Computer network answer

1. What is computer Network? Give the classification and goals of computer network.

A computer network is a set of computers connected together for the purpose of sharing resources. The most common resource shared today is connection to the Internet. Other shared resources can include a printer or a file server.

Types of networks:

I. Personal Area Network (PAN)II. Local Area Network (LAN)

III. Metropolitan Area Network (MAN)IV. Wide Area Network (WAN)

The main goal of networking is "Resource sharing". And this goal is to make all programs, equipment, and especially data available to anyone on the network without regard to the physical location of the resource or the user. An obvious and widespread example is having a group of office workers share a common printer.

A second goal of setting up a computer network has to do with people rather than information or even computers. A computer network can provide a powerful communication medium among employees. Virtually every company that has two or more computers now has email (electronic mail), which employees generally use for a great deal of daily communication.

Telephone calls between employees may be carried by the computer network instead of by the phone company. This technology is called IP telephony or VoIP when Internet technology is used. In modern times, Video can also be added to audio so that employees at distant locations can see and hear each other as they hold a meeting. This technique is a powerful tool for eliminating the cost and time previously devoted to travel.

Desktop sharing lets remote workers see and interact with a graphical computer screen. This makes it easy for two or more people who work far apart to read and write a shared blackboard or write a report together. When one worker makes a change to an online document, the others can see the change immediately, instead of waiting several days for a letter.

A third goal for many companies is doing business electronically, especially with customers and suppliers. This new model is called e-commerce (electronic commerce) and it has grown rapidly in recent years. Airlines, bookstores, and other retailers have discovered that many customers like the convenience of shopping from home. Consequently, many companies provide catalogs of their goods and services online and take orders online. Manufacturers of automobiles, aircraft, and computers, among others, buy subsystems from a variety of suppliers and then assemble the parts. Using computer networks, manufacturers can place orders electronically as needed. This reduces the need for large inventories and enhances efficiency.

2. What is point- to- Point subnet? Draw the possible topologies for point- to- Point subnet.

Point-to-point links connect individual pairs of machines. To go from the source to the destination on a network made up of point-to-point links, short messages, called packets in certain contexts, may have to first visit one or more intermediate machines. Often multiple

routes, of different lengths, are possible, so finding good ones is important in point-to-point networks.

Point to point Subnet

3. Write the principles for designing the layered architecture of a network.

Reliability is the design issue of making a network that operates correctly even though it is made up of a collection of components that are themselves unreliable. Finding errors in received information, finding a working path through a network are the segments of reliability.

A second design issue concerns the evolution of the network. Over time, networks grow larger and new designs emerge that need to be connected to the existing network.

A third design issue is resource allocation. Networks provide a service to hosts from their underlying resources, such as the capacity of transmission lines. To do this well, they need mechanisms that divide their resources so that one host does not interfere with another too much.

The last major design issue is to secure the network by defending it against different kinds of threats. One of the major threats is eavesdropping on communications.

Ref: CN, Tanenbaum (page-33)

4. Write short note on: NSFNET, ARPANET, and Internet.

NSFNET: A wide-area network developed under the auspices of the National Science Foundation (NSF). NSFnet replaced ARPANET as the main government network linking universities and research facilities. In 1995, however, the NSF dismantled NSFnet and replaced it with a commercial Internet backbone. At the same time, the NSF implemented a new backbone called very high-speed Backbone Network Service (vBNS), which serves as a testing ground for the next generation of Internet technologies.

ARPANET was the network that became the basis for the Internet. Based on a concept first published in 1967, ARPANET was developed under the direction of the U.S. Advanced Research Projects Agency (ARPA). In 1969, the idea became a modest reality with the interconnection of four university computers.ARPANET was the network that became the basis for the Internet. Based on a concept first published in 1967, ARPANET was developed under the direction of the U.S. Advanced Research Projects Agency (ARPA). In 1969, the idea became a modest reality with the interconnection of four university computers. The initial purpose was to communicate with and share computer resources among mainly scientific users at the connected institutions. ARPANET took advantage of the new idea of sending information in small units called packets that could be routed on different paths and reconstructed at their destination. The development of the TCP/IP protocols in the 1970s made it possible to expand the size of the network, which now had become a network of networks, in an orderly way.

INTERNET: A means of connecting a computer to any other computer anywhere in the world via dedicated routers and servers. When two computers are connected over the Internet, they can send and receive all kinds of information such as text, graphics, voice, video, and computer programs.

No one owns Internet, although several organizations the world over collaborate in its functioning and development. The high-speed, fiber-optic cables (called backbones) through which the bulk of the Internet data travels are owned by telephone companies in their respective countries.

The Internet grew out of the Advanced Research Projects Agency's Wide Area Network (then called ARPANET) established by the US Department Of Defense in 1960s for collaboration in military research among business and government laboratories. Later universities and other US institutions connected to it. This resulted in ARPANET growing beyond everyone’s expectations and acquiring the name 'Internet.'

The development of hypertext based technology (called World Wide web, WWW, or just the Web) provided means of displaying text, graphics, and animations, and easy search and navigation tools that triggered Internet's explosive worldwide growth.

5. Define layer, protocol, interfaces. Show how communication is provided to the top layer in the layer based network?

Layers: To reduce network design complexity, most networks are organized as a stack of layers or levels, each one built upon the one below it. The number of layers, the name of each layer, the contents of each layer, and the function of each layer differ from network to network. The purpose of each layer is to offer certain services to the higher layers while shielding those layers from the details of how the offered services are actually implemented. In a sense, each layer is a kind of virtual machine, offering certain services to the layer above it.Protocol: a protocol is an agreement between the communicating parties on how communication is to proceed. In another word, Protocol is set of some rules and regulations that govern data communication over network.Interface: Between each pair of adjacent layers is an interface. The interface defines which primitive operations and services the lower layer makes available to the upper one. When network designers decide how many layers to include in a network and what each one should do, one of the most important considerations is defining clean interfaces between the layers. Doing so, in turn, requires that each layer perform a specific collection of well-understood functions.

A five-layer network is illustrated in below fig. The entities comprising the corresponding layers on different machines are called peers. The peers may be software processes, hardware devices, or even human beings. In other words, it is the peers that communicate by using the protocol to talk to each other.

In reality, no data are directly transferred from layer n on one machine to layer n on another machine. Instead, each layer passes data and control information to the layer immediately below it, until the lowest layer is reached. Below layer 1 is the physical medium through which actual communication occurs. In Fig. 1-13, virtual communication is shown by dotted lines and physical communication by solid lines.

6. Define connection oriented and connectionless networks. Explain how packets are sent in a simple client server interaction on connection oriented network? Connection-oriented Requires a session connection (analogous to a phone call) be

established before any data can be sent. This method is often called a "reliable" network service. It can guarantee that data will arrive in the same order. Connection-oriented services set up virtual links between end systems through a network, as shown in Figure 1. Note that the packet on the left is assigned the virtual circuit number 01. As it moves through the network, routers quickly send it through virtual circuit 01.

Connectionless Does not require a session connection between sender and receiver. The sender simply starts sending packets (called datagram’s) to the destination. This service does not have the reliability of the connection-oriented method, but it is useful for periodic burst transfers. Neither system must maintain state information for the systems that they send transmission to or receive transmission from. A connectionless network provides minimal services.

7. Name and define services of data link layer. Show the environment of data link layer. Explain how data link layer work in Internet with diagram.

The data link layer can be designed to offer various services. The actual services that are offered vary from protocol to protocol. Three reasonable possibilities that we will consider in turn are:1. Unacknowledged connectionless service.2. Acknowledged connectionless service.3. Acknowledged connection-oriented service.

Unacknowledged connectionless service consists of having the source machine send independent frames to the destination machine without having the destination machine acknowledge them.

When reliability service is offered, there are still no logical connections used, but each frame sent is individually acknowledged. In this way, the sender knows whether a frame has arrived correctly or been lost.

In Acknowledged connection-oriented service, the source and destination machines establish a connection before any data are transferred. Each frame sent over the connection is numbered, and the data link layer guarantees that each frame sent is indeed received. Furthermore, it guarantees that each frame is received exactly once and that all frames are received in the right order.

Fig: Internal Environment of Data Link Layer

Fig: The Data Link Layer in the Internet

8. Name and define key assumptions for dynamic channel allocation protocol. Define CSMA/ CD protocol with algorithm.

Independent Traffic: The model consists of N independent stations (e.g., computers, telephones), each with a program or user that generates frames for transmission. The expected number of frames generated in an interval of length Δt is λΔt, where λ is a constant (the arrival rate of new frames). Once a frame has been generated, the station is blocked and does nothing until the frame has been successfully transmitted.

Single Channel Assumption: A single channel is available for all communication. All stations can transmit on it and all can receive from it. The stations are assumed to be equally capable, though protocols may assign them different roles (e.g., priorities).

Collision Assumption: If two frames are transmitted simultaneously, they overlap in time and the resulting signal is garbled. This event is called a collision.

Continuous time: Frames transmission can begin at any instant of time.

Slotted Time: Time is divided into discrete intervals which are called slots and frame transmission can always begin at start of a slot.

Carrier sense: stations can tell if the channel is in use. If busy, no other station can use it until it is idle. The term ‘carrier’ here refers to an electrical signal on the cable.

No Carrier Sense: Stations cannot sense the channel’s status. They just transmit the data. Only later they can determine whether the transmission was successful or not. For example, satellite networks.

9. What is error control? Error correction and detection using hamming code (odd/ even parity for the given code).

Network is responsible for transmission of data from one device to another device. The end to end transfer of data from a transmitting application to a receiving application involves many steps, each subject to error. With the error control process, we can be confident that the transmitted and received data are identical. Error control is the process of detecting and correcting both the bit level and packet level errors.

Error correction and detection using Hamming code (Check Assignment).

10. Name and define the fields of Ethernet frame format.

Preamble & Start of Frame: The format used to send frames is shown in Fig. 4-14. First comes a Preamble of 8 bytes, each containing the bit pattern 10101010 (with the exception of the last byte, in which the last 2 bits are set to 11). This last byte is called the Start of Frame delimiter for 802.3.

Destination address: Hardware address (MAC address) of the destination station (usually 48 bits i.e. 6 bytes).

Source address: Hardware address of the source station (usually 48 bits i.e. 6 bytes).

Type: Specifies the protocol sending the packet such as IP or IPX (only applies to DIX frame).

Length: Specifies the length of the data segment, actually the number of LLC data bytes, (only applies to 802.3 frame and replaces the Type field).

Data: This block contains the data and it may be up to 1500 bytes long.

Pad: If the length of the field is less than 46 bytes, then padding data is added to bring its length up to the required minimum of 46 bytes.

Checksum: The final field is the Checksum. It is a 32-bit CRC to detect transmission errors. If the receiving host detects a wrong CRC, it will throw away that packet.

11. Draw a typical ADSL arrangement and explain the function of its.

A typical ADSL arrangement is shown in Fig. 2-35. In this scheme, a telephone company technician must install a NID (Network Interface Device) on the customer’s premises. This small plastic box marks the end of the telephone company’s property and the start of the customer’s property. Close to the NID (or sometimes combined with it) is a splitter, an analog filter that separates the 0–4000-Hz band used by POTS from the data. The POTS signal is routed to the existing telephone or fax machine. The data signal is routed to an ADSL modem, which uses digital signal processing to implement OFDM. Since most ADSL modems are external, the computer must be connected to them at high speed. Usually, this is done using Ethernet, a USB cable, or 802.11.

At the other end of the wire, on the end office side, a corresponding splitter is installed. Here, the voice portion of the signal is filtered out and sent to the normal voice switch. The signal above 26 kHz is routed to a new kind of device called a DSLAM (Digital Subscriber Line Access Multiplexer), which contains the same kind of digital signal processor as the ADSL modem. Once the bits have been recovered from the signal, packets are formed and sent off to the ISP.

12. What is Bluetooth technology? Define the goals of Bluetooth. Explain the Bluetooth architecture. Name five Bluetooth profiles. Define different types of multiplexing.

Bluetooth is a wireless standard for interconnecting communication and computing devices and accessories using short-range, low-power, inexpensive wireless radios.

Goals of Bluetooth:• Open spec.• Low cost.

– In order to replace cables, should have similar cost.– Cell phone cable is ~ $10.

• Power efficiency.• Lightweight and small form factor.• Easy to use.• Reliable and resilient to failures.•

Bluetooth Architecture: The basic unit of a Bluetooth system is a piconet, which consists of a master node and up to seven active slave nodes within a distance of 10 meters. Multiple piconets can exist in the same (large) room and can even be connected via a bridge node that takes part in multiple piconets, as in Fig. 4-34. An interconnected collection of piconets is called a scatternet.

In addition to the seven active slave nodes in a piconet, there can be up to 255 parked nodes in the net. These are devices that the master has switched to a lowpower state to reduce the drain on their batteries. In parked state, a device cannot do anything except respond to an activation or beacon signal from the master.

Bluetooth Profiles:

01. Intercom profile allows two telephones to connect as walkie-talkies.02. The headset and hands-free profiles both provide voice communication between a headset and its base station.03. The human interface device profile is for connecting keyboards and mice to computers.

04. Some profiles let a mobile phone or other computer receive images from a camera or send images to a printer.05. One profile uses a mobile phone as a remote control for a (Bluetooth-enabled) TV.

Frequency-Division Multiplexing (FDM) is a technique by which the total bandwidth available in a communication medium is divided into a series of non-overlapping frequency sub-bands, each of which is used to carry a separate signal. The most natural example of frequency-division multiplexing is radio and television broadcasting.

Time-division multiplexing (TDM) is a method of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line so that each signal appears on the line only a fraction of time in an alternating pattern. TDM is used widely as part of the telephone and cellular networks.

Code Division Multiplexing (CDM) is a technique in which each channel transmits its bits as a coded channel-specific sequence of pulses.

13. Define the characteristic properties of twisted pair and coaxial cables with diagrams?

Twisted Pair: A twisted pair consists of two insulated copper wires, typically about 1 mm thick. The wires are twisted together in a helical form, just like a DNA molecule. When the twisted pair cable is more twisted then it reduces electromagnetic interference. Twisted pairs can be used for transmitting either analog or digital information. The bandwidth depends on the thickness of the wire and the distance traveled, but several megabits/sec can be achieved for a few kilometers in many cases.A category 5 twisted pair consists of two insulated wires gently twisted together. Four such pairs are typically grouped in a plastic sheath to protect the wires and keep them together. This arrangement is shown in Fig. 2-3.Different LAN standards may use the twisted pairs differently. For example, 100-Mbps Ethernet uses two (out of the four) pairs, one pair for each direction. To reach higher speeds, 1-Gbps Ethernet uses all four pairs in both directions simultaneously; this requires the receiver to factor out the signal that is transmitted locally.

Coaxial cable: It has better shielding and greater bandwidth than unshielded twisted pairs, so it can span longer distances at higher speeds. Two kinds of coaxial cable are widely used. One kind, 50-ohm cable, is commonly used when it is intended for digital transmission from the start. The other kind, 75-ohm cable, is commonly used for analog transmission and cable television.

A coaxial cable consists of a stiff copper wire as the core, surrounded by an insulating material. The insulator is encased by a cylindrical conductor, often as a closely woven braided mesh. The outer conductor is covered in a protective plastic sheath. A cutaway view of a coaxial cable is shown in Fig. 2-4.

14. What are the major components of an optical system? Write the working principles of fiber optic. Explain the ways of connecting a fiber.

An optical transmission system has three key components: the light source, the transmission medium, and the detector. Conventionally, a pulse of light indicates a 1 bit and the absence of light indicates a 0 bit. The transmission medium is an ultra-thin fiber of glass. The detector generates an electrical pulse when light falls on it. By attaching a light source to one end of an optical fiber and a detector to the other, we have a unidirectional data transmission system that accepts an electrical signal, converts and transmits it by light pulses, and then reconverts the output to an electrical signal at the receiving end.

When a light ray passes from one medium to another—for example, from fused silica to air—the ray is refracted (bent) at the silica/air boundary, as shown in Fig. 2-6(a). Here we see a light ray incident on the boundary at an angle α1 emerging at an angle β1 . The amount of refraction depends on the properties of the two media (in particular, their indices of refraction). For angles of incidence above a certain critical value, the light is refracted back into the silica; none of it escapes into the air. Thus, a light ray incident at or above the critical angle is trapped inside the fiber, as shown in Fig. 2-6(b), and can propagate for many kilometers with virtually no loss.

Fibers can be connected in three different ways. First, they can terminate in connectors and be plugged into fiber sockets. Second, they can be spliced mechanically. Third, two pieces of fiber can be fused (melted) to form a solid connection.

15. Define cabling & cable topologies for Ethernet?

Point to point Topology: The network consists of a direct link between two computers.

Bus Topology: Bus topology uses one main cable to which all nodes are directly connected. The main cable acts as a backbone for the network.

Star Topology: In star topology, each computer is connected to a central hub using a point-to-point connection.

Ring Topology: In ring topology, the computers in the network are connected in a circular fashion, and the data travels in one direction.

Mesh Topology: In mesh topology, every node has a direct point-to-point connection to every other node.

16. Write short note on Gigabit, Switched Ethernet & Bluetooth technology?

Gigabit Ethernet: Gigabit Ethernet increase performance tenfold while maintaining compatibility with all existing Ethernet standards. In particular, gigabit Ethernet offer unacknowledged datagram service with both unicast and broadcast, use the same 48-bit addressing scheme already in use, and maintain the same frame format, including the minimum and maximum frame sizes. The final standard met all these goals.Like fast Ethernet, all configurations of gigabit Ethernet use point-to-point links. In the simplest configuration, illustrated in Fig. 4-20(a), two computers are directly connected to each other. The more common case, however, uses a switch or a hub connected to multiple computers and possibly additional switches or hubs, as shown in Fig. 4-20(b). In both configurations, each individual Ethernet cable has exactly two devices on it, no more and no fewer.Also like fast Ethernet, gigabit Ethernet supports two different modes of operation: full-duplex mode and half-duplex mode. The ‘‘normal’’ mode is fullduplex mode, which allows traffic in both directions at the same time.The other mode of operation, half-duplex, is used when the computers are connected to a hub rather than a switch. A hub does not buffer incoming frames. Instead, it electrically connects all the lines internally, simulating the multidrop cable used in classic Ethernet.Gigabit Ethernet supports both copper and fiber cabling, as listed in Fig. 4-21.

Switched Ethernet:

19. How DNS works? Define DNS name space with diagram.

To map a name onto an IP address, an application program calls a library procedure called the resolver, passing it the name as a parameter. We saw an example of a resolver, gethostbyname, in Fig. 6-6. The resolver sends a query containing the name to a local DNS server, which looks up the name and returns a response containing the IP address to the resolver, which then returns it to the caller. The query and response messages are sent as UDP packets. Armed with the IP address, the program can then establish a TCP connection with the host or send it UDP packets.

For the Internet, the top of the naming hierarchy is managed by an organization called ICANN (Internet Corporation for Assigned Names and Numbers). The Internet is divided into over 250 top-level domains, where each domain covers many hosts. Each domain is partitioned into sub domains, and these are further partitioned, and so on. All these domains can be represented by a tree, as shown in Fig. 7-1. The top-level domains come in two flavors: generic and countries. The generic domains, listed in Fig. 7-2

20. What are resource records? Explain with example.

Every domain, whether it is a single host or a top-level domain, can have a set of resource records associated with it. These records are the DNS database. A resource record is a five-tuple.

The format we will use is as follows:

Domain_name Time_to_live Class Type Value

The Domain name tells the domain to which this record applies.The Time to live field gives an indication of how stable the record is.The third field of every resource record is the Class. For Internet information, it is always IN.The Type field tells what kind of record this is. There are many kinds of DNS records. The important types are listed in Fig. 7-3.Finally, we have the Value field. This field can be a number, a domain name, or an ASCII string.

An entry like this one might do the job:

cs.mit.edu 86400 IN CNAME csail.mit.edu

21. What is name server? Explain with example how a resolver look-up a remote name in DNS name space.In theory at least, a single name server could contain the entire DNS database and respond to all queries about it. In practice, this server would be so overloaded as to be useless. Furthermore, if it ever went down, the entire Internet would be crippled.To avoid the problems associated with having only a single source of information, the DNS name space is divided into nonoverlapping zones. One possible way to divide the name space of Fig. 7-1 is shown in Fig. 7-5. Each circled zone contains some part of the tree.

Where the zone boundaries are placed within a zone is up to that zone’s administrator. This decision is made in large part based on how many name servers are desired, and where. For example, in Fig. 7-5, the University of Washington has a zone for washington.edu that handles eng.washington.edu but does not handle cs.washington.edu. That is a separate zone with its own name servers. Such a decision might be made when a department such as English does not wish to run its own name server, but a department such as Computer Science does.

What happens when the domain is remote, such as when flits.cs.vu.nl wants to find the IP address of robot.cs.washington.edu at UW (University of Washington)? In this case, and if there is no cached information about the domain available locally, the name server begins a remote query. This query follows the process shown in Fig. 7-6. Step 1 shows the query that is sent to the local name server. The query contains the domain name sought, the type (A), and the class(IN).

24. What is flow control? Name and define different types of flow control?

Flow control is the process of managing the rate of data transmission between two nodes to prevent a fast sender from overwhelming a slow receiver.

Stop-and-waitStop-and-wait flow control is the simplest form of flow control. In this method, the message is broken into multiple frames. The sender waits for an ACK (acknowledgement) from receiver after every frame for specified time (called time out). It is sent to ensure that the receiver has received the frame correctly. It will then send the next frame only after the ACK has been received. If a frame or ACK is lost during transmission then it has to be transmitted again by sender. This retransmission process is known as ARQ (automatic repeat request).

Sliding Window

A method of flow control in which a receiver gives a transmitter permission to transmit data until a window is full. When the window is full, the transmitter must stop transmitting until the receiver advertises a larger window.

27. What is CRC? Explain CRC method using Modulo-2 arithmetic. Give example.

A cyclic redundancy check (CRC) is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data. Blocks of data entering these systems get a short check value attached, based on the remainder of a polynomial division of their contents.

28. Name different types of modulation with necessary diagram. Explain switching techniques.

Circuit Switching:

Conceptually, when you or your computer places a telephone call, the switching equipment within the telephone system seeks out a physical path all the way from your telephone to the receiver’s telephone. This technique is called circuit switching. It is shown schematically in Fig. 2-42(a). Each of the six rectangles represents a carrier switching office (end office, toll office, etc.). In this example, each office has three incoming lines and three outgoing lines. When a call passes through a switching office, a physical connection is (conceptually) established between the line on which the call came in and one of the output lines, as shown by the dotted lines.

Packet Switching:

The alternative to circuit switching is packet switching, shown in Fig. 2- 42(b). With this technology, packets are sent as soon as they are available. There is no need to set up a dedicated path in advance, unlike with circuit switching. It is up to routers to use store-and-forward transmission to send each packet on its way to the destination on its own.