:

Will an external transceiver (MAU) be used to connect to the medium?

How many pins do the connectors have and what is each pin used for?

Transmission technique: determines whether the encoded bits will be transmitted by baseband (digital) or broadband (analog) signaling.

Physical medium transmission: transmits bits as electrical or optical signals appropriate for the physical medium, and determines:

What physical medium options can be used

How many volts/db should be used to represent a given signal state, using a given physical medium

DATA LINK LAYER

The data link layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link. To do this, the data link layer provides: 

Link establishment and termination: establishes and terminates the logical link between two nodes.Frame traffic control: tells the transmitting node to "back-off" when no frame buffers are available.

Frame sequencing: transmits/receives frames sequentially.

Frame acknowledgment: provides/expects frame acknowledgments. Detects and recovers from errors that occur in the physical layer by retransmitting non-acknowledged frames and handling duplicate frame receipt.

Frame delimiting: creates and recognizes frame boundaries.

Frame error checking: checks received frames for integrity.

Media access management: determines when the node "has the right" to use the physical medium.

NETWORK LAYER

The network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors. It provides: Routing: routes frames among networks.

Subnet traffic control: routers (network layer intermediate systems) can instruct a sending station to "throttle back" its frame transmission when the router's buffer fills up.

Frame fragmentation: if it determines that a downstream router's maximum transmission unit (MTU) size is less than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station.

Logical-physical address mapping: translates logical addresses, or names, into physical addresses.

Subnet usage accounting: has accounting functions to keep track of frames forwarded by subnet intermediate systems, to produce billing information.

Communications Subnet

The network layer software must build headers so that the network layer software residing in the subnet intermediate systems can recognize them and use them to route data to the destination address. 

This layer relieves the upper layers of the need to know anything about the data transmission and intermediate switching technologies used to connect systems. It establishes, maintains and terminates connections across the intervening communications facility (one or several intermediate systems in the communication subnet). 

In the network layer and the layers below, peer protocols exist between a node and its immediate neighbor, but the neighbor may be a node through which data is routed, not the destination station. The source and destination stations may be separated by many intermediate systems.

TRANSPORT LAYER

The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers. 

The size and complexity of a transport protocol depends on the type of service it can get from the network layer. For a reliable network layer with virtual circuit capability, a minimal transport layer is required. If the network layer is unreliable and/or only supports datagrams, the transport protocol should include extensive error detection and recovery. 

The transport layer provides:

Message segmentation: accepts a message from the (session) layer above it, splits the message into smaller units (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.

Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments.

Message traffic control: tells the transmitting station to "back-off" when no message buffers are available.

Session multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions (see session layer).

Typically, the transport layer can accept relatively large messages, but there are strict message size limits imposed by the network (or lower) layer. Consequently, the transport layer must break up the messages into smaller units, or frames, prepending a header to each frame. 

The transport layer header information must then include control information, such as message start and message end flags, to enable the transport layer on the other end to recognize message boundaries. In addition, if the lower layers do not maintain sequence, the transport header must contain sequence information to enable the transport layer on the receiving end to get the pieces back together in the right order before handing the received message up to the layer above.

End-to-end layers

Unlike the lower "subnet" layers whose protocol is between immediately adjacent nodes, the transport layer and the layers above are true "source to destination" or end-to-end layers, and are not concerned with the details of the underlying communications facility. Transport layer software (and software above it) on the source station carries on a conversation with similar software on the destination station by using message headers and control messages.

SESSION LAYER

The session layer allows session establishment between processes running on different stations. It provides: 

Session establishment, maintenance and termination: allows two application processes on different machines to establish, use and terminate a connection, called a session.

• Session support: performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging, and so on.

APPLICATION LAYER

The application layer serves as the window for users and application processes to access network services. This layer contains a variety of commonly needed functions:

Resource sharing and device redirection

• Remote file access

• Remote printer access

• Inter-process communication

• Network management

• Directory services

• Electronic messaging (such as mail)

• Network virtual terminals

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The fundamental purpose of data communications is to exchange information between user's computers, terminals and applications programs. It has the aim of relaying the message effectively and socialization between people.

9. The two different computer network connection types are

Point-to-Point Connection –

A point-to-point connection is a direct link between two devices such as a computer and a printer. It uses dedicated link between the devices. The entire capacity of the link is used for the transmission between those two devices. Most of today's point-to-point connections are associated with modems and PSTN (Public Switched Telephone Network) communications. In point to point networks, there exist many connections between individual pairs of machines.

To move from sources to destination, a packet (short message) may follow different routes. In networking, the Point-to-Point Protocol (PPP) is a data link protocol commonly used in establishing a direct connection between two networking nodes. It can provide connection authentication, transmission encryption, and compression PPP is used over many types of physical networks including serial cable, phone line, trunk line, cellular telephone, specialized radio links, and fiber optic links such as SONET. PPP is also used over Internet access connections (now marketed as "broadband").

Multipoint Connection –

A multipoint connection is a link between three or more devices. It is also known as Multi-drop configuration. The networks havjng multipoint configuration are called Broadcast Networks. In broadcast network, a message or a packet sent by any machine is received by all other machines in a network. The packet contains address field that specifies the receiver. Upon receiving a packet, every machine checks the address field of the packet. If the transmitted packet is for that

particular machine, it processes it; otherwise it just ignores the packet. Broadcast network provides the provision for broadcasting & multicasting. Broadcasting is the process in which a single packet is received and processed by all the machines in the network. It is made possible by using a special code in the address field of the packet. When a packet is sent to a subset of the machines i.e. only to few machines in the network it is known as multicasting. Historically, multipoint connections were used to attach central CPs to distributed dumb terminals. In today's LAN environments, multipoint connections link many network devices in various configurations.

10.

LAN has the following characteristics:

• Coverage area is generally a few kilometers.

• Using different dedicated transmission medium you can achieve the transmission rate of 1 Mb/s to 100 Mbit / sec or higher, with the further development of LAN technology is currently being developed toward higher speed (e.g. 155Mbps, 655Mbps and 1000Mbps etc.).

• In LAN you can run the multiple devices to share a transmission medium.

• You can use the different topology mainly bus and ring in LAN.

• The communication quality is better IN LAN, the transmission error rate are low as compare to WAN.

• LAN support a variety of communications transmission mediumsuch as a cable (thin cable, thick cable, and twisted pair), fiber and wireless transmission.

• A LAN usually has low cost, installation, expansion and maintenance and LAN installation is relatively simple, good scalability.

11

A computer network that spans a relatively large geographical area. Typically, a WAN consists of two or more local-area networks (LANs). Computers connected to a wide-area network are often

connected through public networks, such as the telephone system. They can also be connected through leased lines or satellites.

Terms associated with WAN –Customer premises equipment (CPE) -Customer premises equipment (CPE) is equipment that's owned by the subscriber and located on the subscriber's premises.Demarcation point - The demarcation point is the precise spot where the service provider's responsibility ends and the CPE begins. It's generally a device in a telecommunications closet owned and installed by the telecommunications company (telco). It's your responsibility to cable (extended demarc) from this box to the CPE, which is usually a connection to a CSU/DSU or ISDN interface.Local loop -The local loop connects the demarc to the closest switching office, which is called a central office.Central office (CO) -This point connects the customer's network to the provider's switching network.Toll network -The toll network is a trunk line inside a WAN provider's network. This network is a collection of switches and facilities owned by the ISP. Definitely familiarize yourself with these terms because they're crucial to understanding WAN technologies.

12)A metropolitan area network (MAN) is a computer network larger than a local area network, covering an area of a few city blocks to the area of an entire city, possibly also including the surrounding areas

A metropolitan area network (MAN) is a network that interconnects users with computer resources in a geographic area or region larger than that covered by even a large local area network (LAN) but smaller than the area covered by a wide area network (WAN). The term is applied to the interconnection of networks in a city into a single larger network (which may then also offer efficient connection to a wide area network). It is also used to mean the interconnection of several local area networks by bridging them with backbone lines. The latter usage is also sometimes referred to as a campus network.

13)

A LAN (local area network) is a group of computers and network devices connected together, usually within the same building. By definition, the connections must be high speed and relatively inexpensive (e.g., token ring orEthernet). Most Indiana University Bloomington departments are on LANs.

A LAN connection is a high-speed connection to a LAN. On the IUB campus, most connections are either Ethernet (10 Mbps) or Fast Ethernet (100 Mbps), and a few locations have Gigabit Ethernet (1000 Mbps) connections.

A MAN (metropolitan area network) is a larger network that usually spans several buildings in the same city or town. The IUB network is an example of a MAN.

A WAN (wide area network), in comparison to a MAN, is not restricted to a geographical location, although it might be confined within the bounds of a state or country. A WAN connects several LANs, and may be limited to an enterprise (a corporation or an organization) or accessible to the public. The technology is high speed and relatively expensive. The Internet is an example of a worldwide public WAN.

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TRANSPORT LAYER

The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers.

The size and complexity of a transport protocol depends on the type of service it can get from the network layer. For a reliable network layer with virtual circuit capability, a minimal transport layer is required. If the network layer is unreliable and/or only supports datagrams, the transport protocol should include extensive error detection and recovery.

The transport layer provides:

• Message segmentation: accepts a message from the (session) layer above it, splits the message into smaller units (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.

• Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments.

• Message traffic control: tells the transmitting station to "back-off" when no message buffers are available.

• Session multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions (see session layer).

Typically, the transport layer can accept relatively large messages, but there are strict message size limits imposed by the network (or lower) layer. Consequently, the transport layer must break up the messages into smaller units, or frames, prepending a header to each frame.

The transport layer header information must then include control information, such as message start and message end flags, to enable the transport layer on the other end to recognize message boundaries. In addition, if the lower layers do not maintain sequence, the transport header must contain sequence information to enable the transport layer on the receiving end to get the pieces back together in the right order before handing the received message up to the layer above.

End-to-end layers

Unlike the lower "subnet" layers whose protocol is between immediately adjacent nodes, the transport layer and the layers above are true "source to destination" or end-to-end layers, and are

not concerned with the details of the underlying communications facility. Transport layer software (and software above it) on the source station carries on a conversation with similar software on the destination station by using message headers and control messages.

15)

Computer network topology is the way various components of a network (like nodes, links, peripherals, etc) are arranged. Network topologies define the layout, virtual shape or structure of network, not only physically but also logically. The way in which different systems and nodes are connected and communicate with each other is determined by topology of the network. Topology can be physical or logical. Physical Topology is the physical layout of nodes, workstations and cables in the network; while logical topology is the way information flows between different components.

Bus topology –

Bus Topology is the simplest of network topologies. In this type of topology, all the nodes (computers as well as servers) are connected to the single cable (called bus), by the help of interface connectors. This central cable is the backbone of the network and is known as Bus (thus the name). Every workstation communicates with the other device through this Bus.

A signal from the source is broadcasted and it travels to all workstations connected to bus cable. Although the message is broadcasted but only the intended recipient, whose MAC address or IP address matches, accepts it. If the MAC /IP address of machine doesn’t match with the intended address, machine discards the signal.

A terminator is added at ends of the central cable, to prevent bouncing of signals. A barrel connector can be used to extend it. Below I have given a basic diagram of a bus topology and then have discussed advantages and disadvantages of Bus Network Topology

Star topology –

In Star topology, all the components of network are connected to the central device called “hub” which may be a hub, a router or a switch. Unlike Bus topology (discussed earlier), where nodes were connected to central cable, here all the workstations are connected to central device with a

point-to-point connection. So it can be said that every computer is indirectly connected to every other node by the help of “hub”.

All the data on the star topology passes through the central device before reaching the intended destination. Hub acts as a junction to connect different nodes present in Star Network, and at the same time it manages and controls whole of the network. Depending on which central device is used, “hub” can act as repeater or signal booster. Central device can also communicate with other hubs of different network. Unshielded Twisted Pair (UTP) Ethernet cable is used to connect workstations to central node.

In Ring Topology, all the nodes are connected to each-other in such a way that they make a closed loop. Each workstation is connected to two other components on either side, and it communicates with these two adjacent neighbors. Data travels around the network, in one direction. Sending and receiving of data takes place by the help of TOKEN.

Token Passing : Token contains a piece of information which along with data is sent by the source computer. This token then passes to next node, which checks if the signal is intended to it. If yes, it receives it and passes the empty to into the network, otherwise passes token along with the data to next node. This process continues until the signal reaches its intended destination.

The nodes with token are the ones only allowed to send data. Other nodes have to wait for an empty token to reach them. This network is usually found in offices, schools and small buildings.

In a mesh network topology, each of the network node, computer and other devices, are interconnected with one another. Every node not only sends its own signals but also relays data from other nodes. In fact a true mesh topology is the one where every node is connected to every other node in the network. This type of topology is very expensive as there are many redundant connections, thus it is not mostly used in computer networks. It is commonly used in wireless networks. Flooding or routing technique is used in mesh topology.

17)

Transmission mode means transferring of data between two devices. It is also called communication mode. These modes direct the direction of flow of information. There are three types of transmission mode. They are :

• Simplex Mode

• Half duplex Mode

• Full duplex Mode

Simplex

In this type of transmission mode data can be sent only through one direction i.e. communication is unidirectional. We cannot send a message back to the sender. Unidirectional communication is done in Simplex Systems.

Examples of simplex Mode is loudspeaker, television broadcasting, television and remote, keyboard and monitor etc.

Half duplex

In half duplex system we can send data in both directions but it is done one at a time that is when the sender is sending the data then at that time we can’t send the sender our message. The data is sent in one direction.

Example of half duplex is a walkie- talkie in which message is sent one at a time and messages are sent in both the directions.

Full deplex mode

In full duplex system we can send data in both directions as it is bidirectional. Data can be sent in both directions simultaneously. We can send as well as we receive the data.

Example of Full Duplex is a Telephone Network in which there is communication between two persons by a telephone line, through which both can talk and listen at the same time.

In full duplex system there can be two lines one for sending the data and the other for receiving data.

18)

Physical layer

The physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers. It provides:

Data encoding: modifies the simple digital signal pattern (1s and 0s) used by the PC to better accommodate the characteristics of the physical medium, and to aid in bit and frame synchronization. It determines:

What signal state represents a binary 1

How the receiving station knows when a "bit-time" starts

How the receiving station delimits a frame

Physical medium attachment, accommodating various possibilities in the medium:

Will an external transceiver (MAU) be used to connect to the medium?

How many pins do the connectors have and what is each pin used for?

Transmission technique: determines whether the encoded bits will be transmitted by baseband (digital) or broadband (analog) signaling.

Physical medium transmission: transmits bits as electrical or optical signals appropriate for the physical medium, and determines:

What physical medium options can be used

How many volts/db should be used to represent a given signal state, using a given physical medium

DATA LINK LAYER

The data link layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link. To do this, the data link layer provides:

• Link establishment and termination: establishes and terminates the logical link between two nodes.

• Frame traffic control: tells the transmitting node to "back-off" when no frame buffers are available.

• Frame sequencing: transmits/receives frames sequentially.

• Frame acknowledgment: provides/expects frame acknowledgments. Detects and recovers from errors that occur in the physical layer by retransmitting non-acknowledged frames and handling duplicate frame receipt.

• Frame delimiting: creates and recognizes frame boundaries.

• Frame error checking: checks received frames for integrity.

• Media access management: determines when the node "has the right" to use the physical medium.

NETWORK LAYER

The network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors. It provides:

Routing: routes frames among networks.

• Subnet traffic control: routers (network layer intermediate systems) can instruct a sending station to "throttle back" its frame transmission when the router's buffer fills up.

• Frame fragmentation: if it determines that a downstream router's maximum transmission unit (MTU) size is less than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station.

• Logical-physical address mapping: translates logical addresses, or names, into physical addresses.

• Subnet usage accounting: has accounting functions to keep track of frames forwarded by subnet intermediate systems, to produce billing information.

Communications Subnet

The network layer software must build headers so that the network layer software residing in the subnet intermediate systems can recognize them and use them to route data to the destination address.

This layer relieves the upper layers of the need to know anything about the data transmission and intermediate switching technologies used to connect systems. It establishes, maintains and terminates connections across the intervening communications facility (one or several intermediate systems in the communication subnet).

In the network layer and the layers below, peer protocols exist between a node and its immediate neighbor, but the neighbor may be a node through which data is routed, not the destination station. The source and destination stations may be separated by many intermediate systems.

TRANSPORT LAYER

The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers.

The size and complexity of a transport protocol depends on the type of service it can get from the network layer. For a reliable network layer with virtual circuit capability, a minimal transport layer is required. If the network layer is unreliable and/or only supports datagrams, the transport protocol should include extensive error detection and recovery.

The transport layer provides:

• Message segmentation: accepts a message from the (session) layer above it, splits the message into smaller units (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.

• Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments.

• Message traffic control: tells the transmitting station to "back-off" when no message buffers are available.

• Session multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions (see session layer).

Typically, the transport layer can accept relatively large messages, but there are strict message size limits imposed by the network (or lower) layer. Consequently, the transport layer must break up the messages into smaller units, or frames, prepending a header to each frame.

The transport layer header information must then include control information, such as message start and message end flags, to enable the transport layer on the other end to recognize message boundaries. In addition, if the lower layers do not maintain sequence, the transport header must contain sequence information to enable the transport layer on the receiving end to get the pieces back together in the right order before handing the received message up to the layer above.

End-to-end layers

Unlike the lower "subnet" layers whose protocol is between immediately adjacent nodes, the transport layer and the layers above are true "source to destination" or end-to-end layers, and are not concerned with the details of the underlying communications facility. Transport layer software (and software above it) on the source station carries on a conversation with similar software on the destination station by using message headers and control messages.

SESSION LAYER

The session layer allows session establishment between processes running on different stations. It provides:

• Session establishment, maintenance and termination: allows two application processes on different machines to establish, use and terminate a connection, called a session.

• Session support: performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging, and so on.

APPLICATION LAYER

The application layer serves as the window for users and application processes to access network services. This layer contains a variety of commonly needed functions:

Resource sharing and device redirection

• Remote file access

• Remote printer access

• Inter-process communication

• Network management

• Directory services

• Electronic messaging (such as mail)

• Network virtual terminals

19)

The Application Layer, also known as Layer 7 of the OSI Reference Model, is the top layer of the model and provides the user interface to a variety of network-related information services. Together with the Presentation Layer, it provides the interoperability element of the internetwork. Where the Presentation Layer is about syntax, the Application Layer is about semantics. It defines generalized services that applications need. Note that this layer is not typically where the applications themselves reside. The applications you use every day sit above the OSI Reference Model and draw services from the interface to the software that implements the model in your computer system.

• Applications services include:

• File Transfer, Access, and Management (FTAM) services

• General document and message interchange services, such as email.

• The Application Layer also provides other services.

• Identification of communication partner by name or address (directory services)

• Selection of dialogue discipline and agreement of Presentation Layer services

• Support for network standard virtual terminals

• Printing services

There are many different types of Application Layer services. Each service may invoke other Application Layer services. This is a unique feature of the Application Layer. For instance, before a file can be sent, there must be an association between the two Application Layers. The term “association” is used to describe a connection between two Application Layer entities. This terminology differs from lower layers where the term “connection” is used. The meaning, however, is the same. Both file transfer (FTAM) and association establishment are Application Layer services and both services are required for successful transfer of information.

The services provided by the Application Layer break into two groups: common and specific. Common services are those that are generally useful to many different types of applications. An example is association establishment and termination. Specific services are typically associated with a single or small group of applications. An example is a message handling service.

A digression comparing Application and Presentation Layer protocols is in order. A common distinction between Levels 6 and 7 is based on the generality of services provided; Presentation Layer protocols are general in nature and Application Layer protocols are typically specific in nature. The common services (common application service elements) are an exception to this rule. This distinction is still made in this text because the well-known and commonly implemented Application Layer protocols (e.g., FTAM and X.400) follow this rule.

Given this architecture, different application programs can call these services via a published interface. This obviates the need for a programmer to write a message handling system, for example. It also means these services can interoperate between different applications.

20)

IP Address –

An IP address is a 32-bit number that uniquely identifies a host (computer or other device, such as a printer or router) on a TCP/IP network.

IP addresses are normally expressed in dotted-decimal format, with four numbers separated by periods, such as 192.168.123.132. To understand how subnet masks are used to distinguish between hosts, networks, and subnetworks, examine an IP address in binary notation.

For example, the dotted-decimal IP address 192.168.123.132 is (in binary notation) the 32 bit number 110000000101000111101110000100. This number may be hard to make sense of, so divide it into four parts of eight binary digits.

Subnet mask

The second item, which is required for TCP/IP to work, is the subnet mask. The subnet mask is used by the TCP/IP protocol to determine whether a host is on the local subnet or on a remote network.

In TCP/IP, the parts of the IP address that are used as the network and host addresses are not fixed, so the network and host addresses above cannot be determined unless you have more information. This information is supplied in another 32-bit number called a subnet mask. In this example, the subnet mask is 255.255.255.0. It is not obvious what this number means unless you know that 255 in binary notation equals 11111111; so, the subnet mask is

11111111.11111111.11111111.00000000

A media access control address (MAC address) is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used as a network address for most IEEE 802 network technologies, including Ethernet and WiFi. Logically, MAC addresses are used in the media access control protocol sublayer of the OSI reference model.

MAC addresses are most often assigned by the manufacturer of a network interface controller (NIC) and are stored in its hardware, such as the card's read-only memory or some other firmware mechanism. If assigned by the manufacturer, a MAC address usually encodes the manufacturer's registered identification number and may be referred to as the burned-in address (BIA). It may also be known as an Ethernet hardware address (EHA), hardware address or physical address. This can be contrasted to a programmed address, where the host device issues commands to the NIC to use an arbitrary address.