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Computer networks lan

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Lan architecture
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Computer networks lan

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Local Area Networks: LAN protocol architecture (IEEE - 802 reference model),
Topologies - Bus, tree, ring and star. Logic link control. Medium access control:-Random
access- Aloha, CSMA, CSMA/CD, Exponential Back off algorithm ,CSMA/CA,
controlled access-Reservation, Polling, Token Passing.
LAN systems: Traditional Ethernet:-MAC sub layer access method(CSMA/CD) ,IEEE
802.3 MAC frame, Addressing physical layer, Physical Layer, Physical Layer,
Implementation, Bridged Ethernet, Switched Ethernet, Full-Duplex Ethernet.
FAST ETHERNET:- Mac Sublayer, Physical Layer, Physical Layer Implementation,
GIGABIT ETHERNET:- MAC Sublayer, Physical Layer, Physical Layer
Implementation.
LAN Connecting Devices-Repeaters, Hubs, Bridges:- filtering, Transparent Bridges,
Spaning Tree Algorithm.Two-Layer Switch.
Backbone Networks- Bus Backbone, Star Backbone, Connecting Remote LANs.

Local Area Networks: LAN protocol architecture (IEEE - 802 reference model),
Topologies - Bus, tree, ring and star. Logic link control. Medium access control:-Random
access- Aloha, CSMA, CSMA/CD, Exponential Back off algorithm ,CSMA/CA,
controlled access-Reservation, Polling, Token Passing.
LAN systems: Traditional Ethernet:-MAC sub layer access method(CSMA/CD) ,IEEE
802.3 MAC frame, Addressing physical layer, Physical Layer, Physical Layer,
Implementation, Bridged Ethernet, Switched Ethernet, Full-Duplex Ethernet.
FAST ETHERNET:- Mac Sublayer, Physical Layer, Physical Layer Implementation,
GIGABIT ETHERNET:- MAC Sublayer, Physical Layer, Physical Layer
Implementation.
LAN Connecting Devices-Repeaters, Hubs, Bridges:- filtering, Transparent Bridges,
Spaning Tree Algorithm.Two-Layer Switch.
Backbone Networks- Bus Backbone, Star Backbone, Connecting Remote LANs.

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Computer networks lan

  1. 1. Computer Networks -LAN Deepak John Department Of MCA,SJCET-Pala
  2. 2. Local Area Network
  3. 3. Attributes Of A LAN i. Transmission technology ii. Signalling methods iii. Transmission medium iv. Access Control v. Topologies
  4. 4. i. Transmission Technology Two Transmission Technologies used by LANs are : a. Broad cast: A single communication channel is shared by all the machines on the network b. Point to Point: consists of many connections between individual pairs of machines.Packets have to follow multiple routes of different lengths ii. Signalling Methods  Two signaling methods used by LANs are : a. Broadband: The bandwidth of the transmission medium is subdivided by frequency to form two or more sub-channels b. Base-Band: only one signal is transmitted at any point in time
  5. 5. iii. Transmission Medium  Transmission medium can be classified into 4 from physical cable connection to wireless systems. a. Twisted Pair: consists of two individual insulated copper wires physically twisted together to minimize unwanted electromagnetic signals from interfering with or radiating from the pair b. Coaxial cables: consists of a central copper core surrounded by a layer of insulating material. c. Optical fiber cables: contains glass fiber and signals are transmitted in the form of light pulses . d.Wireless transmission
  6. 6. iv. Access control  represents how the devices get permission to communicate on the network.  Some of the access methods primarily employed in local area networks are :  Polling  Token-passing  Slotted ring  CSMA/CD  Switching
  7. 7. v. Topology  refers to the way in which the end points, or stations, attached to the network are interconnected. The common topologies for LANs are: i. Bus ii. Tree iii. Ring iv. Star
  8. 8. LAN PROTOCOL ARCHITECTURE  The LAN protocol architecture consists of layering of protocols that contribute to the basic functions of a LAN  The standardized LAN protocol architecture encompasses 3 layers i. Physical layer ii. Medium Access control layer (MAC) iii.Logical Link control
  9. 9. IEEE 802 Reference Model  The layers of OSI Reference Model can be classified as Network Support Layers and User support layers  LAN protocols are concerned with the network support layers, mainly Physical and Data Link Layer  IEEE 802 Reference Model is a modified reference model suitable for LANs built by IEEE.  It corresponds to the lower 2 layers of OSI reference model  Here the data link layer has been divided into two sub layers: logical link control (LLC) and media access control (MAC)
  10. 10. Physical layer  includes functions such as : i. Encoding/decoding of signals ii. Specification of the transmission medium and the topology Logical Link Layer Acts as the interface between the Network layer and the MAC sub-layer  Functions: i. Error Control ii. Flow Control iii. Sequencing & User Addressing Functions
  11. 11.  LLC standard is common to all LAN’s and offers three types of services for controlling the exchange of data between two users. i. Unacknowledged connectionless service: does not involve any of the flow- and error control mechanisms and delivery of data is not guaranteed. ii. Connection-mode service:A logical connection is set up between two users exchanging data, and flow control and error control are provided. iii. Acknowledged connectionless service:datagram are to be acknowledged, but no prior logical connection is set up.
  12. 12. LLC Protocol Data Unit (PDU)  which contains 4 fields  Destination Service Access Point (DSAP), Source Service Access Point (SSAP) : address fields which specify the destination and source users of LLC.These identify the network protocol entities which use the link layer service  LLC control field: describes type of PDU(U,I and S) and includes other information such as sequencing and flow control information
  13. 13. Medium Access Control  It is lower sub-layer of data link layer closer to the physical layer  Key parameters of MAC technique is where and how  a.Where: refers to whether control is exercised in a centralized or distributed fashion.  Central: A controller is designated that has the authority to grant access to the network.  Distributed:The stations collectively perform a MAC function to determine the order in which stations transmit.  b. How: is constrained by the topology and competing factors like cost, performance, and complexity.
  14. 14. Basic functions of MAC Sub Layer are: i. Media Access Control ii. Error Detection iii. Station Addressing  Defines two medium access control techniques specific for each LAN. i. Synchronous techniques: a specific capacity is dedicated to a connection. ii. Asynchronous: allocate capacity in an asynchronous (dynamic) fashion, more or less in response to immediate demand.
  15. 15. Asynchronous techniques can be classified into three: i. Round Robin ii. Reservation iii. Contention i.Round Robin  Each station in turn is given the opportunity to transmit.  When it is finished, relinquishes its turn, and the right to transmit passes to the next station in logical sequence.  Control of sequence may be centralized or distributed.  Efficient when many stations have data to transmit over an extended period of time
  16. 16. ii. Reservation Time on the medium is divided into slots. A station wishing to transmit reserves future slots for an extended or even an indefinite period.  Reservations may be made in a centralized or distributed fashion. iii. Contention  Useful for bursty type traffic  No control is exercised to determine whose turn it is.  Stations send data by taking risk of collision (with others’ packets).however they understand collisions by listening to the channel, so that they can retransmit.  Efficient under light or moderate load,bad under heavy load  Round-Robin and Contention techniques are the most commonly used in LANs.
  17. 17. MAC- Frame Format  The MAC layer is responsible for performing functions related to medium access and for transmitting the data.  MAC Control: contains any protocol control information needed for the functioning of the MAC protocol  Destination MAC Address: The destination physical attachment point on the LAN for this frame.  Source MAC Address: The source physical attachment point on the LAN for this frame.  LLC PDU: The LLC data from the next higher layer.  CRC: The Cyclic Redundancy Check field
  18. 18. Standard Title Description 802.1 High-level Interface Specification of standards for LAN architecture, interconnection, management 802.2 Logical Link Control Specification of standards for the LLC layer 802.3 Ethernet(CSMA/CD) Specification of standards for CSMA/CD architectures 802.4 Token Bus Specification of standards for token bus architectures 802.5 Token Ring Specification of standards for token ring architectures 802.6 Metropolitan Area Networks Specification of standards for MANs 802.7 Broadband Technical Advisory Group Provision of guidance to other groups working on broadband LANs 802.8 Fiber Optic Technical P i i f id h ki Advisory Group Provision of guidance to other groups working on fiber optic-based LANs 802.9 Integrated Data and Specification of standards for interfaces to ISDN Voice Networks
  19. 19. Multiple Access Protocols  A MAC (Media Access Control) protocol is a set of rules to control access to a shared communication medium among various users.
  20. 20. RANDOMACCESS  In random access or contention methods, no station is superior to another station and none is assigned the control over another.  Two features give this method its name:  First, there is no scheduled time for a station to transmit.  Second, no rules specify which station should send next. Stations compete with one another to access the medium
  21. 21. ALOHA  When the user simply transmits a frame, there are chances of collision  The user could simply retransmit, but this would not help, other user involved in the collision will also retransmit, resulting in another collision  One way to avoid this is to wait a random amount of time before retransmitting which forms the basis of ALOHA  There are two versions of ALOHA: pure and slotted.
  22. 22. Pure ALOHA The system is working as follows: 1. let users transmit whenever they have data to be sent. 2. collisions will occur. 3. using a feedback mechanism to know about the status of frame. 4. the collided frames will be destroyed. 5. retransmit the destroyed frame.  the number of collisions rise rapidly with increased load.  After a maximum number of retransmission attempts Kmax station must give up and try later.
  23. 23. Frames in a pure ALOHA network
  24. 24. Suppose L: the average frame length, R: rate, X=L/R: frame time 1. Transmit a frame at t=t0 (and finish transmission of the frame at t0+X ) 2. If ACK does not come after t0+X+2tprop or detect collision, wait for random time: B 3. Retransmit the frame at t0+X+2tprop+B First transmission Retransmission t t0-X t0 t0+X t +X+2t t0+X+2tprop +B 0+2tprop t0 X Vulnerable Time-out Backoff period: B Retransmission period if necessary
  25. 25. Vulnerable period: t0-X to t0+X, if any other frames are transmitted during the period, the collision will occur. Therefore the probability of a successful transmission is the probability that there is no additional transmissions in the vulnerable period. Therefore, if a station generates only one frame in this vulnerable time (and no other stations generate a frame during this time), the frame will reach its destination successfully.  Max channel utilization is 18% - very bad
  26. 26. Slotted ALOHA  Slotted ALOHA was invented to improve the efficiency of pure ALOHA.  Here, we divide the time into slots and force the station to send only at the beginning of the time slot  If a station misses this moment, it must wait until the beginning of the next time slot.  There is still the possibility of collision if two stations try to send at the beginning of the same time slot
  27. 27. First transmission Retransmission t t0 -X t0 t0+X+2tprop t0+X t0+X+2tprop +B Time-out Backoff period: B Retransmission if necessary Vulnerable period: t0-X to t0 , i.e., X seconds long  Max channel utilization is 37%,doubles Normal ALOHA, but still low
  28. 28. Carrier Sense Multiple Access(CSMA)  A station wishing to transmit first listens to the medium if another transmission is in progress (carrier sense).  If the medium is in use, station waits  if the medium is idle, station may transmit  Collision probability depends on the propagation delay  Longer propagation delay, worse the utilization  Collisions can occur only when more than one user begins transmitting within the period of propagation delay.  The vulnerable time for CSMA is the propagation time .  If collision occurs  Wait random time and retransmit
  29. 29.  Suppose tprop is propagation delay from one extreme end to the other extreme end of the medium. When transmission is going on, a station Station A begins g g , can listen to the medium and detect it. Vulnerable period = tprop A g transmission at t=0 sense sense Station A captures channel A at t=tprop CSMA random access scheme sense sense  After tprop, A’s transmission will arrive the other end; every station will hear it and refrain from the transmission, so A captures the medium and can finish its transmission.
  30. 30.  Following are some versions of CSMA protocol Based on how to do when medium is busy • 1-Persistent CSMA • Non-Persistent CSMA • p-Persistent CSMA
  31. 31.  1-persistent CSMA  if the medium is idle, transmit.  if the medium is busy, continue to listen until the channel is sensed idle; then transmit immediately.  If more than one station are Sense carrier yes Busy? sensing, then they will begin transmission the same time when no Send the frame h lb idl lli i channel becomes idle, so collision. At this time, each station wait for a random time and then re-senses with probability 1 time, re the channel again.  Problem with 1-persistent CSMA is “high collision rate”.
  32. 32.  Nonpersistent  If the channel is busy the station does not continually check it for detecting the end of ongoing transmission. It waits for a random time then checks the channel. If the channel is idle, sends the frame. Sense carrier Wait random time… yes Busy? no Send the frame
  33. 33.  p-persistent CSMA  if medium is idle, station transmits with a probability p. otherwise it defers to the next slot with probability 1-p. the process repeat until either the frame has been transmitted or another station has begun transmission. Sense carrier Busy? yes no Send the frame with p probability p yp
  34. 34. Behaviour of three persistence methods
  35. 35. CSMA/CD (CSMA with Collision Detection)  Drawback of CSMA: when two frames collide, the medium remains unusable for the duration of transmission of both damaged frames.  CSMA/CD:  1. if the medium is idle, transmit; otherwise, go to step 2.  2. if the medium is busy, continue to listen until the channel is idle, then transmit.  3. if a collision is detected during transmission, transmit a brief jamming signal  4. after transmitting a jamming signal, wait a random amount of time, then attempt to transmit.
  36. 36. CSMA/CD efficiency  tprop = max prop between 2 nodes in LAN  ttrans = time to transmit max-size frame  Efficiency = 1/(1+5 * tprop / ttrans)  For 10 Mbit Ethernet, tprop = 51.2 us, ttrans = 1.2 ms  Efficiency is 82.6%!  Much better than ALOHA,  simple, and cheap  Efficiency goes to 1 as tprop goes to 0  Goes to 1 as ttrans goes to infinity
  37. 37. Exponential Back off Algorithm  used by a transmitting station to determine how long to wait following a collision before attempting to retransmit the frame  Each station generate a random number that falls within a specified range of values. which determines the length of time it must wait before testing the carrier. The range of values increases exponentially after each failed retransmission.  After c collisions, the range is between 0 and 2c – 1,It then waits that number of slot times before attempting retransmission.  If repeated collisions occur, the range continues to expand ,until after 10 attempts when it reaches 1023. After that the range of values stays fixed .If a station is unsuccessful in transmitting after 16 attempts, then gives up if cannot transmit  low delay with small amount of waiting stations  large delay with large amount of waiting stations
  38. 38. Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)  In CSMA/CA once the channel is clear, it again waits for an additional time period before performing the transmission.  Before sending a frame, source senses the medium  Backoff until the channel is idle.  After the channel is found idle, the station waits for a period of time called the Distributed Inter Frame Space (DIFS); then the station sends a control frame called Request To Send (RTS).  After receiving RTS, the destination waits for a period called Short Inter Frame Space (SIFS), the destination station sends a control frame, called Clear To Send (CTS) to source. This control frame indicates that the destination station is ready to receive data.  Source sends data after waiting for SIFS  Destination sends ACK after waiting for SIFS.
  39. 39.  RTS frame indicates the duration of time that the source needs to occupy the channel.  Stations that are affected by this transmission create a timer called a Network Allocation Vector (NAV) that shows how much time must pass before these stations are allowed to check the channel for idleness.
  40. 40. CONTROLLED ACCESS  In controlled access, the stations consult one another to find which station has the right to send  A station cannot send unless it has been authorized by other stations.  Three popular control access methods are:  Polling  Reservation  Token Passing
  41. 41. Polling  Stations take turns accessing the medium  Two models: Centralized and distributed polling  Centralized polling  One device is assigned as primary station and the others as secondary stations  All data exchanges are done through the primary.  If the primary wants to receive data, it asks the secondary's if they have anything to send; this is called poll function.  If the primary wants to send data, it tells the secondary to get ready to receive; this is called select function  Polling can be done in order (Round-Robin) or based on predetermined order
  42. 42. Primary is sending to Secondary Secondary is sending to Primary  ACK is the acknowledgment of the secondary's ready status.  Secondary responds either with a NAK frame if it has nothing to send or with data (in the form of a data frame) if it does
  43. 43.  Distributed polling No primary and secondary Stations have a known polling order list which is made based on some protocol. station with the highest priority will have the access right first, then it passes the access right to the next station (it will send a pulling message to the next station in the pulling list), which will passes the access right to the following next station,…
  44. 44. Reservation  A station needs to make a reservation before sending data.  Transmissions are organized into variable length cycles.  Each cycle begins with a reservation interval that consists of (N) minislots. One minislot for each of the N stations.  When a station needs to send a data frame, it makes a reservation in its own minislot.  By listening to the reservation interval, every station knows which stations will transfer frames, and in which order.  The stations that made reservations can send their data frames after the reservation frame.
  45. 45. Token Passing  Here the stations in a network are organized in a logical ring.  In this method, a special packet called a token circulates through the ring.  token gives the station the right to access the channel and send its data  When a station receives the token and has no data to send, it just passes the data to the next station. token
  46. 46. LAN SYSTEMS
  47. 47. Ethernet  Ethernet is a dominant physical and data link layer technology for local area networks (LANs).  It is a bus based broadcast network using co-axial cable operating at 10 or 100 Mbps.  It uses a control method called Carrier Sense Multiple Access/Collision Detection (CSMA/CD) to transmit data Ethernet Evolution Standard Ethernet Fast Ethernet Gigabit Ethernet Ten-Gigabit Ethernet 100-Gigabit Ethernet (10 Mbps) (100 Mbps)
  48. 48. IEEE 802.3 MAC Frame
  49. 49.  Preamble: Alternating 0s and 1s; used for synchronizing; 7bytes (56 bits).  Start Frame Delimiter (SFD): 10101011 indicates the start of the frame. Last two bits alerts that the next field is destination address.  Destination Address (DA): 6 bytes (48 bits) physical address of destination station(s)  Source Address (SA): 6 bytes (48 bits) physical address of sender  Length/Type: if less than 1500, it indicates the length of data field. If greater than 1536, it indicates the type of PDU.  Data: 46 to 1500 bytes;  CRC: CRC-32 for error detection
  50. 50. Addressing  Each station on an Ethernet network has its own network interface card( NIC) fits inside and provides a 6-byte physical address Eg:06:01:02:01:2C:4B. First three bytes from left specify the vendor. (Cisco 00-00-0C, 3Com 02-60-8C) and the last 24 bit should be created uniquely by the company
  51. 51. A source address is always a unicast address where as destination address can be unicast, multicast, or broadcast. Unicast: defines one recipient ,second digit from left is even  Multicast: defines a group of recipients ,Second digit from left is odd  Broadcast : defines a group of all stations in the same LAN ,All ones The transmission is left-to-right, byte by byte; however, for each byte, the least significant bit is sent first and the most significant bit is sent last.
  52. 52. Physical Layer Implementation  The Standard Ethernet defines several physical layer implementations; four of the most common, are
  53. 53. Physical Layer Signaling  Uses Manchester encoding.  At the sender, data are converted to a digital signal using the Manchester scheme; at the receiver, the received signal is interpreted as Manchester and decoded into data.  Helps synchronize sender and recvr.
  54. 54. 10Base5: Thick Ethernet  use a bus topology with an external transceiver (transmitter/receiver) connected via a tap to a thick coaxial cable.  10-Mbps transmission speed and 5 represents 500 meters maximum cable segment length.  The transceiver is responsible for transmitting, receiving, and detecting collisions
  55. 55. 10Base2: Thin Ethernet  uses a bus topology, but the cable is much thinner and more flexible.  The transceiver is normally part of the network interface card (NIC), which is installed inside the station.  can transmit 10 Mbps digital signals over coaxial cable.  more cost effective than 10Base5 because thin coaxial cable is less expensive than thick coaxial.  The length of each segment cannot exceed 185 m (close to 200 m).
  56. 56. 10Base-T: Twisted-Pair Ethernet  Uses a physical star topology.  The stations are connected to a hub via two unshielded twisted pair cables; One for transmitting data, and the other for receiving data  Maximum length of the cable segment can be 100 meters.
  57. 57. 10Base-F: Fiber Ethernet  Although there are several types of optical fiber l0-Mbps Ethernet, the most common is called 10Base-F.  It uses a star topology to connect stations to a hub.  The stations are connected to the hub using two fiber-optic cables.
  58. 58. Bridged Ethernet  LAN can be divided using bridges. Bridges have two effects on an Ethernet LAN: • They raise the bandwidth • They separate collision domains  Raising the Bandwidth  each network is independent.  Suppose there are 12 stations. And bandwidth is 10 Mbps.  If we divide the network into 2 networks using bridge, each network has a capacity of 10 Mbps.  The 10 Mbps capacity is shared between 7 stations, 6+1(bridge acts as a station in each segment), not 12 stations.
  59. 59. A network with and without a bridge
  60. 60.  Separating collision domains:  Collisions domains become much smaller and possibility of collision is reduced.  With bridging, lesser number of channels compete for access to the medium.
  61. 61. Switched Ethernet cards,  The heart of the system is a switch containing room for typically 4 to 32 plug-in cards each containing one to eight connectors that allows faster handling of packets.  When a station wants to transmit a frame, it outputs a frame to switch.  Half duplex
  62. 62.  All ports on the same card are wired together to form a local on-card LAN.  Collisions on this on-card LAN are detected and handled using CSMA/CD protocol.  One transmission per card is possible at any instant. All the cards can transmit in parallel.  With this design each card forms its own collision domain.
  63. 63. Full-duplex switched Ethernet Each station is connected to the switch through two links: one to transmit and one to receive. increases the capacity of each domain from 10 to 20 Mbps. no chances of collision, so CSMA/CD is not used
  64. 64. Fast Ethernet  IEEE 802.3 u.  same frame format, media access, and collision detection rules as 10 Mbps Ethernet  data transfer rate of 100 Mb/s .  compatible with Standard Ethernet.
  65. 65. MAC Sub Layer  The only two changes made in the MAC layer are the data rate and the collision domain  A new feature added called Auto negotiation;allows two devices to negotiate the mode or data rate of operation.  For example, a device with a maximum capacity of 10 Mbps can communicate with a device with a 100 Mbps capacity.
  66. 66. Physical Layer  Implementation
  67. 67. Topologies Fast Ethernet Topologies
  68. 68. Gigabit Ethernet  IEEE 802.3z.  All config rations of gigabit Ethernet are point to point configurations point.  Point-to-point, between two computers or one computer – to –switch.  Compatible with 100BASE-T and 10BASE-T MAC Sublayer
  69. 69.  It supports two different modes of medium access : full duplex mode and half duplex mode.  Half duplex is used when computers are connected by a hub. Collision in hub is possible and so CSMA/CD is required.  Full duplex is used when computers are connected by a switch. No collision is there and so CSMA/CD is not used.
  70. 70.  Carrier Extension tells the hardware to add its own padding bits after the normal frame to extend the frame to 512 bytes. RRRRRRRRRRRRR Carrier Extension Frame 512 bytes  Frame Bursting allows a sender to transmit a concatenated sequence of multiple frames in a single transmission. If the total burst is less than 512 bytes, the hardware pads it again. Frame Extension Frame Frame Frame 512 bytes Frame burst
  71. 71. Physical Layer  Topology.
  72. 72. LAN CONNECTING DEVICES
  73. 73. Connecting Devices Networking Devices Internetworking Devices Repeaters Bridges Routers Gateways
  74. 74. Network Connecting Devices
  75. 75. Repeaters  A repeater (or regenerator) is an electronic device that operates on only the physical layer of the OSI model.  A repeater installed on a link receives the signal before it becomes too weak or corrupted, regenerates the original pattern, and puts the refreshed copy back on the link.
  76. 76.  A repeater does not actually connect two LANS; it connects two segments of the same LAN.  A repeater forwards every frame; it has no filtering capability Function of repeater
  77. 77. Hubs Passive Hubs  A passive hub is just a connector which connects the wires coming from different branches Active Hub  A Hub is a multiport repeater. used to create connections between stations in a physical star topology.  Connection to the hub consists of two pairs of twisted pair wire one for transmission and the other for receiving.  it copy the received frame onto all other links
  78. 78. Bridges  Bridges operate in both the physical and the data link layers of the OSI model.  Bridges can divide a large network into smaller segments.  When a frame (or packet) enters a bridge, the bridge not only regenerates the signal but checks the destination address and forwards the new copy only to the segment the address belong.  This is done by a bridge table (forwarding table) that contains entries for the nodes on the LAN  The bridge table is initially empty and filled automatically by learning from frames movements in the network  A bridge runs CSMA/CD before sending a frame onto the channel
  79. 79. Types of Bridges 1. Simple Bridge 2. Multiport Bridge 3. Transparent Bridge Simple Bridge  The address table must be entered manually  Whenever a new station is added or removed, the table must modified.  Installation and maintenance of simple bridges are time-consuming and potentially more. Multiport bridges  Amultiport bridge can be used to connect more than two LANs.
  80. 80. Transparent Bridges  A transparent, or learning, bridge builds its table of station addresses on its own as it performs its bridge functions.  the stations are completely unaware of the bridge’s existence.  A transparent bridge must meet three criteria: 1. Frames must be forwarded from one station to another. 2. The forwarding table is automatically made by learning frame movements in the network. 3. Loops in the system must be prevented.
  81. 81. Learning Bridge
  82. 82. Loop Problem  multiple paths of bridges and local- local-area networks (LANs) exist between any two LANs in the internetwork.  Having more than one transparent bridge between a pair of LAN segments can create loops in the system.  Bridging Loops Can Result in Inaccurate Forwarding and Learning in Transparent Bridging Environments .
  83. 83. Step 1. Station-A sends a frame to Station-B. Both the bridges forward the frame to LAN Y and update the table with the source address of A. Step 2. Now there are two copies of the frame on LAN-Y. The copy sent by Bridge-a is received by Bridge-b and vice versa. As both the bridges have no information about Station B, both will forward the frames to LAN-X. Step 3. Again both the bridges will forward the frames to LAN-Y because of the lack of information of the Station B in their database and again Step-2 will be repeated, and so on. So, the frame will continue to loop around the two LANs indefinitely.  Looping problem Is avoided by using Blocking ports (no frame is send out of these ports).
  84. 84. Spanning Tree Algorithm  IEEE 802.1d  In graph theory, a spanning tree is a graph in which there is no loop  In a bridged LAN, this means creating a topology in which each LAN can be reached from any other LAN through one path only  A LAN can be depicted as a graph, whose nodes are bridges and LAN segments (or cables), and whose edges are the interfaces connecting the bridges to the LAN segments
  85. 85. STEPS  Every bridge has a built-in ID and the bridge with smallest ID is selected as the root bridge  The algorithm tries to find the shortest path (a path with the shortest cost) from the root bridge to every other bridge or LAN  The combination of the shortest paths creates the shortest tree  Based on the spanning tree, we mark the forwarding ports and blocking ports
  86. 86. The forwarding ports are shown as solid lines, whereas the blocked ports are shown as dotted lines. Spanning tree of a network of bridges
  87. 87. Two-Layer Switches  Two-layer switch performs at the physical and data link layers.  is a bridge, with many ports and allows better (faster) performance.  A bridge with many ports may be able to allocate a unique port to each station, with each station on its own independent entity.  It makes a filtering decision based on the MAC address of the frame it received.  It can have a buffer to hold the frames for processing.  More than one station transmitting at a time.  It can have a switching factor that forwards the frames faster.
  88. 88. Types Store-and-forward switch • Accepts frame on input line • Buffers it briefly, • Then routes it to appropriate output line • Delay between sender and receiver Cut-through switch • Takes advantage of destination address appearing at beginning of frame • Switch begins repeating frame onto output line as soon as it recognizes destination address
  89. 89. Routers  Capable of connecting networks of different types  Routers separate networks into different broadcast domains.  They use the “logical address” of packets and routing tables to determine the best path for data delivery.  RIP(Routing Information Protocol) 98
  90. 90. Back Bone Networks  A backbone network allows several LANs to be connected.  In a backbone network, no station is directly connected to the backbone; the stations are part of a LAN, and the backbone connects the LANs.  The backbone is itself a LAN that uses a LAN protocol such as Ethernet and each connection to the backbone is itself another LAN.  The two most common architectures are the bus backbone and the star backbone.
  91. 91. Bus Backbone  In a bus backbone, the topology of the backbone is a bus.  Bus backbones are normally used as a distribution backbone to connect different buildings in an organization.
  92. 92. Star Backbone  the topology of the backbone is a star; the backbone is just a switch.  mostly used as a distribution backbone inside a building.
  93. 93. Connecting Remote LANs  Consider a situation where 2 LANS are located at some distance.  It is possible to connect these LANS by taking a full duplex leased connection from telephone network operator.  Use 2 bridges, one at each end of leased connection  These bridges are called remote bridges  Useful when a company has several offices with LANs and needs to connect them.  These bridges have ports that have data rate and signal levels compatible to telephone network standard.  The bridges establish a data link connection through the leased circuit and then carry out bridge operation

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