COMPUTER NETWORK [IT-503]

IT- Vsem

Examination June 2015

UNIT – 1

Que 1a) What do you mean by the term “clock based framing”?

Ans. Clock-based framingIn a clock-based system, a series of repetitive pulses are used to maintain a constant bit rate and keep the digital bits aligned in the data stream. SONET (Synchronous Optical Network) is a synchronous system in which all the clocks in the network are synchronized back to a master clock reference. SONET frames are then positioned within the clocked stream.

Que 1 b) List the common types of cables and fibres available for local links ?

Ans. The following are the types of cables used in networks

  • Unshielded Twisted Pair (UTP) Cable
  • Shielded Twisted Pair (STP) Cable
  • Coaxial Cable
  • Fiber Optic Cable
  • Cable Installation Guides
  • Wireless LANs
  • Unshielded Twisted Pair (UTP) Cable

Que 1 c) Given the token ring frame format of 802.5 ?

Ans.Token Ring and IEEE 802.5 support two basic frame types: tokens and data/command frames. Tokens are 3 bytes in length and consist of a start delimiter, an access control byte, and an end delimiter. Data/command frames vary in size, depending on the size of the Information field. Data frames carry information for upper-layer protocols, while command frames contain control information and have no data for upper-layer protocols.

Token Frame Fields

Start Delimiter / Access Control / Ending delimiter

Start delimiter (1 byte): Alerts each station of the arrival of a token (or data/command frame). This field includes signals that distinguish the byte from the rest of the frame by violating the encoding scheme used elsewhere in the frame.

•Access-control (1 byte): Contains the Priority field (the most significant 3 bits) and the Reservation field (the least significant 3 bits), as well as a token bit (used to differentiate a token from a data/command frame) and a monitor bit (used by the active monitor to determine whether a frame is circling the ring endlessly).

•End delimiter (1 byte): Signals the end of the token or data/command frame. This field also contains bits to indicate a damaged frame and identify the frame that is the last in a logical sequence.

Data/Command Frame Fields

Start Delimiter / Access Control / Frame Control / Destination address / Source address / Data / Frame check sequence / End Delimiter / Frame Status

Data/command frames have the same three fields as Token Frames, plus several others. The Data/command frame fields are described below:

Frame-control byte (1 byte)—Indicates whether the frame contains data or control information. In control frames, this byte specifies the type of control information.

Destination and source addresses (2-6 bytes)—Consists of two 6-byte address fields that identify the destination and source station addresses.

Data (up to 4500 bytes)—Indicates that the length of field is limited by the ring token holding time, which defines the maximum time a station can hold the token.

Frame-check sequence (FCS- 4 byte)—Is filed by the source station with a calculated value dependent on the frame contents. The destination station recalculates the value to determine whether the frame was damaged in transit. If so, the frame is discarded.

Frame Status (1 byte)—This is the terminating field of a command/data frame. The Frame Status field includes the address-recognized indicator and frame-copied indicator.

Qus.1 e) Compare and contrast stop and wait and sliding window protocol in detail with an illustration ?

Ans. In a stop-and-wait protocol, the sender waits until a packet is correctly acknowledged before sending the next packet. When the round trip time is large relative to the packet transmission time, this protocol has low efficiency. In sliding window protocols the sender is allowed to transmit more that one packet before receiving an acknowledgment.

The total number of unacknowledged packets that may be sent is referred to as the sender's maximum window size. For example, suppose that the sender has a window of size W. In this case, the transmitter can send packets numbered 1,2,…,W, before receiving an acknowledgment for packet 1.

To maximize efficiency, the window should be chosen big enough to minimize the time the transmitter is idle. We considered an example of a satellite link with a RTT of 500msec and a packet transmission time of 20 seconds. Thus in a RTT, the transmitter can send 25 additional packets. If no errors occur, the acknowledgment from the first frame should arrive immediately after the 26th frame is sent. This allows the transmitter to send frame 27.With no errors, this gives good efficiency, because it allows the sender to transmit continuously.

When a sliding window protocol is used, we still want to have a retransmission protocol that provides a reliable service, i.e. delivers packets in order, correctly, and only once. There are two classes of such protocols that are commonly used, the first is called goback-n, and the second is called selective repeat.

For both types of protocols, as with stop-and-wait, the sender numbers each packet, and the receiver returns numbered acknowledgments when a packet is correctly received. However, notice that with a sliding window protocol, as opposed to stop-and-wait, more than 1 bit will be needed for sequence numbers. The sender's maximum window size is the maximum number of unacknowledged frames it is allowed to have sent at any time.

UNIT – II

Que 2 a) Give the example for the bit/byte oriented protocols?

Ans. Byte oriented protocols are BISYNC, PPP, LCP and DDCMP

Bit-oriented protocols are HDLC and SDLC

Que 2 b) Explain hidden/exposed terminal problem ?

Ans. Hidden Terminal Problem: In wireless networking, the hidden node problem or hidden terminal problem occurs when a node is visible from a wireless access point (AP), but not from other nodes communicating with said AP. This leads to difficulties in media access control. Hidden nodesin awireless networkrefer to nodes that are out of range of other nodes or a collection of nodes.

Que 2 c) Explain what congestion control strategy is adapted in TCP?

Ans.Jacobson and Karels developed a congestion control mechanism for TCP following a congestion collapse on the internet. Prior to this no congestion control mechanism was specified for TCP. Their method is based on ensuring the 'conservation of packets,' i.e., that the packets are entering the network at the same rate that they are exiting with a full window of packets in transit. A connection in this state is said to be in equilibrium. If all connections are in equilibrium, congestion collapse is unlikely The authors identified three ways for packet conservation to be violated:

  1. The connection never reaches equilibrium.
  2. A source sends a new packet before an old one exits.
  3. Congestion in the network prevents a connection from reaching equilibrium.

TCP is 'self clocking,' i.e., the source sends a new packet only when it receives an ack for an old one and the rate at which the source receives acks is the same rate at which the destination receives packets. So the rate at which the source sends matches the rate of transmission over the slowest part of the connection.

Que 2 d) Describe in detail about the physical properties and collision avoidance scheme present in wi-fi?

Ans.The IEEE 802.11 specification (ISO/IEC 8802-11) is an international standard describing the characteristics of awireless local area network(WLAN). The nameWi-Fishort for "Wireless Fidelity”. Today, due to misuse of the terms, the name of the standard is often confused with the name of the certification. A Wi-Fi network, in reality, is a network that complies with the 802.11 standard.

Wi-Fi is the name of the industry consortium that decides implementations of the wireless Ethernet standards IEEE802.11. So, explaining the multiple access scheme that Wi-Fi uses. In a Wi-Fi network, the devices communicate via a special node called the access point (AP). All the devices share the same communication channel.The basic issue is how to regulate access to this shared channel in a distributed way.

Wi-Fi uses a scheme called carrier sense multiple access with collision avoidance (CSMA/CA). The principle is listen before you talk, if you collide by speaking at the same time as someone else, wait a random time before you try again. This is essentially a cocktail party protocol.

The devices compute random wait times. The wait time of a device is the number of idle time slots it must wait before transmitting, counted D seconds after then end of the previous transmission. The device must add D seconds to its wait time whenever that wait time is interrupted by a transmission.
If devices compute different random times, they do not collide after their wait times. If the channel is busy at the end of a wait time, the device computes a new random wait time and starts waiting again.
In Wi-Fi, the random wait time is zero for a node that just receives a new packet to transmit while it is not waiting to transmit and the channel is idle. If the channel is busy, the node computes a wait time that is uniform in time slots. If the node tries to transmit a packet and does not receive an ACK, it doubles the set of time slots among which it picks uniformly a wait

To connect to a Wi-Fi LAN, a computer has to be equipped with awireless network interface controller. The combination of computer and interface controller is called astation. All stations share a single radio frequency communication channel. Transmissions on this channel are received by all stations within range. The hardware does not signal the user that the transmission was delivered and is therefore called abest-effort deliverymechanism. A carrier wave is used to transmit the data in packets, referred to as "Ethernet frames". Each station is constantly tuned in on the radio frequency communication channel to pick up available transmissions.

Que 2 e) Explain in detail about Ethernet 802.3 with its access protocol and addressing mechanism?

Ans.Ethernet Physical Layer

A Comparison of Various Ethernet and IEEE 802.3 Physical-Layer Specifications

Characteristic / Ethernet Value / IEEE 802.3 Values
10Base5 / 10Base2 / 10BaseT / 10BaseF / 10 Base -TX / 100BaseT4
Data rate (Mbps) / 10 / 10 / 10 / 10 / 10 / 100 / 100
Signaling method / Baseband / Baseband / Baseband / Baseband / Baseband / Baseband / Baseband
Maximum segment length (m) / 500 / 500 / 185 / 100 / 2,000 / 100 / 100
Media / 50-ohm coax (thick) / 50-ohm coax (thick) / 50-ohm coax (thin) / Unshielded twisted-pair cable / Fiber-optic / Cat 5 UTP / Unshielded twisted-pair cable
Nodes/segment / 100 / 100 / 30 / 1024 / 1024
Topology / Bus / Bus / Bus / Star / Point-to-point / Bus / Bus

10Base5 means it operates at 10 Mbps, uses baseband signaling and can support segments of up to 500 meters. The 10Base5 cabling is popularly called the Thick Ethernet. Vampire taps are used for their connections where a pin is carefully forced halfway into the co-axial cable's core as shown in the figure below. The 10Base2 or Thin Ethernet bends easily and is connected using standard BNC connectors to form T junctions (shown in the figure below). In the 10Base-T scheme a different kind of wiring pattern is followed in which all stations have a twisted-pair cable running to a central hub (see below). The difference between the different physical connections is shown below:


Fig. (a) 10Base5 (b)10Base2 (c)10Base-T

All 802.3 baseband systems use Manchester encoding , which is a way for receivers to unambiguously determine the start, end or middle of each bit without reference to an external clock. There is a restriction on the minimum node spacing segment length between two nodes in 10Base5 and 10Base2 and that is 2.5 meter and 0.5 meter respectively. The reason is that if two nodes are closer than the specified limit then there will be very high current which may cause trouble in detection of signal at the receiver end. Connections from station to cable of 10Base5 i.e. Thick Ethernet are generally made using vampire taps and to 10Base2 i.e. Thin Ethernet are made using industry standard BNC connectors to form T junctions. To allow larger networks, multiple segments can be connected by repeaters as shown. A repeater is a physical layer device. It receives, amplifies and retransmits signals in either direction.

Addressing mechanism: The local network addresses used inIEEE 802networks andFDDInetworks are calledMAC addresses; they are based on the addressing scheme used in earlyEthernetimplementations. A MAC address is a unique serial number. Once a MAC address has been assigned to a particular network interface (typically at time of manufacture), that device should be uniquely identifiable amongst all other network devices in the world. This guarantees that each device in a network will have a different MAC address analogous to a street address. This makes it possible for data packets to be delivered to a destination within asub-network, i.e.hostsinterconnected by some combination ofrepeaters,hubs,bridgesandswitches, but not bynetwork layerrouters. Thus, for example, when anIPpacket reaches its destination sub-network, the destination IP address a layer 3 or network layer concept is resolved with theAddress Resolution ProtocolforIPv4, or byNeighbor Discovery Protocol(IPv6) into the MAC address (a layer 2 concept) of the destination host.

Examples of physical networks areEthernetnetworks andWi-Finetworks, both of which are IEEE 802 networks and use IEEE 802 48-bit MAC addresses.A MAC layer is not required infull-duplexpoint-to-pointcommunication, but address fields are included in some point-to-point protocols for compatibility reasons.

UNIT -3

Que 3 b) Explain the token early/delayed released?

Ans. If a node transmits the token and nobody wants to send the data the token comes back to the sender. If the first bit of the token reaches the sender before the transmission of the last bit, then error situation araises. So to avoid this we should have:

propogation delay + transmission of n-bits (1-bit delay in each node ) > transmission of the token time.A station may hold the token for the token-holding time. which is 10 ms unless the installation sets a different value. If there is enough time left after the first frame has been transmitted to send more frames, then these frames may be sent as well. After all pending frames have been transmitted or the transmission frame would exceed the token-holding time, the station regenerates the 3-byte token frame and puts it back on the ring.

Que 3 c) Differentiate secret key and public key encryption technique?

Ans. Secret Key Encryption:Secret key encryption uses a single key to both encrypt and decrypt messages. As such it must be present at both the source and destination of transmission to allow the message to be transmitted securely and recovered upon receipt at the correct destination. The key must be kept secret by all parties involved in the communication. If the key fell into the hands of an attacker, they would then be able to intercept and decrypt messages, thus thwarting the attempt to attain secure communications by this method of encryption.

Secret key algorithms like DES assert that even although it is theoretically possible to derive the secret key from the encrypted message alone, the quantities of computation involved in doing so make any attempts infeasible with current computing hardware. The Kerberos architecture is a system based on the use of secret key encryption.

Public Key Encryption: Public key systems use a pair of keys, each of which can decrypt the messages encrypted by the other. Provided one of these keys is kept secret (theprivatekey), any communication encrypted using the correspondingpublickey can be considered secure as the only person able to decrypt it holds the corresponding private key.

The algorithmic properties of the encryption and decryption processes make it infeasible to derive a private key from a public key, an encrypted message, or a combination of both. RSA is an example of a public key algorithm for encryption and decryption. It can be used within a protocol framework to ensure that communication is secure and authentic.

Que 3 d) Explain in detail about the congestion avoidance mechanism?

Ans.TCP congestion-avoidance algorithm:Transmission Control Protocol(TCP) uses anetwork congestion-avoidancealgorithm that includes various aspects of anadditive increase/multiplicative decrease(AIMD) scheme, with other schemes such asslow-startto achieve congestion avoidance.TheTCP congestion-avoidance algorithmis the primary basis forcongestion controlin the Internet.

TCP Tahoe and Reno: To avoidcongestion collapse, TCP uses a multi-faceted congestion-control strategy. For each connection, TCP maintains acongestion window, limiting the total number of unacknowledged packets that may be in transit end-to-end. This is somewhat analogous to TCP'ssliding windowused forflow control. TCP uses a mechanism calledslow startto increase the congestion window after a connection is initialized and after a timeout. It starts with a window of two times themaximum segment size(MSS). Although the initial rate is low, the rate of increase is very rapid:for every packetacknowledged, the congestion window increases by 1 MSS so that the congestion window effectively doubles for everyround-trip time(RTT). When the congestion window exceeds a thresholdssthreshthe algorithm enters a new state, calledcongestion avoidance. In some implementations (e.g., Linux), the initial ssthresh is large, and so the first slow start usually ends after a loss. However, ssthresh is updated at the end of each slow start, and will often affect subsequent slow starts triggered bytimeouts.

Congestion avoidance: As long as non-duplicate ACKs are received, the congestion window is additively increased by one MSS every round trip time. When a packet is lost, the likelihood of duplicate ACKs being received is very high (it's possible though unlikely that the stream just underwent extreme packet reordering, which would also prompt duplicate ACKs). The behavior of Tahoe and Reno differ in how they detect and react to packet loss:

Tahoe: Common Tahoe implementations detect congestion only by setting a timer for receiving a related ACK. Tahoe sets the slow start threshold to half of the current congestion window, reduces the congestion window to 1 MSS, and resets to slow-start state.[8]