Data Communication & Computer Network
1. Explain the various classifications of Computer Networks based on size, topology etc. Compare and contrast the advantages and disadvantages in each classification.
Topology is a characteristic of a Local Area Network. It is both the physical configuration of the cabling used to connect computers in the network, and the logical way in which the system views the structure of the network. Topology is therefore the physical or logical arrangement of computers.
Factors to Consider When Selecting a Topology
Cost. The transmission medium chosen for a Local Area Network has to be physically installed in the building, using cables and raceways. To make the network cost effective, it´s desirable to minimize installation cost. This may be done by using the correct hardware to link the cables, good modems, and cost-effective computers.
Flexibility. One of the main benefits of a Local Area Network is the ability to distribute the data processing and peripheral nodes around a given area. It can locate the computing power and equipment close to the ultimate users. Because in an office, the arrangement of furniture and internal walls is often subject to change, the topology should allow for easy reconstruction of the network when nodes are moved or added.
Reliability – The topology chosen for the network can help locate faults by allowing the fault to be detected and isolated.
Types of Network Topology
Below we discuss the three main types of network topology.
Star Topology (Also Called Radial Topology)
This topology includes a central node to which all other nodes are connected. Star topology is used in most existing information networks that involve data processing and voice communication. In many cases when a building is wired with a star topology, faced cables radiate out from the center to intermediate connection points to wiring cables. This allows sufficient connection points to be provided for one subarea, while providing flexibility in the allocation of the connection points within that area.
Advantages of Star Topology
Easy diagnosis and isolation of problems
Easy to add a new computer system to the network
Failure of one workstation does not affect the entire network
Uses a single access protocol
Ease of service
Disadvantages of Star Topology
Depends on a central node
Adding nodes can be costly
Requires long cables to connect many nodes
Bus Topology (Also Called "Linear Topology")
Bus topology is the simplest method of networking computers. It consists of a single cable, known as a trunk, backbone, or segment, that connects all the computers in the network. Each system is directly attached to a common communication channel. Messages consist of a signal transmitted over the channel. As the message passes along the channel each system receives it and then examines the destination address contained in the message. If the destination address tells a particular system that the message is addressed to it, that system accepts and processes the message; if the message address tells the computer that the message is intended for another system, the computer will ignore the message.
On a bus topology, signals are sent to all the computers in the network. To keep the signal from bouncing back and forth along the cable a terminator (a British Naval Connector) is placed at the end of the cable. In a bus topology only one computer can send data at a time; therefore, the more computers in a bus, the slower data is transmitted in the network.
Advantages of Bus Topology
Cheap because of its simplicity
Requires a short cable length
Easy to expand the network
Simple to set up compared to Star and Ring topology
No chance of data collision, since one computer transmits at a time
Locating a cable fault is relatively easy
Ideal for one-to-many data transmission
Signals on the cable are bidirectional, so they reach all the nodes
Disadvantages of Bus Topology
Fault diagnosis is difficult, because fault detection may have to be performed from any point in the network.
Fault isolation is difficult, because if the fault is detected in a node the node can simply be removed, but if the fault is in the network medium itself, an entire segment of the bus must be disconnected to isolate the fault
Repeater configuration may be necessary. When the backbone of a bus type network uses a repeater, it may be necessary to tailor the cable length and adjust the terminator.
Computer nodes must be intelligent. Because each node on the network is directly connected to the central bus, each node must have a method of identifying its own data.
In ring topology, each node is connected to form a single closed data path. Data from one node is passed along to the next node, which examines it. If that node is not the intended destination, then the data is transmitted to the next node until the destination is reached. A token (a special bit pattern) is circulated in the network to enable a node to capture the data. Ring topology might be structured so that there are a number of information frames or slots in constant circulation. A node wishing to transmit first detects the arrival of an empty slot, then inserts the data it wishes to send, and marks the frame as full. The receiving node takes the data and then marks the frame as empty. In implementation, when the network is first constituted, one particular node is given the responsibility for generating the token or slot.
Advantages of Ring Topology
The ability to achieve transmission rates on the order of 10 million bits per second
Provision of local communication via a single channel
No central server (which reduces the cost)
Disadvantages of Ring Topology
Failure of one node results in failure of the entire network
Detection of a fault is very difficult
Isolation of a fault is not easy
Based on size
Types of Networks
There are several different types of computer networks. Computer networks can be characterized by their size as well as their purpose.
The size of a network can be expressed by the geographic area they occupy and the number of computers that are part of the network. Networks can cover anything from a handful of devices within a single room to millions of devices spread across the entire globe.
Some of the different networks based on size are:
Personal area network, or PAN
Local area network, or LAN
Metropolitan area network, or MAN
Wide area network, or WAN
In terms of purpose, many networks can be considered general purpose, which means they are used for everything from sending files to a printer to accessing the Internet. Some types of networks, however, serve a very particular purpose. Some of the different networks based on their main purpose are:
Storage area network, or SAN
Enterprise private network, or EPN
Virtual private network, or VPN
Let´s look at each of these in a bit more detail.
Personal Area Network
A personal area network, or PAN, is a computer network organized around an individual person within a single building. This could be inside a small office or residence. A typical PAN would include one or more computers, telephones, peripheral devices, video game consoles and other personal entertainment devices.
If multiple individuals use the same network within a residence, the network is sometimes referred to as a home area network, or HAN. In a very typical setup, a residence will have a single wired Internet connection connected to a modem. This modem then provides both wired and wireless connections for multiple devices. The network is typically managed from a single computer but can be accessed from any device.
This type of network provides great flexibility. For example, it allows you to:
Send a document to the printer in the office upstairs while you are sitting on the couch with your laptop.
Upload the photo from your cell phone to your desktop computer.
Watch movies from an online streaming service to your TV.
If this sounds familiar to you, you likely have a PAN in your house without having called it by its name.
Local Area Network
A local area network, or LAN, consists of a computer network at a single site, typically an individual office building. A LAN is very useful for sharing resources, such as data storage and printers. LANs can be built with relatively inexpensive hardware, such as hubs, network adapters and Ethernet cables.
The smallest LAN may only use two computers, while larger LANs can accommodate thousands of computers. A LAN typically relies mostly on wired connections for increased speed and security, but wireless connections can also be part of a LAN. High speed and relatively low cost are the defining characteristics of LANs.
LANs are typically used for single sites where people need to share resources among themselves but not with the rest of the outside world. Think of an office building where everybody should be able to access files on a central server or be able to print a document to one or more central printers. Those tasks should be easy for everybody working in the same office, but you would not want somebody just walking outside to be able to send a document to the printer from their cell phone! If a local area network, or LAN, is entirely wireless, it is referred to as a wireless local area network, or WLAN.
Metropolitan Area Network
A metropolitan area network, or MAN, consists of a computer network across an entire city, college campus or small region. A MAN is larger than a LAN, which is typically limited to a single building or site. Depending on the configuration, this type of network can cover an area from several miles to tens of miles. A MAN is often used to connect several LANs together to form a bigger network. When this type of network is specifically designed for a college campus, it is sometimes referred to as a campus area network, or CAN.
Wide Area Network
A wide area network, or WAN, occupies a very large area, such as an entire country or the entire world. A WAN can contain multiple smaller networks, such as LANs or MANs. The Internet is the best-known example of a public WAN.
2 Explain ISO – OSI, seven layer network architecture giving the functions of each layer.
The Open System Interconnection (OSI) model defines a networking framework to implement protocols in seven layers. Use this handy guide to compare the different layers of the OSI model and understand how they interact with each other.
Open System Interconnection (OSI) model defines a networking framework to implement protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, and proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.
There is really nothing to the OSI model. In fact, it´s not even tangible. The OSI model doesn´t perform any functions in the networking process. It is a conceptual framework so we can better understand complex interactions that are happening.
The OSI model takes the task of internetworking and divides that up into what is referred to as a vertical stack that consists of the following layers.
Physical (Layer 1)
This layer conveys the bit stream - electrical impulse, light or radio signal — through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.
Layer 1 Physical examples include Ethernet, FDDI, B8ZS, V.35, V.24, RJ45.
Data Link (Layer 2)
At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.
Layer 2 Data Link examples include PPP, FDDI, ATM, IEEE 802.5/ 802.2, IEEE 802.3/802.2, HDLC, Frame Relay.
Network (Layer 3)
This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.
Layer 3 Network examples include AppleTalk DDP, IP, IPX.
Transport (Layer 4)
This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.
Layer 4 Transport examples include SPX, TCP, UDP.
Session (Layer 5)
This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.
Layer 5 examples include NFS, NetBios names, RPC, SQL.
Presentation (Layer 6)
This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.
Layer 6 Presentation examples include encryption, ASCII, EBCDIC, TIFF, GIF, PICT, JPEG, MPEG, MIDI.
Application (Layer 7)
This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.
Layer 7 Application examples include WWW browsers, NFS, SNMP, Telnet, HTTP, FTP
3. What is Routing, Explain Shortest Path Routing algorithm with example?
In internetworking, the process of moving a packet of data from source to destination. Routing is usually performed by a dedicated device called a router. Routing is a key feature of the Internet because it enables messages to pass from one computer to another and eventually reach the target machine. Each intermediary computer performs routing by passing along the message to the next computer. Part of this process involves analysing a routing tablet determine the best path.
Routing is often confused with bridging, which performs a similar function. The principal difference between the two is that bridging occurs at a lower level and is therefore more of a hardware function whereas routing occurs at a higher level where the software component is more important. And because routing occurs at a higher level, it can perform more complex analysis to determine the optimal path for the packet.
Links between routers have a cost associated with them. In general it could be a function of distance, bandwidth, average traffic, communication cost, mean queue length, measured delay, router processing speed, etc.
The shortest path algorithm just finds the least expensive path through the network, based on the cost function.
Dijkstra´s algorithm is an example. You can start from source/destination (doesn´t matter in a unidirectional graph).
Set probe node to starting node.
Probe neighbouring nodes and tentatively label them with (probe node, cumulative distance from start).
Search all tentatively labelled nodes (and not just the nodes labelled from the current probe) for the minimum label, make this minimum node´s label permanent, and make it the new probe node.
If the probe node is the destination/source, stop, else go to 2.
The distance part of the node labels is cumulative distance from the starting node, not simply distance from the last probe node.
The key to discovering that you´ve gone down a bad (greater distance) path is that you examine all nodes with temporary labels in step 3. This means that you switch the probe node to another, shorter path if the you run into a high cost link.
If you label each node with its predecessor on the path, and the distance to that node, then you can easily find the route you desire (albeit backwards) by starting at the destination and following the trail of predecessors backwards. You´ll also know the distance from source to destination from the label on the destination.
Read the case study and answer the accompanied questions.
Congestion control refers to the techniques and mechanism that can either prevent congestion, before it happens, or remove congestion, after it has happened. In general, we can divide congestion control mechanism into two broad categories: Open loop congestion control (Prevention) and Closed-loop Congestion Control (Removal).
1. In how many way congestion control techniques can be applied? Explain.
Congestion typically occurs where multiple links feed into a single link, such as where internal LANs are connected to WAN links. Congestion also occurs at routers in core networks where nodes are subjected to more traffic than they are designed to handle. TCP/IP networks such as the Internet are especially susceptible to congestion because of their basic connection- less nature. There are no virtual circuits with guaranteed bandwidth. Packets are injected by any host at any time, and those packets are variable in size, which make predicting traffic patterns and providing guaranteed service impossible. While connectionless networks have advantages, quality of service is not one of them.
Shared LANs such as Ethernet have their own congestion control mechanisms in the form of access controls that prevent multiple nodes from transmitting at the same time. See "Access Methods" and "MAC (Media Access Control)."
The following basic techniques may be used to manage congestion.
End-system flow control this is not a congestion control scheme, but a way to prevent the sender from overrunning the buffers of the receiver. See "Flow-Control Mechanisms."
Network congestion control in this scheme, end systems throttle back in order to avoid congesting the network. The mechanism is similar to end-to-end flow controls, but the intention is to reduce congestion in the network, not the receiver.
Network-based congestion avoidance in this scheme, a router detects that congestion may occur and attempts to slow down senders before queues become full.
Resource allocation this technique involves scheduling the use of physical circuits or other resources, perhaps for a specific time period. A virtual circuit, built across a series a switches with a guaranteed bandwidth is a form of resource allocation. This technique is difficult, but can eliminate network congestion by blocking traffic that is in excess of the network capacity. A list of related topics may be found on the related entries page.
Caching is probably the ultimate congestion control scheme. By moving content closer to users, a majority of traffic is obtained locally rather than being obtained from distant servers along routed paths that may experience congestion. Caching has become a serious business on the Internet, as discussed under "Content Distribution."
Queuing and Congestion
Any discussion of congestion naturally involves queuing. Buffers on network devices are managed with various queuing techniques. Properly managed queues can minimize dropped packets and network congestion, as well as improve network performance.
The most basic technique is FIFO (first-in, first-out), where packets are processed in the order in which they arrive in the queue. Going beyond this, a priority queuing scheme uses multiple queues with different priority levels so that the most important packets are sent first.
Congestion Control in Frame Relay
While this topic is primarily about congestion problems in connectionless packet-switched networks, it is useful to examine the way congestion is handled in a connection-oriented network. Frame relay provides a good example.
Frame relay subscribers negotiate a CIR (committed information rate) with the service provider. The CIR is the guaranteed level of service, but providers usually allow subscribers to burst over this level if network capacity is available. However, frames in excess of the CIR are marked as discard eligible. If a switch on the network becomes congested, it will drop discard eligible frames. This ensures that the service providers can meet their negotiated CIR levels for subscribers.
Dropping frames is never a good idea, so two congestion avoidance mechanisms are available:
BECN (backward explicit congestion notification) When a switch starts to experience congestion (i.e., the buffers/queues are getting full), it can send a frame in the backward direction to senders with the BECN bit set to inform senders to slow down.
FECN (forward explicit congestion notification) When a switch starts congesting, it can send a frame in the forward direction to receiving nodes with the FECN bit set. This informs the forward nodes that they should inform the sender to slow down.
Slow Start Congestion Control
Slow start reduces the burst affect when a host first transmits. It requires a host to start its transmissions slowly and then build up to the point where congestion starts to occur. The host does not initially know how many packets it can send, so it uses slow start as a way to gauge the network´s capacity. A host starts a transmission by sending two packets to the receiver. When the receiver receives the segments, it returns ACKs (acknowledgements) as confirmation. The sender increments its window by two and sends four packets. This build-up continues with the sender doubling the number of packets it sends until an ACK is not received, indicating that the flow has reached the network´s ability to handle traffic or the receiver’s ability to handle incoming traffic.
Fast Retransmit and Fast Recovery (Reno)
Fast retransmit and fast recovery are algorithms that are designed to minimize the effect that dropping packets has on network throughput. The fast retransmit mechanism infers information from another TCP mechanism that a receiver uses to signal to the sender that it has received packets out of sequence. The technique is to send several duplicate ACKs to the sender.
Active Queue Management and Congestion Avoidance
Dropping packets is inefficient. If a host is bursting and congestion occurs, a lot of packets will be lost. Therefore, it is useful to detect impending congestion conditions and actively manage congestion before it gets out of hand.
Active queue management is a technique in which routers actively drop packets from queues as a signal to senders that they should slow down. RFC 2309 lists the following advantages of active queue management:
Burst are inevitable. Keeping queue size small and actively managing queues improves a router´s ability to absorb bursts without dropping excessive packets.
If a source overflows a shared queue, all the devices sharing that queue will slow down (the "global synchronization" problem).
Recovering from many dropped packets is more difficult than recovering from a single dropped packet.
Large queue can translate into delay. Active queue management allows queues to be smaller, which improves throughput.
Lock-out occurs when a host fills a queue and prevents other hosts from using the queue. Active queue management can prevent this condition.
2. In the given TCP network, R1 and R2 are two routers and network 1 and network 2 are the part of this TCP network. The input data rate of router R1 is 7 mb/s and output data rate of same router is 6.53 mb/s. Will there be congestion? If yes then how you will control the congestion in this TCP network?
The standard fare in TCP implementations today can be found in RFC 2581. This reference document specifies four standard congestion control algorithms that are now in common use. Each of the algorithms noted within that document was actually designed long before the standard was published. Their usefulness has passed the test of time.
The four algorithms, Slow Start, Congestion Avoidance, Fast Retransmit and Fast
One is described below.
Slow Start, a requirement for TCP software implementations is a mechanism used by the sender to control the transmission rate, otherwise known as sender-based flow control. This is accomplished through the return rate of acknowledgements from the receiver. In other words, the rate of acknowledgements returned by the receiver determine the rate at which the sender can transmit data. When a TCP connection first begins, the Slow Start algorithm initializes a congestion window to one segment, which is the maximum segment size (MSS) initialized by the receiver during the connection establishment phase. When acknowledgements are returned by the receiver, the congestion window increases by one segment for each acknowledgement returned. Thus, the sender can transmit the minimum of the congestion window and the advertised window of the receiver, which is simply called the transmission window.
Slow Start is actually not very slow when the network is not congested and network response time is good. For example, the first successful transmission and acknowledgement of a TCP segment increases the window to two segments. After successful transmission of these two segments and acknowledgements completes, the window is increased to four segments. Then eight segments, then sixteen segments and so on, doubling from there on out up to the maximum window size advertised by the receiver or until congestion finally does occur. At some point the congestion window may become too large for the network or network conditions may change such that packets may be dropped. Packets lost will trigger a timeout at the sender. When this happens, the sender goes into congestion avoidance mode as described in the next section.
Question No: 1
The transmission signal coding method of TI carrier is called
Question No: 2
Which of the following signal is not standard RS-232-C signal?
Question No: 3
Which of the following communication modes support two-way traffic but in only one direction at a time?
B. Half duplex
C. Three-quarters duplex
D. All of the above
Question No: 4
What is the number of separate protocol layers at the serial interface gateway specified by the X.25 standard?
Question No: 5
What is max data capacity for optical fiber cable?
A. 10 mbps
B. 100 mbps
C. 1000 mbps
D. 10000 mbps
Question No: 6
For large networks, which topology is used?
Question No: 7
Layer one of the OSI model is--
A. Physical layer
B. Link layer
C. Transport layer
D. Network layer
Question No: 8
In which topology, if a computer’s network cable is broken, whole network goes down.
Question No: 9
In OSI network architecture, the dialogue control and token management are responsibilities of-
A. Session layer
B. Network layer
C. Transport layer
D. Data link layer
Question No: 10
ISO OSI model is used in--
A. Stand alone PC
B. Network environment
C. Sub network access
D. None of above
Question No: 11
If a computer on the network shares resources for others to use, it is called--
D. None of above
Question No: 12
ISO stands for--
A. International Standard Organization
B. International Student Organization
C. Integrated Services Organization
D. None of above
Question No: 13
Which of the following is used for modulation and demodulation?
Question No: 14
Which layer decides which physical pathway the data should take?
Question No: 15
A standalone program that has been modified to work on a LAN by including concurrency controls such as file and record locking is an example of--
A. LAN intrinsic software
B. LAN aware software
D. LAN ignorant software
Question No: 16
ISDN is an example of which network?
A. Circuit switched
B. Packet switched
C. Both of above
D. None of above
Question No: 17
X.25 is an example of which network?
A. Circuit switched
B. Packet switched
C. Both of above
D. None of above
Question No: 18
The process of converting analog signals into digital signals so they can be processed by a receiving computer is referred to as--
Question No: 19
What is the max cable length of STP?
A. 200 ft
B. 100 ft
C. 100 m
D. 200 m
Question No: 20
In which portion of LAN management software restricts access, records user activities and audit data etc?
A. Configuration management
B. Security management
C. Performance management
D. Switch Management
Question No: 21
Which connector is used in STP?
Question No: 22
Network cable lies on layer--
D. Data link
Question No: 23
What is the central device in star topology?
A. STP server
Question No: 24
What is the max data capacity of STP?
A. 10 mbps
B. 100 mbps
C. 1000 mbps
D. 10000 mbps
Question No: 25
Which of the following performs modulation and demodulation?
A. Fibre optics
C. Coaxial cable
Question No: 26
Which server allows LAN users to share computer programs and data?
A. Communication server
B. Print server
C. File server
D. Mail Server
Question No: 27
In OSI network architecture, the routing is performed by—
A. Network layer
B. Data link layer
C. Transport layer
D. Session layer
Question No: 28
Which of the following architecture uses CSMA/CD access method?
A. ARC net
C. Both of above
D. None of above
Question No: 29
Which buffer holds the data before it is sent to the printer through print server?
D. None of above
Question No: 30
The x.25 standard specifies a--
A. Technique for start-stop data
B. Technique for dial access
C. Dte/dce interface
D. Data bit rate
Question No: 31
Which of the following might be used by a company to satisfy its growing communications needs?
A. Front end processor
D. All of the above
Question No: 32
A band is always equivalent to
A. A byte
B. A bit
C. 100 bits
D. None of above
Question No: 33
Which of the following does not allow multiple uses or devices to share one communication line?
A. Double plexer
Question No: 34
Communiction circuits that transmit data in both directions but not at the same time are operating in
A. A simplex mode
B. A half duplex mode
C. A full duplex mode
D. An asynchronous mode
Question No: 35
How many OSI layers are covered in the X.25 standard?
Question No: 36
Which of the following is considered a broad band communication channel?
A. Coaxial cable
B. Fibre optics cable
C. Microwave circuits
D. All of above
Question No: 37
Which of the following is not a transmission medium?
A. Telephone lines
B. Coaxial cables
D. Microwave systems
Question No: 38
Which of the following is required to communicate between two computers?
A. Communications software
C. Communication hardware
D. All of above including access to transmission medium
Question No: 39
Which of the following is an advantage to using fibre optics data transmission?
A. Resistance to data theft
B. Fast data transmission rate
C. Low noise level
D. All of above
Question No: 40
Electronic systems that transmit data over communications lines from one location to another are called ________.
A. transmission systems
C. microcomputer systems
D. LAN transmission systems
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