Unit II. Part A (2 Marks)

Transcription

1 Unit II Part A (2 Marks) 1. Differentiate fast Ethernet and Gigabit Ethernet. Fast Ethernet increased speed from 10 to 100 megabits per second (Mbit/s). Gigabit Ethernet was the next iteration, increasing the speed to 1000 Mbit/s. 2. What is the difference between switch and bridge? Bridge A bridge goes one step up on a hub in that it looks at the destination of the packet before sending. If the destination address is not on the other side of the bridge it will not transmit the data. A bridge only has one incoming and one outgoing port. Switch A switch has multiple ports. When a packet comes through a switch it is read to determine which computer to send the data to. This leads to increased efficiency in that packets are not going to computers that do not require them. 3. What is CSMA / CD? Carrier sense multiple access (CSMA) is a media access control (MAC) protocol in which a node verifies the absence of other traffic before transmitting on a shared transmission medium, such as an electrical bus, or a band of the electromagnetic spectrum. CSMA/CD is used to improve CSMA performance by terminating transmission as soon as a collision is detected, thus shortening the time required before a retry can be attempted. 4. What is CSMA /CA? Carrier sense multiple access (CSMA) is a media access control (MAC) protocol in which a node verifies the absence of other traffic before transmitting on a shared transmission medium, such as an electrical bus, or a band of the electromagnetic spectrum. In CSMA/CA collision avoidance is used to improve the performance of CSMA by attempting to be less "greedy" on the channel. If the channel is sensed busy before transmission then the transmission is deferred for a "random" interval. This reduces the probability of collisions on the channel.

2 5. What is Ethernet? Ethernet is the most widely-installed local area network ( LAN) technology. Specified in a standard, IEEE 802.3, Ethernet was originally developed by Xerox from an earlier specification called Alohanet (for the Palo Alto Research Center Aloha network) and then developed further by Xerox, DEC, and Intel. An Ethernet LAN typically uses coaxial cable or special grades of twisted pair wires. Ethernet is also used in wireless LANs. The most commonly installed Ethernet systems are called 10BASE-T and provide transmission speeds up to 10 Mbps. Devices are connected to the cable and compete for access using a Carrier Sense Multiple Access with Collision Detection (CSMA/CD ) protocol. 6. What are the functions of MAC and LLC? The standard has sub-divided the Data Link Layer into two parts, viz: LLC (Link Layer Control) and MAC (Media Access Control) Layer LLC: Flow control, error control, and part of the framing duties are all brought together in this LLC sub-layer. Framing is also performed in the MAC sub-layer. The LLC provides a single data link control protocol for all IEEE LANs. MAC: In IEEE 802, there is a sub-layer called Media Access Control (MAC) that defines the specific access method for each LAN. For example: It defines CSMA/CD as the media access method for Ethernet LANs. It defines the token passing method for Token Ring and Token Bus LANs. 7. What are the access methods used by wireless LANs? The access methods used by wireless LANs are: i. Frequency Hopping Spread Spectrum (FHSS) ii. Direct sequence Spread Spectrum (DSSS) 8. What are the functions of bridges? A network bridge, also known as a layer 2 switch, is a hardware device used to create a connection between two separate computer networks or to divide one network into two. Both networks usually use the same protocol; Ethernet is an example of a protocol. The principal function of a network bridge is to forward data based on the MAC address of the sending and receiving devices. This operation helps to eliminate what are known as collision domains. 9. What is the size of Ethernet address? The size of the Ethernet address is 48 bits or 6 bytes. 10. What is the advantage of FDDI over a basic Token Ring?

3 Similarities - FDDI uses a rotating ring setup in the same way as the token ring protocol. - FDDI's ring operation is basically very similar to the Token Ring early release operation in the way that tokens are passed on the network. Differences - As opposed to Token Ring's single ring, FDDI, uses two to achieve better results and less chance of failure. - In a basic Token Ring network, at any instant there is a single active ring monitor which supplies the master clock for the ring, whereas in FDDI this approach isn't ideal because of the high data rates. Instead, each ring interface has its own local clock, and outgoing data is transmitted using this clock. - Unlike the basic Token Ring, which is based on the use of priority and reservation bits, the priority operation of the FDDI ring uses a principle that is based on a parameter known as the Token Rotation Time, or TRT. - FDDI uses a timed token protocol where Token Ring uses priority/reservation token access, leading to differences in frame format and how station traffic is handled Comparison Chart Protocol Data Rate Segment Length Media Rings FDDI 100 Unlimited Optical Fiber 2 IEEE / Not Specified 1 IBM Token Ring 4/ Twisted Pair 1

4 Part B (16 Marks) 1. Explain the physical properties of Ethernet (IEEE 802.3) with necessary diagrams. ETHERNET (IEEE 802.3) Digital Equipment and Intel Corporation joined Xerox to define a 10-Mbps Ethernet standard in It then formed the basis for IEEE standard and its features are: Ethernet uses the Manchester encoding scheme Ethernet is limited to supporting a maximum of 1024 hosts. Ethernet has a total reach of only 2500 m (with the use of repeaters) Ethernets works best under lightly loaded conditions (less than 30%). Easy to administer and maintain Ethernet has evolved to Fast Ethernet (100 Mbps), Gigabit Ethernet (1 Gbps) and Ten- Gigabit Ethernet (10 Gbps). Physical Properties All standard implementations use digital signaling (baseband) at 10 Mbps The implementations of Standard Ethernet are 10Base5, 10Base2, 10Base-T & 10Base-F 10Base5: Thick Ethernet Thick Ethernet uses bus topology with an external transceiver connected via a tap to a thick coaxial cable. The transceiver is responsible for transmitting, receiving, and detecting collisions. Collision occurs only in the coaxial cable. The maximum length of the cable must not exceed 500m. 10Base2: Thin Ethernet Thin Ethernet also uses bus topology, but the cable is much thinner and more flexible. The transceiver is part of the network interface card (NIC). Thin coaxial cable is less expensive The length of each segment should not exceed 185m. 10Base-T: Twisted-Pair Ethernet 10Base-T uses a star topology. The stations are connected to a hub via two pairs of twisted cable. The maximum length of the twisted cable is 100m Any collision happens in the hub only. 10Base-F: Fiber Ethernet 10Base-F uses star topology to connect stations to a hub. The stations are connected to the hub using two fiber-optic cables with a maximum length of 2000m. Frame Format

5 Preamble contains 7 bytes of alternating 0s and 1s that alerts the receiving system and enables it to synchronize its input timing. Start frame delimiter the second field ( ) signals the beginning of the frame. Destination address the 6 bytes contains the physical address of the destination station or stations to receive the packet. Source address the 6 bytes contains the physical address of the sender of the packet. Length/type it either represents length or type of the upper layer protocol. Minimum frame length is 64 bytes. This restriction is required for operation of CSMA/CD. Data this field carries data encapsulated from the upper-layer protocols. It is a minimum of 46 and a maximum of 1500 bytes. CRC the last field contains error detection information (CRC-32). Addressing Each station on Ethernet network has its own network interface card (NIC). The NIC provides the station a globally unique with a 6-byte physical address (shown in hex, separated by colon). Each manufacturer is given a unique prefix (3 bytes). If the least significant bit of the first byte in a destination address is 0, the address is unicast; otherwise, it is multicast. In broadcast address, all bits are 1s. Receiver Algorithm Each frame transmitted on an Ethernet is received by every adaptor connected to that Ethernet. Ethernet receives broadcast frames, frames addressed to it and multicast frames if it belongs to that group. Otherwise frames are discarded Receives all frames, if it runs in promiscuous mode. Ethernet does not provide any mechanism for acknowledging received frames, making it as an unreliable medium. Transmitter Algorithm Ethernet follows Carrier Sense Multiple Access with Collision Detect (CSMA/CD) Ethernet is said to be a 1-persistent protocol. When the adaptor has a frame to send: o If line is idle, it transmits the frame immediately. o If line is busy, it waits for the line to go idle and then transmits immediately. It is possible for two (or more) adaptors to begin transmitting at the same time. o In such case, the frames collide o They transmit a 32-bit jamming sequence and then stop the transmission. o Retransmits after a back-off procedure 2. Explain in detail about Token Ring and its frame format.

6 TOKEN RING (IEEE 802.5) Developed by IBM and later became standard A token ring network consists of a set of nodes connected in a ring Data flow is unidirectional The token ring technique is based on the use of a small frame (24 bits) called a token that circulates around the ring. A station can transmit data only if it has a token Each node along the ring simply forwards the frame, with the destination node alone saving a copy Physical Properties In a ring topology, any link or node failure would render the whole network useless This problem is addressed by connecting each station into the ring using an electromechanical relay. o When the station is healthy, relay is open and the station is included in the ring. o If the station goes down, relay closes and the ring bypasses the station. Relays packed into a single box are known as a multi-station access unit (MSAU). MSAUs makes it easy to add stations to and remove stations from the network The data rate is 4 / 16 Mbps and uses differential Manchester encoding. Max. number of stations is 250 per ring

7 Medium Access Control The network adaptor contains a receiver, a transmitter, and one or more bits of data storage between them As the token circulates around the ring, any station that has data to send, seizes the token by changing one bit in the token, which transforms the token into a start-of-frame sequence for a data frame Once a station has the token, it is allowed to send one or more packets. Each transmitted packet contains the destination address of the intended receiver As the packet flows past each node on the ring, each node looks inside the packet to see if it is the intended recipient. If so, it copies the packet into a buffer as it flows through the network adaptor The sending station has the responsibility of removing the packet from the ring. It inserts a new token on the ring after the frame reaches it back Token Holding Time (THT) specifies how long a given node is allowed to hold the token o Allowing a node to send as much as it wants will result in better utilization of the ring. This strategy will fail if multiple nodes have data to send o The default THT is 10 ms The token rotation time (TRT) is estimated as o TRT ActiveNodes THT + RingLatency o Active nodes is number of nodes that wish to transmit data o Ring latency refers to time taken for token to circulate the ring when no station wishes to transmit data Reliable delivery is provided using 2 bits in the packet trailer, the A and C bits. o The intended recipient sets the A bit when the frame comes to it o It sets the C bit after copying the frame. o Destination station nonexistent or not active (A = 0, C = 0) o Destination station exists but frame not copied (A = 1, C = 0) o Frame received (A = 1, C = 1) The token contains a 3-bit priority field o Each device that wants to send a packet assigns a priority to that packet o The device can seize the token only if the packet s priority the token s priority o A station can change the token priority to its priority if it has higher priority. By doing so, it can seize the token. o When it releases the token, the priority is reset to its original value. Monitor Token rings have one of the stations designated as a monitor The monitor s job is to ensure the health of the ring A monitor periodically announces its presence with a special control message If a station fails that message for some period of time, it assumes that the monitor has failed and will try to become the monitor o It transmits a claim token frame o If that token circulates back to itself, then the station becomes monitor o If more than one station competes to become monitor, then highest address wins Monitor ensures that either token circulates in the ring or is held by some station To detect missing token, the monitor watches for a passing token within the interval NumStations THT + RingLatency. If it fails to see, it inserts a new token

8 The monitor also checks for corrupted or orphaned frames. o The monitor sets a header bit in the data frame when it passes through once. o When monitor sees the header bit in a frame, it indicates orphaned frame and is removed from the ring Detection of dead stations is done by sending a beacon frame to the suspect destination. Based on how far this frame gets, the status of the ring can be established Frame Format Start delimiter contains Manchester codes that indicates start of frame Access control includes 3 priority bits (P) and 3 reservation bits (R). The T bit indicates whether the frame is token or data. M is monitor bit. Frame control is a demux key that identifies the higher-layer protocol Dest addr and Src addr 6 byte source and destination address Checksum is a 32-bit CRC End delimiter same as start delimiter Frame status Contains the address recognized (A) and frame-copied (C) bits The token frame is a 3-byte frame containing the first three fields of the general frame format 3. Discuss the various aspects of FDDI in details with neat sketch. FIBER DISTRIBUTED DATA INTERFACE (FDDI) Physical Properties FDDI network consists of two independent rings that transmit data in opposite directions The second ring is used only if the primary fails FDDI network is able to tolerate a break in the ring or a station failure. Normal operation Failure of primary ring FDDI allows station to connect to the network by means of a single cable. Such stations are known as single attachment station (SAS) Dual connected stations are called dual attachment stations (DAS) A concentrator is used to attach several SASs to the dual ring If an SAS fails, the concentrator detects and isolates it, thereby keeps the ring connected

9 The buffer can vary for each station in the range 9 80 bits FDDI is a 100-Mbps network with a 10 ns bit time A FDDI network can have at most 500 stations with a maximum distance of 2 km between any pair of stations. The network is limited to a total of 200 km of fiber (100km due to dual nature of ring) The physical medium is not mandated to be fiber, could be coax or twisted pair. FDDI uses 4B/5B encoding Timed Token Algorithm The THT for each node is defined according to the network. All nodes agree to a target token rotation time (TTRT) Each node measures the time between successive arrivals of token, known as measured TRT. If a node's measured TRT > TTRT, then the token is late, and the node does not transmit any data. If a node's measured TRT < TTRT, then the token is early, and the node is allowed to hold the token for the duration TRT - TTRT. It transmits data, if it can send a full frame. FDDI defines two classes of traffic namely synchronous and asynchronous: o When a node receives a token, it is allowed to send synchronous (delay sensitive) data, even if the token is late. o The total amount of synchronous data that can be sent during one token rotation is also bounded by TTRT o A node can send asynchronous traffic only when the token is early. o When a node has both types of data to transmit, it transmits asynchronous traffic up to a TTRT time and then synchronous traffic for another TTRT o Thus a single rotation of token can take a maximum 2 TTRT Token Maintenance All nodes on an FDDI ring monitor the ring to be sure that the token has not been lost. The maximum idle time that a node should experience must not be more than 2.5 ms. Each node sets a timer to 2.5 ms after it notices a data frame or token frame. If this timer expires, the node suspects that something has gone wrong and transmits a claim frame. The claim frame contains the node's TTRT estimate based on requirements of application running on it. When a node receives a claim frame o If the TTRT bid in the frame is less than its own, then it's estimate is changed to TTRT bid and forwards the claim frame.

10 o If the bid TTRT is greater than the node s estimate, then claim frame is removed and the node inserts its own claim frame. o If the TTRT bid and the node's estimate are the same, then one with the higher address wins. If this claim frame makes it all the way around the ring, then o The node removes the claim frame and inserts a new token on the ring o The node's estimate becomes TTRT. Frame Format Starting delimiter Indicates start of frame. It contains uses 4B/5B control symbol Frame control indicates whether the traffic is synchronous or asynchronous Destination address The ring may contain a mixture of 16 and 48-bit address Source address Specifies the station that sent the frame Frame check sequence A 32-bit cyclic redundancy check Ending delimiter marks end of frame. Same as start delimiter Frame status Contains the error detected (E), address recognized (A), and frame copied (F) indicators The token frame format is 4. Write in briefly about CSMA. Carrier sense multiple access (CSMA) is a probabilistic media access control (MAC) protocol in which a node verifies the absence of other traffic before transmitting on a shared transmission medium, such as an electrical bus, or a band of the electromagnetic spectrum. Carrier sense means that a transmitter uses feedback from a receiver to determine whether another transmission is in progress before initiating a transmission. That is, it tries to detect the presence of a carrier wave from another station before attempting to transmit. If a carrier is sensed, the station waits for the transmission in progress to finish before initiating its own transmission. In other words, CSMA is based on the principle "sense before transmit" or "listen before talk". Multiple access means that multiple stations send and receive on the medium. Transmissions by one node are generally received by all other stations connected to the medium. CSMA with collision detection CSMA/CD is used to improve CSMA performance by terminating transmission as soon as a collision is detected, thus shortening the time required before a retry can be attempted. CSMA with collision avoidance In CSMA/CA collision avoidance is used to improve the performance of CSMA by attempting to be less "greedy" on the channel. If the channel is sensed busy before transmission then the transmission is deferred for a "random" interval. This reduces the probability of collisions on the channel.

11 CSMA access modes[edit source editbeta] 1-persistent When the sender (station) is ready to transmit data, it checks if the transmission medium is busy. If so, it then senses the medium continually until it becomes idle, and then it transmits the message (a frame). In case of a collision, the sender waits for a randomperiod of time and attempts to transmit again. 1-persistent CSMA is used in CSMA/CD systems including Ethernet. P-persistent This is a sort of trade-off between 1 and non-persistent CSMA access modes. When the sender is ready to send data, it checks continually if the medium is busy. If the medium becomes idle, the sender transmits a frame with a probability p. If the station chooses not to transmit (the probability of this event is 1-p), the sender waits until the next available time slot and transmits again with the same probability p. This process repeats until the frame is sent or some other sender starts transmitting. In the latter case the sender monitors the channel, and when idle, transmits with a probability p, and so on. p-persistent CSMA is used in CSMA/CA systems including Wi-Fi and other packet radio systems. Non-persistent Non persistent CSMA is less aggressive compared to P persistent protocol. In this protocol, before sending the data, the station senses the channel and if the channel is idle it starts transmitting the data. But if the channel is busy, the station does not continuously sense it but instead of that it waits for random amount of time and repeats the algorithm. Here the algorithm leads to better channel utilization but also results in longer delay compared to 1 persistent. O-persistent Each station is assigned a transmission order by a supervisor station. When medium goes idle, stations wait for their time slot in accordance with their assigned transmission order. The station assigned to transmit first transmits immediately. The station assigned to transmit second waits one time slot (but by that time the first station has already started transmitting). Stations monitor the medium for transmissions from other stations and update their assigned order with each detected transmission (i.e. they move one position closer to the front of the queue). [2] O-persistent CSMA is used by CobraNet, LonWorks and the controller area network. 5. How does a bridge learn on which port a host resides? Explain with an example. Static Bridges Bridges during setup is configured with a forwarding table manually by the administrator. When a frame arrives, the bridge checks the table The outgoing port for the destination is obtained and the frame is sent on that port. The disadvantages are: o Addition/deletion of stations must be updated manually Learning Bridges

12 Bridges can build the forwarding table gradually by learning from frame movements. The bridge inspects both the destination and the source addresses. o The destination address is used for the forwarding decision o The source address is used for adding entries to the table and for updating The table is empty when the bridge boots up The bridge inspects both the destination and the source addresses. o The destination address is used for the forwarding decision o The source address is used for adding entries to the table and for updating The table is empty when the bridge boots up. When station A sends a frame to station D, o The bridge has no entry for either D or A o From source address, the bridge learns that station A must be located on the LAN connected to port 1, i.e., frames destined for A must be sent out through port 1. o The bridge adds this entry to its table o The frame is flooded on the other two ports When station E sends a frame to station A o The bridge has an entry for A, so it forwards the frame only to port 1

13 o There is no flooding o It uses the source address of the frame, E, to add a second entry to the table When station B sends a frame to C o The bridge has no entry for C o It floods the network and adds one more entry to the table If the network id of source and destination IP address matches then there is no forwarding but an entry is added for the destination host. The process of learning continues as the bridge forwards frames and optimizes its forwarding decision When the table becomes full, new addresses are not added The table is discarded periodically and rebuilt 6. Explain in detail about wireless LAN. A wireless local area network (WLAN) links two or more devices using some wireless distribution method (typically spread-spectrum or OFDM radio), and usually providing a connection through an access point to the wider Internet. This gives users the mobility to move around within a local coverage area and still be connected to the network. Most modern WLANs are based on IEEE standards, marketed under the Wi-Fi brand name. WLANs were once called LAWNs (for local area wireless network) by the Department of Defense. Wireless LANs have become popular in the home due to ease of installation, and in commercial complexes offering wireless access to their customers; often for free. New York City, for instance, has begun a pilot program to provide city workers in all five boroughs of the city with wireless Internet access. Architecture Stations All components that can connect into a wireless medium in a network are referred to as stations. All stations are equipped withwireless network interface controllers (WNICs). Wireless stations fall into one of two categories: access points, and clients. Access points (APs), normally routers, are base stations for the wireless network. They transmit and receive radio frequencies for wireless enabled devices to communicate with. Wireless clients can be mobile devices such as laptops, personal digital assistants, IP phonesand other smartphones, or fixed devices such as desktops and workstations that are equipped with a wireless network interface. Basic service set The basic service set (BSS) is a set of all stations that can communicate with each other. Every BSS has an identification (ID) called the BSSID, which is the MAC address of the access point servicing the BSS. There are two types of BSS: Independent BSS (also referred to as IBSS), and infrastructure BSS. An independent BSS (IBSS) is an ad hoc network that contains no access points, which means they can not connect to any other basic service set. Extended service set

14 An extended service set (ESS) is a set of connected BSSs. Access points in an ESS are connected by a distribution system. Each ESS has an ID called the SSID which is a 32-byte (maximum) character string. Distribution system A distribution system (DS) connects access points in an extended service set. The concept of a DS can be used to increase network coverage through roaming between cells. DS can be wired or wireless. Current wireless distribution systems are mostly based on WDS or MESH protocols, though other systems are in use.