Subject Name: Computer Forensic & Cyber Applications. Subject Teacher:- Prof. Pansare R.B.

Transcription

1 Que 1 SND College of Engineering & Research Center, Yeola Department of Computer Engineering Subject Name: Computer Forensic & Cyber Applications Subject Teacher:- Prof. Pansare R.B. Important Questions & Answers Describe layers of OSI Model with neat suitable Diagram Marks-8 (June 2015, May 2016) Application Layer Application layer provides platform to send and receive data over the network. All applications and utilities that communicate with network fall in this layer. For examples. Browsers :- Mozilla Firefox, Internet Explorer, Google Chrome etc clients: - Outlook Express, Mozilla Thunderbird etc. FTP clients :- Filezilla, sftp, vsftp

2 Application layer protocols : SNMP (Simple Network Management Protocol) Used to control the connected networking devices. TFTP (Trivial File Transfer Protocol) Used to transfer the files rapidly. DNS (Domain Naming System) Used to translate the name with IP address and vice versa. DHCP (Dynamic Host Configuration Protocol) Used to assign IP address and DNS information automatically to hosts. Telnet used to connect remote devices. HTTP (Hypertext Transfer Protocol) Used to browse web pages. FTP (File Transfer Protocol) Used to reliably sends/retrieves files. SMTP (Simple Mail Transfer Protocol) Used to sends . POP3 (Post Office Protocol v.3) Used to retrieves . NTP (Network Time Protocol) Used to synchronizes clocks. Presentation layer Presentation layer prepares the data. It takes data from application layer and marks it with formatting code such as.doc,.jpg,.txt,.avi etc. These file extensions make it easy to realize that particular file is formatted with particular type of application. With formatting presentation layer also deals with compression and encapsulation. It compresses (on sending computer) and decompresses (on receiving computer) the data file. This layer can also encapsulate the data, but it s uncommon as this can be done by lower layers more effectively. The Session Layer Session layer deals with connections. It establishes, manages, and terminates sessions between two communicating nodes. This layer provides its services to the presentation layer. Session layer also synchronizes dialogue between the presentation layers of the two hosts and manages their data exchange. For example, web servers may have many users communicating with server at a given time. Therefore, keeping track of which user communicates on which path is important and session layer handle this responsibility accurately. Transport Layer Transport layer provides following services: - It sets up and maintains the connection between two devices. It multiplexes connections that allow multiple applications to simultaneously send and receive data. According to requirement data transmission method can be connection oriented or connection less. For unreliable data delivery connection less method is used.

3 Connection less method uses UDP protocol. For reliable data delivery connection oriented method is used. Connection oriented method uses TCP protocol. When Implemented a reliable connection, sequence numbers and acknowledgments (ACKs) are used. Reliable connection controls flow through the uses of windowing or acknowledgements. Network Layer The network layer is responsible for the source-to-destination delivery of a packet, possibly across multiple networks (links). Whereas the data link layer oversees the delivery of the packet between two systems on the same network (links), the network layer ensures that each packet gets from its point of origin to its final destination. If two systems are connected to the same link, there is usually no need for a network layer. However, if the two systems are attached to different networks (links) with connecting devices between the networks (links), there is often a need for the network layer to accomplish source-to-destination delivery. Figure shows the relationship of the network layer to the data link and transport layers. Other responsibilities of the network layer include the following: Logical addressing- The physical addressing implemented by the data link layer handles the addressing problem locally. If a packet passes the network boundary, we need another addressing system to help distinguish the source and destination Systems. The network layer adds a header to the packet coming from the upper layer that, among other things, includes the logical addresses of the sender and Receiver. Routing- When independent networks or links are connected to create internetworks (network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination. One of the functions of the network layer is to provide this mechanism. Data Link layer The data link layer transforms the physical layer, a raw transmission facility, to a reliable link. It makes the physical layer appear error-free to the upper layer Responsibilities of the data link layer include the following:

4 Framing. The data link layer divides the stream of bits received from the network layer into manageable data units called frames. Physical addressing:- If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the sender and/or receiver of the frame. If the frame is intended for a system outside the sender's network, the receiver address is the address of the device that connects the network to the next one. Flow control:- If the rate at which the data are absorbed by the receiver is less than the rate at which data are produced in the sender, the data link layer imposes a flow control mechanism to avoid overwhelming the receiver. Error control:- The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames. Error control is normally achieved through a trailer added to the end of the frame. Access control:- When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time. Physical Layer The physical layer coordinates the functions required to carry a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium. It also defines the procedures and functions that physical devices and interfaces have to perform for transmission to Occur. The physical layer is also concerned with the following: Physical characteristics of interfaces and medium. Representation of bits. Data rate. Synchronization of bits. Line configuration. Physical topology. Transmission mode. Summary Of all Layers.

5 Q.2 Explain TCP/IP Model Marks-8 The TCP/IP protocol suite was developed prior to the OSI model. Therefore, the layers in the TCP/IP protocol suite do not exactly match those in the OSI model. The original TCP/IP protocol suite was defined as having four layers: 1) Host-to-network. 2) Internet 3) Transport 4) Application. Figure: TCP/IP Model TCP/IP is a hierarchical protocol made up of interactive modules, each of which provides a specific functionality. The modules are not necessarily interdependent. Whereas the OSI model specifies which functions belong to each of its layers, the layers of the TCP/IP protocol suite contain relatively independent protocols that can be mixed and matched depending on the needs of the system. The term hierarchical means that each upper-level protocol is supported by one or more lower-level protocols. At the transport layer, TCP/IP defines three protocols: Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Stream Control Transmission Protocol (SCTP). At the network layer, the main protocol defined by TCP/IP is the Internetworking Protocol (IP). There are also some other protocols that support data movement in this layer. Physical and Data Link Layers:-At the physical and data link layers, TCPIIP does not define any specific protocol. It supports all the standard and proprietary protocols. A network in a TCP/IP internetwork can be a local-area network or a wide-area network. Network Layer: - At the network layer, TCP/IP supports the Internetworking Protocol. It uses four supporting protocols: ARP, RARP, ICMP, and IGMP. Internetworking Protocol (IP) The Internetworking Protocol (IP) is the transmission mechanism used by the TCP/IP protocols. It is an unreliable and connectionless protocol-a best-effort delivery service. The term best effort means that IP provides no error checking or tracking. IP assumes the unreliability of the underlying layers and does its best to get a transmission through to its destination, but with no guarantees. IP transports data in packets called datagrams, each of which is transported separately.

6 The Address Resolution Protocol (ARP) is used to associate a logical address with a physical address. On a typical physical network, such as a LAN, each device on a link is identified by a physical or station address, usually imprinted on the network interface card (NIC). ARP is used to find the physical address of the node when its Internet address is known. The Reverse Address Resolution Protocol (RARP) allows a host to discover its Internet address when it knows only its physical address. It is used when a computer is connected to a network for the first time or when a diskless computer is booted. The Internet Control Message Protocol (ICMP) is a mechanism used by hosts and gateways to send notification of datagram problems back to the sender. ICMP sends query and error reporting messages. The Internet Group Message Protocol (IGMP) is used to facilitate the simultaneous transmission of a message to a group of recipients. Transport Layer The transport layer was represented in TCP/IP by two protocols: TCP and UDP. IP is a host-to-host protocol, meaning that it can deliver a packet from one physical device to another. UDP and TCP are transport level protocols responsible for delivery of a message from a process (running program) to another process. A new transport layer protocol, SCTP, has been devised to meet the needs of some newer applications. The User Datagram Protocol (UDP) is the simpler of the two standard TCPIIP transport protocols. It is a process-to-process protocol that adds only port addresses, checksum error control, and length information to the data from the upper layer. The Transmission Control Protocol (TCP) provides full transport-layer services to applications. TCP is a reliable stream transport protocol. The term stream, in this context, means connection-oriented: A connection must be established between both ends of a transmission before either can transmit data. At the sending end of each transmission, TCP divides a stream of data into smaller units called segments. Each segment includes a sequence number for reordering after receipt, together with an acknowledgment number for the segments received. Segments are carried across the internet inside of IP datagrams. At the receiving end, TCP collects each datagram as it comes in and reorders the transmission based on sequence numbers. The Stream Control Transmission Protocol (SCTP) provides support for newer applications such as voice over the Internet. It is a transport layer protocol that combines the best features of UDP and TCP. Application Layer The application layer in TCP/IP is equivalent to the combined session, presentation, and application layers in the OSI model Many protocols are defined at this layer Q.3 Explain Protocols and Standard Marks- 5,2,3 Protocols In computer networks, communication occurs between entities in different systems. An entity is anything capable of sending or receiving information. However, two entities cannot simply send bit streams to each other and expect to be understood. For communication to occur, the entities must agree on a protocol. A protocol is a set of rules that govern data communications. A protocol defines what is communicated, how it is communicated, and when it is communicated. The key elements of a protocol are syntax, semantics, and timing.

7 Syntax. The term syntax refers to the structure or format of the data, meaning the order in which they are presented. For example, a simple protocol might expect the first 8 bits of data to be the address of the sender, the second 8 bits to be the address of the receiver, and the rest of the stream to be the message itself. Semantics. The word semantics refers to the meaning of each section of bits. How is a particular pattern to be interpreted, and what action is to be taken based on that interpretation? For example, does an address identify the route to be taken or the final destination of the message? Timing. The term timing refers to two characteristics: when data should be sent and how fast they can be sent. For example, if a sender produces data at 100 Mbps but the receiver can process data at only 1 Mbps, the transmission will overload the receiver and some data will be lost. Standards Standards provide guidelines to manufacturers, vendors, government agencies, and other service providers to ensure the kind of interconnectivity necessary in today's marketplace and in international communications. Data communication standards fall into two categories: de facto (meaning "by fact" or "by convention") and de jure (meaning "by law" or "by regulation"). De facto. Standards that have not been approved by an organized body but have been adopted as standards through widespread use are de facto standards. De facto standards are often established originally by manufacturers who seek to define the functionality of a new product or technology. De jure. Those standards that have been legislated by an officially recognized body are de jure standards. Standards Organizations Standards are developed through the cooperation of standards creation committees, forums, and government regulatory agencies. Standards Creation Committees While many organizations are dedicated to the establishment of standards, data telecommunications in North America rely primarily on those published by the following: International Telecommunication Union-Telecommunication Standards Sector (ITU-T) International Organization for Standardization (ISO). American National Standards Institute (ANSI) Institute of Electrical and Electronics Engineers (IEEE). Electronic Industries Association (EIA). Regulatory Agencies All communications technology is subject to regulation by government agencies such as the Federal Communications Commission (FCC) in the United States. The purpose of these agencies is to protect the public interest by regulating radio, television, and wire/cable communications. The FCC has authority over interstate and international commerce as it relates to communications. Internet Standards An Internet standard is a thoroughly tested specification that is useful to and adhered to by those who work with the Internet. It is a formalized regulation that must be followed. There is a strict procedure by which a specification attains Internet standard status. A specification begins as an Internet draft. An Internet draft is a working document (a work in progress) with no official status and a 6-month lifetime. Upon recommendation from the Internet authorities, a draft may be published as a Request for Comment (RFC). Each RFC is edited,

8 assigned a number, and made available to all interested parties. RFCs go through maturity levels and are categorized according to their requirement level. Q.4 Define network topology and explain various network topologies Marks-9 The term physical topology refers to the way in which a network is laid out physically. One or more devices connect to a link; two or more links form a topology. The topology of a network is the geometric representation of the relationship of all the links and linking devices (usually called nodes) to one another. There are four basic topologies possible: Mesh, Star, Bus, and Ring Dec Mesh Topology, Every device has a dedicated point-to-point link to every other device. The term dedicated means that the link carries traffic only between the two devices it connects. To find the number of physical links in a fully connected mesh network with n nodes, we first consider that each node must be connected to every other node. Node 1 must be connected to n - I nodes, node 2 must be connected to n - 1 nodes, and finally node n must be connected to n - 1 nodes. We need n(n - 1) physical links. However, if each physical link allows communication in both directions (duplex mode), we can divide the number of links by 2. In other words, we can say that in a mesh topology, we need n (n -1) /2 duplex-mode links. To accommodate that many links, every device on the network must have n - 1 input/output (VO) ports to be connected to the other n - 1 station. Figure: Mesh Topology Advantages The use of dedicated links guarantees that each connection can carry its own data load, thus eliminating the traffic problems that can occur when links must be shared by multiple devices. A mesh topology is robust. There is the advantage of privacy or security Disadvantages Mesh are related to the amount of cabling and the number of I/O ports required. Finally, the hardware required to connect each link (I/O ports and cable) can be prohibitively expensive. 2. Star Topology In a star topology, each device has a dedicated point-to-point link only to a central controller, usually

9 called a hub. The devices are not directly linked to one another. Unlike a mesh topology, a star topology does not allow direct traffic between devices. The controller acts as an exchange: If one device wants to send data to another, it sends the data to the controller, which then relays the data to the other connected device and the hub. Advantages Include robustness. A star topology connecting four stations Hub fault isolation. As long as the hub is working, it can be used to monitor link problems and bypass defective links. Disadvantage a star topology is the dependency of the whole topology on one single point, the hub. If the hub goes down, the whole system is dead. Although a star requires far less cable than a mesh, each node must be linked to a central hub. For this reason, often more cabling is required in a star than in some other topologies (such as ring or bus). The star topology is used in local-area networks (LANs), High-speed LANs often use a star topology with a central hub. 3. Bus Topology The preceding examples all describe point-to-point connections. A bus topology, on the other hand, is multipoint. One long cable acts as a backbone to link all the devices in a network. Figure: Bus Topology Nodes are connected to the bus cable by drop lines and taps. A drop line is a connection running between the device and the main cable. A tap is a connector that either splices into the main cable or punctures the sheathing of a cable to create a contact with the metallic core. As a signal travels along the backbone, some of its energy is transformed into heat. Therefore, it becomes weaker and weaker as it travels farther and farther. For this reason there is a limit on the number of taps a bus can support and on the distance between those taps. Advantages Bus topology includes ease of installation. Backbone cable can be laid along the most efficient path, and then connected to the nodes by drop lines of various lengths. In this way, a bus uses less cabling

10 than mesh or star topologies. In a star, for example, four network devices in the same room require four lengths of cable reaching all the way to the hub. In a bus, this redundancy is eliminated. Only the backbone cable stretches through the entire facility. Each drop line has to reach only as far as the nearest point on the backbone. Disadvantages Include difficult reconnection and fault isolation. A bus is usually designed to be optimally efficient at installation. It can therefore be difficult to add new devices. 4. Ring Topology Figure: Ring Topology In a ring topology, each device has a dedicated point-to-point connection with only the two devices on either side of it. A signal is passed along the ring in one direction, from device to device, until it reaches its destination. Each device in the ring incorporates a repeater. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them A ring is relatively easy to install and reconfigure. Each device is linked to only its immediate neighbors (either physically or logically). To add or delete a device requires changing only two connections. The only constraints are media and traffic considerations (maximum ring length and number of devices). In addition, fault isolation is simplified. Generally in a ring, a signal is circulating at all times. If one device does not receive a signal within a specified period, it can issue an alarm. The alarm alerts the network operator to the problem and its location. Ring topology was prevalent when IBM introduced its local-area network Token Ring. Today, the need for higher-speed LANs has made this topology less popular. 5. Hybrid Topology A network can be hybrid. For example, we can have a main star topology with each branch connecting several stations in a bus topology.

11 Figure: Hybrid Topology Q. 5 Explain Transmission media A transmission medium can be broadly defined as anything that can carry information from a source to a destination. For example, the transmission medium for two people having a dinner conversation is the air. The air can also be used to convey the message in a smoke signal or semaphore. For a written message, the transmission medium might be a mail carrier, a truck, or an airplane. In telecommunications, transmission media can be divided into two broad categories: guided and unguided. Guided media include twisted-pair cable, coaxial cable, and fiber-optic cable. Unguided medium is free space. Figure:- Transmission Media GUIDED MEDIA Guided media, which are those that provide a conduit from one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable. A signal traveling along any of these media is directed and contained by the physical limits of the medium. Twisted-pair and coaxial cable use metallic (copper) conductors that accept and transport signals in the form of electric current. Optical fiber is a cable that accepts and transports signals in the form of light. 1. Twisted-Pair Cable:- o A twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted together. o One of the wires is used to carry signals to the receiver, and the other is used only as a ground reference. The receiver uses the difference between the two. o In addition to the signal sent by the sender on one of the wires, interference (noise) and crosstalk may affect both wires and create unwanted signals. o

12 Figure: Twisted Pair Cable structure Unshielded Versus Shielded Twisted-Pair Cable The most common twisted-pair cable used in communications is referred to as unshielded twisted-pair (UTP). IBM has also produced a version of twisted-pair cable for its use called shielded twisted-pair (STP). STP cable has a metal foil or braided mesh covering that encases each pair of insulated conductors. Although metal casing improves the quality of cable by preventing the penetration of noise or crosstalk, it is bulkier and more expensive. Figure shows the difference between UTP and STP. Our discussion focuses primarily on UTP because STP is seldom used outside of IBM. Figure: Difference between UTP & STP Connectors The most common UTP connector is RJ45 (RJ stands for registered jack. The RJ45 is a keyed connector, meaning the connector can be inserted in only one way. Applications Twisted-pair cables are used in telephone lines to provide voice and data channels. The local loop-the line that connects subscribers to the central telephone office---commonly consists of unshielded twisted-pair cables. The DSL lines that are used by the telephone companies to provide high-data-rate connections also use the high-bandwidth capability of unshielded twisted-pair cables. Local-area networks, such as 10Base-T and 100Base-T, also use twisted-pair cables. 2. Coaxial Cable Coaxial cable (or coax) carries signals of higher frequency ranges than those in twisted pair cable, in part because the two media are constructed quite differently. Instead of having two wires, coax has a central core conductor of solid or stranded wire (usually copper) enclosed in an insulating sheath, which is, in turn, encased in an outer conductor of metal foil, braid, or a combination of the two. The outer metallic wrapping serves both as a shield against noise and as the second conductor, which completes the circuit. This outer conductor is also enclosed in an insulating sheath, and the whole cable is protected by a plastic cover.

13 Figure:- Coaxial Cable Core Coaxial Cable Standards- Coaxial cables are categorized by their radio government (RG) ratings. Each RG number denotes a unique set of physical specifications, including the wire gauge of the inner conductor, the thickness and type of the inner insulator, the construction of the shield, and the size and type of the outer casing. Each cable defined by an RG rating is adapted for a specialized function. Table: Categories of Coaxial cable. Coaxial Cable Connectors To connect coaxial cable to devices, we need coaxial connectors. The most common type of connector used today is the Bayone-Neill-Concelman (BNe), connector. There are three popular types of these connectors: The BNC connector, the BNC T connector, and the BNC terminator. The BNC connector is used to connect the end of the cable to a device, such as a TV set. The BNC T connector is used in Ethernet networks to branch out to a connection to a computer or other device. The BNC terminator is used at the end of the cable to prevent the reflection of the signal. Figure: BNC Connector

14 Applications Coaxial cable was widely used in analog telephone networks where a single coaxial network could carry 10,000 voice signals. Cable TV networks also use coaxial cables. Hybrid networks use coaxial cable. Fiber-Optic Cable A fiber-optic cable is made of glass or plastic and transmits signals in the form of light. To understand optical fiber, we first need to explore several aspects of the nature of light. Light travels in a straight line as long as it is moving through a single uniform substance. If a ray of light traveling through one substance suddenly enters another substance (of a different density), the ray changes direction. Optical fibers use reflection to guide light through a channel. A glass or plastic core is surrounded by a cladding of less dense glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it. Propagation Modes Figure: Optical Fiber There are two types of Modes:- 1) Multimode 2) Single Mode 1) Multimode is so named because multiple beams from a light source move through the core in different paths. How these beams move within the cable depends on the structure of the core. Multimode can be implemented in two forms: step-index or graded-index. A) In multimode step-index fiber, the density of the core remains constant from the center to the edges. A beam of light moves through this constant density in a straight

15 line until it reaches the interface of the core and the cladding. At the interface, there is an abrupt change due to a lower density; this alters the angle of the beam's motion. The term step index refers to the suddenness of this change, which contributes to the distortion of the signal as it passes through the fiber. B) A second type of fiber, called multimode graded-index fiber, decreases this distortion of the signal through the cable. The word index here refers to the index of refraction. As we saw above, the index of refraction is related to density. A graded-index fiber, therefore, is one with varying densities. Density is highest at the center of the core and decreases gradually to its lowest at the edge. 2) Single-Mode uses step-index fiber and a highly focused source of light that limits beams to a small range of angles, all close to the horizontal. The single mode fiber itself is manufactured with a much smaller diameter than that of multimode fiber, and with substantially lowers density (index of refraction). The decrease in density results in a critical angle that is close enough to 90 to make the propagation of beams almost horizontal. In this case, propagation of different beams is almost identical, and delays are negligible. All the beams arrive at the destination "together" and can be recombined with little distortion to the signal.

16 Fiber-Optic Cable Connectors There are three types of connectors for fiber-optic cables 1) The subscriber channel (SC) connector is used for cable TV. It uses a push/pull locking system. 2) The straight-tip (ST) connector is used for connecting cable to networking devices. It uses a bayonet locking system and is more reliable than SC. 3) MT-RJ is a connector that is the same size as RJ45. Applications Fiber-optic cable is often found in backbone networks because its wide bandwidth is cost-effective. Some cable TV companies use a combination of optical fiber and coaxial cable, thus creating a hybrid network. Local-area networks such as 100Base-FX network (Fast Ethernet) and 1000Base-X also use fiber-optic cable. Q. 6 Explain Circuit switching, Packet switching and message switching Mark 8 May 2016 Circuit Switching In circuit switching network dedicated channel has to be established before the call is made between users. The channel is reserved between the users till the connection is active. For half duplex communication, one channel is allocated and for full duplex communication, two channels are allocated. It is mainly used for voice communication requiring real time services without any much delay. As shown in the figure, if user-a wants to use the network; it need to first ask for the request to obtain the one and then user-a can communicate with user-c. During the connection phase if user-b tries to call/communicate with user-d or any other user it will get busy signal from the network. Packet Switching

17 In packet switching network unlike CS network, it is not required to establish the connection initially. The connection/channel is available to use by many users. But when capacity or number of users increases then it will lead to congestion in the network. Packet switched networks are mainly used for data and voice applications requiring non-real time scenarios. As shown in the figure 2, if user-a wants to send data/information to user-c and if user-b wants to send data to user-d, it is simultaneously possible. Here information is padded with header which contains addresses of source and destination. This header is sniffed by intermediate switching nodes to determine their route and destination. In packet switching, station breaks long message into packets. Packets are sent one at a time to the network. Packets are handled in two ways, viz. datagram and virtual circuit. In datagram, each packet is treated independently. Packets can take up any practical route. Packets may arrive out of order and may go missing. In virtual circuit, preplanned route is established before any packets are transmitted. The handshake is established using call request and call accept messages. Here each packet contains virtual circuit identifier (VCI) instead of the destination address. In this type, routing decisions for each packet are not needed. Message Switching With message switching there is no need to establish a dedicated path between two stations. When a station sends a message, the destination address is appended to the message. The message is then transmitted through the network, in its entirety, from node to node. Each node receives the entire message, stores it in its entirety on disk, and then transmits the message to the next node. This type of network is called a store-and-forward network. Message Switching A message-switching node is typically a general-purpose computer. The device needs sufficient secondary-storage capacity to store the incoming messages, which could be long. A time delay is introduced using this type of scheme due to store- and-forward time, plus the time required to find the next node in the transmission path.

18 Q.7 Explain Network Hardware Components.(HUB, REPEATER, SWITCH, ROUTER) Mark 8 Dec 2014 Hubs Passive Hub A passive hub is just a connector. It connects the wires coming from different branches. In a star-topology Ethernet LAN, a passive hub is just a point where the signals coming from different stations collide; the hub is the collision point. This type of a hub is part of the media; its location in the Internet model is below the physical layer. Active Hub An active hub is actually a multipart repeater. It is normally used to create connections between stations in a physical star topology. We have seen examples of hubs in some Ethernet implementations (lobase-t, for example). However, hubs can also be used to create multiple levels of hierarchy, as shown in Figure The hierarchical use of hubs removes the length limitation of 10Base-T (100 m). Repeater A repeater is a device that operates only in the physical layer. Signals that carry information within a network can travel a fixed distance before attenuation endangers the integrity of the data. A repeater receives a signal and, before it becomes too weak or corrupted, regenerates the original bit pattern. The repeater then sends the refreshed signal. A repeater can extend the physical length of a LAN. Figure: Repeater connecting in Two LAN A repeater does not actually connect two LANs; it connects two segments of the same LAN. The segments connected are still part of one single LAN. A repeater is not a device that can connect two LANs of different protocols. A repeater can overcome the 10Base5 Ethernet length restriction. In this standard, the length of the cable is limited to 500 m. To extend this length, we divide the cable into segments and install repeaters between segments. Note that the whole network is still considered one LAN, but the portions of the network separated by repeaters are called segments. The repeater acts as a two-port node, but operates only in the physical layer. When it receives a frame from any of the ports, it regenerates and forwards it to the other port. Bridges A bridge operates in both the physical and the data link layer. As a physical layer device, it regenerates the signal it receives. As a data link layer device, the bridge can check the physical (MAC) addresses (source and

19 destination) contained in the frame. Figure: Hub connecting computer In figure,two LANs are connected by a bridge. If a frame destined for station 712B arrives at port 1, the bridge consults its table to find the departing port. According to its table, frames for 7l2B leave through port 1; therefore, there is no need for forwarding, and the frame is dropped. On the other hand, if a frame for 712B arrives at port 2, the departing port is port 1 and the frame is forwarded. In the first case, LAN 2 remains free of traffic; in the second case, both LANs have traffic. In our example, we show a twoport bridge; in reality a bridge usually has more ports. Routers A router is a three-layer device that routes packets based on their logical addresses (host-to-host addressing). A router normally connects LANs and WANs in the Internet and has a routing table that is used for making decisions about the route. The routing tables are normally dynamic and are updated using routing protocols.

20 Q.8 Explain periodic listen and Sleep operation in S-MAC. Explain schedule selection and Coordination in S-MAC Ans. Periodic Listen and Sleep Operations One of the S-MAC design objectives is to reduce energy consumption by avoiding idle listening. This is achieved by establishing low-duty-cycle operations for sensor nodes. Periodically, nodes move into a sleep state during which their radios are turned off completely. Nodes become active when there is traffic in the network. Marks 8 Dec2014 June2015 The basic periodic listen and sleep scheme is depicted in Figure. Based on this scheme, each node sets a wake-up timer and goes to sleep for the specified period of time. At the expiration of the timer, the node wakes up and listens to determine if it needs to communicate with other nodes. The complete listen- and-sleep cycle is referred to as a frame. Each frame is characterized by its duty cycle, defined as the listening interval-to-frame length ratio. Although the length of the listening interval can be selected independently by sensor nodes, for simplicity the protocol assumes the value to be the same for all nodes. Nodes are free to schedule their own sleep and listen intervals. It is preferable, however, that the schedules of neighboring nodes be coordinated in order to reduce the control overhead necessary to achieve communications between these nodes. Contrary to other protocols in which coordination is achieved through a master node such as a cluster. Schedule Selection and Coordination The neighboring nodes coordinate their listen and sleep schedules such that they all listen at the same time and all sleep at the same time. To coordinate their sleeping and listening, each node selects a schedule and exchanges it with it neighbors during the synchronization period. Each node maintains a schedule table that contains the schedule of all its known neighbors. To select a schedule, a node first listens to the channel for a fixed amount of time, at least equal to the synchronization period. At the expiration of this waiting period, if the node does not hear a schedule from another node, it immediately chooses its own schedule. The node announces the schedule selected by broadcasting a SYNC packet to all its neighbors. It is worth noting that the node must first perform physical carrier sensing before broadcasting the SYNC packet. This reduces the likelihood of SYNC packet collisions among competing nodes.

21 If during the synchronization period the node receives a schedule from a neighbor before choosing and announcing its own schedule, the node sets its schedule to be the same as the schedule received. It is worth noting that a node may receive a different schedule after it chooses and announces its own schedule. This may occur if the SYNC packet is corrupted by either collision or channel interference. If the node has no neighbor with whom it shares a schedule, the node simply discards its own schedule and adopts the new one. On the other hand, if the node is aware of other neighboring nodes that have already adopted its schedule, the node adopts both schedules. The node is then required to wake up at the listen intervals of the two schedules adopted. This is illustrated in Figure, The advantage of carrying multiple schedules is that border nodes are required to broadcast only one SYNC packet. The disadvantage of this approach is that border nodes consume more energy, as they spend less time in the sleep mode. Schedule Synchronization Schedule updating is continuous process and this is accomplished by sending a SYNC packet. For a node to receive both SYNC packets and data packets, the listen interval is divided into two subintervals as depicted in Figure.

22 This figure illustrates three cases. In the first case 1. the sender sends only a SYNC packet; 2. sender sends only a data packet; and 3. Sender sends a SYNC packet in addition to the data packet. Access to the channel by contending nodes during these subintervals is regulated using a multi slotted contention window. The first subinterval is dedicated to the transmission of SYNC packets; the second subinterval is used for the transmission of data packets. In either of these subintervals, a contending station randomly selects a time slot, performs carrier sensing, and starts sending its packet if it detects that the channel is idle. Transmission of data packets uses the RTS/CTS handshake to secure exclusive access to the channel during transmission of the data. This access procedure guarantees that the neighboring nodes receive both the synchronization and data packets. Adaptive Listening A closer look at the periodic listen and sleep scheme reveals that a message may incur increased latency as it is stored and forwarded between adjacent network nodes. If a sensor is to follow its sleep schedule strictly, data packets may be delayed at each hop. To address this shortcoming and improve latency performance, the protocol uses an efficient technique referred to as adaptive listening. Based on this technique, a node that overhears, during its listen period, the exchange of a CTS or RTS packet between a neighboring node and another node assumes that it may be the next hop along the routing path of the overheard RTS/CTS packet, ignores its own wake-up schedule, and schedules an extra listening period around the time the transmission of the packet terminates. The overhearing node determines the time necessary to complete the transmission of the packet from the duration field of the overheard CTS or RTS packet. Immediately upon receiving the data packet, the node issues an RTS packet to initiate an

23 TS/CTS handshake with the overhearing node. Ideally, the latter node is awake, in which case the packet forwarding process proceeds immediately between the two nodes. If the overhearing node does not receive an RTS packet during adaptive listening, it reenters its sleep state until the next scheduled listen interval. Q.9 Explain IEEE protocol Marks 8

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26 Q.10 Explain Naming & Addressing Marks 6

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