Packet Transmission

1. Packet Transmission Computers use data grouped into packets for transmission Local Area Networks Wide Area Networks Addressing and Routing

2. Concepts of Packets Computer networks divide data into small blocks called packets Packets are send individually Often called packet networks and packet switching networks Motivation for using packets Sender and receiver needs to coordinate transmission to ensure that data arrives correctly Helps determine which blocks arrive intact and which do not Computers often share underlying connections and hardware Packet switching helps ensure fairness to access

3. Shared Resources The first networks A 5 MB file at 56 Kbps will take 12 min to transfer from A to D B & C must wait Packet networks Divide data into packets of 1000 bytes each A sends a packet to D taking only 143 ms B transmits data to C A continues No long delays

4. Packets and TDM Time Division Multiplexing Many resources take turns accessing the shared communication resources All sources receive prompt service The source with less data finishes early

5. Packets and Frames Packet refers to a small block of data Each hardware technology uses different packet format Frame denotes packet used with specific type of network EX : RS-232 mechanism Does not include a mechanism that allows a sender to signal the end of a block of characters Sending and receiving computers must agree on such details

6. Packets and Frames (Cont.) Network systems can choose two unused values to define format EX: RS-232 can use frame delimiters Soh �start of header Eot � end of transmission Overhead is an disadvantage An extra, unnecessary character between blocks of data Advantageous when large delays or computer crashes Missing eot indicates sending computer crashed Missing soh indicates receiver missed beginning of frame

7. Byte Stuffing Data and control information must be distinguished Network system change the data slightly before it is sent Termed data stuffing Insert extra bits or bytes to change data Byte stuffing and character stuffing Data stuffing used with character oriented hardware Bit stuffing Data stuffing used with bit oriented hardware

8. Byte Stuffing (Cont) EX : RS-232 soh and eot must not appear in the data Byte stuffing reserves a third character �esc� Marks occurrences of reserved characters

9. Implementing Byte Stuffing Sender must scan and perform mapping before any data is sent Sender replaces characters Receiver looks for a combination of �esc� followed by a x, y or z Replaces combination by appropriate single characters Receiver is sure that soh and eot are frame delimiters

10. Transmission Errors Interference can introduce unwanted electric currents in wires Interference can cause The receiver to misinterpret the data The receiver to lose the data sent by sender The receiver to detect data, although sender did not send any data Termed transmission errors The problem of lost ,changed or spuriously appearing data

11. Parity Even or odd Sender and receiver must agree in which form to use Even parity � the total number of 1 bits (including parity bit) must be even EX : parity bit for 0100101 is 1 Parity bit for 0101101 is 0 Odd parity � total number of 1 bits (including parity bit) must be odd EX : parity bit for 0100101 is 0 Receivers computation of parity must agree to sender�s Else receiver reports parity error

12. Parity Checking Parity check � mechanism requires the sender to compute an additional bit, called parity bit RS-232 circuits uses parity check to ensure that each character arrives intact Attach parity bit to each character before sending Receiver removes the parity bit and performs the same operation as the sender Verifies the result with the value of the parity bit If one of the bits is damaged, receiver reports error

13. Error Detection Parity cannot detect error involving an even number of bits EX : Two 0 bits changed to 1 Two 1 bits changed to 0 One 0 bit changed to 1 and vice versa Parity is preserved even with errors Alternative mechanisms used depending on : The size of the additional information The computational complexity of the algorithm The number of bit errors that can be detected

14. Checksums Checksum : sender treats the data as a sequence of binary integers and computes their sum Carry bits, if any, are added into the final sum Advantages : size and ease of computation and cost of transmission Disadvantages : cannot detect all common errors

15. Cyclic Redundancy Checks (CRC) CRC hardware uses A shift register An exclusive or (xor) unit To compute a CRC Values in shift registers initialized to 0 Bits of message shifted once at a time One bit of message applied at input All shift register perform shift operation

16. CRC (Cont.) Shift registers contain CRC after entire message has been shifted Receiver uses identical hardware and compares CRC To simplify checking CRC Append and additional 16 bits of zeroes to message Receiver computes CRC over incoming message plus incoming CRC If no errors,value should be zero Uses a polynomial expressed as a power of X P( X ) = X 16 + X 12 + X 5 + 1

17. Burst Errors CRC is especially useful with Vertical errors Burst errors Vertical errors appear in a vertical column when characters are arranged in rows EX : Damaged character oriented I/O device Burst errors � involve changes to a small set of bits near a single location Caused by electric interference from lighting , electric motor, etc.

18. Frame Format and Error Detection Networks usually associate error detection with each frame If no characters are lost , byte stuffing of CRC is not required If CRC is not byte stuffed, a single character loss causes the receiver to discard two frames Individual standards specify whether CRC is computed on the message or the encoded frame

19. Local Area Network (LAN) Most networks are local i.e. the network fits inside a building or a single room Permits multiple computers to share resources Ex:a printer accessed by two computers in a network No separate modems and cables Computers must take turns using the shared medium

20. Direct Point to Point Communication Point to point network or mesh network Each communication channel connects and is availabel to two computers Advantages Independent installation facilitates use of appropriate hardware Connected computers decide how to communicate Easy to enforce security and privacy Disadvantages Must provide a separate communication channel for each pair of computers Number of connections grows quickly as the size of set increases

21. Direct Point to Point Communication (Cont.) Number of connections needed for N computers is (N2 � N) /2 Adding Nth computer requires N-1 connections Expenses are high because many connections follow same physical path Ex: In fig., 6 connections pass between two locations If one computer is added to location1 , number of connection become 9

22. Shared Communication Channels LAN developed during the late 1960s and early 1970s Consists of a single shared medium Computers take turns using the medium to send packets Reduces cost Shared network used for only local commnucation Large geographic separation introduces longer delays Shred network with long delays are inefficient Providing high bandwidth communication channel over long distances is expensive

23. Locality of Reference LANs now connect more computers than any other type of network Locality of reference � computer communication follows two patterns Temporal locality of reference: a computer is more likely to communicate with the same set of computer repeatedly Physical locality of reference: a computer tends to communicate with computers that are physically nearby

24. LAN Topologies Star topology :- All computers attach to a central point The center of the star network often called hub Hub accepts and delivers data In practice, star networks seldom have a symmetric shape A hub often resides in a location separate from the computers attached to it

25. LAN Topologies (Cont.) Ring topology:- arranges for computers to be connected in a closed loop A cable connects first computer to second,another cable connects second to third and so on A cable connects the final computer to the first Refers to logical connection not physical orientation

26. LAN Topologies (Cont.) Bus topology:- Consists of a single ,long cable to which computers attach Any computer can send data to any computer Coordination is necessary to ensure that only one computer sends a single at any time

27. Why Multiple Topologies? Each topology has advantages and disadvantages Advantages Ring makes it easy to coordinate access and detect operation Star protects network from damage by single wire Bus requires fewer wires than star Disadvantages Entire ring network is disabled if one of the cables is cut Bus network is disabled if main wire is damaged

28. Ethernet Widely used network topology that employs bus topology Invented at Xerox corporation�s Palo Alto Research center in early 1970s Consists of a single coaxial cable, called the ether, to which multiple computers connect Ethernet coaxial cable also termed segment Length limited to 500 m , minimum separation between pairs is 3 m

29. Ethernet Operation Original Ethernet hardware operated at 10 Mbps Fast Ethernet operates at 100 Mbps Gigabit Ethernet operates at 1000 Mbps or 1 Gbps The Ethernet standard specifies all details Multiple computers share access to a single medium Sending computer has exclusive use of the entire cable

30. Carrier Sense on Multiple Access(CSMA) Networks Ethernet network does not have a centralized controller Ethernet employs CSMA to coordinate transmission among multiple attached computers CSMA :- Idea of using the presence of a signal to determine when to transmit Uses electrical activity on the cable to determine status Signals informally called a carrier If no carrier present , transmit Carrier present, must wait for the sender to finish Technically , Carrier Sense is checking for a carrier wave

31. Collision Detect CSMA cannot prevent all possible conflicts Two computers send a frame at the same time finding the cable idle Interference between two signals is called a collision No hardware damage but produces garbled value Ethernet standards require sending station to monitor signals Technically termed as collision detect Ethernet mechanism known as Carrier Sense Multiple Access with Collision Detect

32. Back Off With CSMA/CD While detecting collisions , CSMA/CD recovers from them To avoid multiple collisions , each computer delays retransmission Computer choose random delay, between 0 and maximum delay , d If choice of delay is nearly same , collisions occur Random delay doubled at each successive collisions 0-d , 0-2d, 0-4d ,� Binary exponential back off Doubling the range of random delay after each collision

33. Wireless LAN Uses antenna to broadcast RF signals Data send at 2 Mbps using 900 Mhz frequency All computers configured to the same frequency Transmitters use low power Enough power to travel a short distance Metallic obstructions can block the signal Cannot use CSMA/CD mechanism

34. CSMA/CA Wireless LANs use Carrier Sense Multiple Access with Collision Avoidance Operation Computer 1 transmits brief control message Computer 2 receives and responds Computer 1 receives response and begins transmission Control message collide Sending station apply random back-off before retransmission

35. Local Talk A LAN that employs bus topology Invented by apple computer corporation Designed for apple Macintosh which includes all required hardware Uses a version of CSMA/CD Disadvantages Lower bandwidth (230.4 kbps) Distance limitations Advantages Almost free Easy to install Available on many computers

36. IBM Token Ring LANs employing ring topology use token passing mechanism Token Ring operates as a single shared medium A special, short message called token coordinates use of the ring A token permits transmission of one frame

37. IBM Token Ring (Cont.) One token exists on the ring t any time Each computer sends one frame before passing token Token cycles around when no data to send Time taken is brief (milliseconds) because Token is small Handled by ring hardware , not CPU IMB Token Ring is best known token passing network Operates at 16 million bps Used with computers from IBM , other vendors and printers

38. Fiber distributed data Interconnect (FDDI) Token Ring technology Transmission rate of 100 million bps Uses optical fibers to interconnect computers Contains two complete rings ( counter rotating) to overcome failures Self healing network Hardware detects a catastrophic failure and recovers automatically

39. Asynchronous Transfer Mode (ATM) A star topology developed by telephone companies One or more interconnected switches form a central hub to which all computers attach Designed to provide high bandwidth ATM switch operates at 155 Mbps or faster Each connection uses a pair of optical fibers

40. Hardware Addressing Any signal sent across a shared network reaches all attached stations Each station on the LAN is assigned a unique numeric value Called physical/hardware/media access address Sender includes hardware address of intended recipients Each frame begins with a header consisting of Destination address fields Source address fields Network interface hardware examines address fields in frames Accepts only those frames where destinations address matches station�s address

41. LAN Hardware Handles details of sending and receiving frames Operates without using the station�s CPU Uses physical addressing to prevent receiving all packets

42. Addressing Schemes Static addressing scheme Hardware manufacturers assign unique physical address Address does not change unless hardware is replaced Easy to use and permanent Configurable addressing scheme Mechanism to set a physical address Used by most network administrators because Address are permanent No large addresses because unique only to a single network Interface can be replaced without changing computer�s physical address

43. Addressing Schemes (Cont.) Dynamic addressing scheme Mechanism that automatically assigns a physical address to the station when the station first boots Tries random numbers until a unique address is found Advantages No need for manufacturers to coordinate in assigning addresses Allows each address to be smaller Uniqueness is only important within a single LAN Disadvantages Lack of permanence Potential conflict

44. Broadcasting Refers to transmissions available to a large audience All stations receive a copy of the signal each time a frame is transmitted To make broadcasting efficient, most LANs use broadcast address Hardware interface recognizes both the special broadcast address and the station�s physical address A frame with either of the two addresses is accepted and delivered to the computer�s operating system Ex: Finding a printer by its name

45. Multicasting Broadcasting is extremely inefficient because Processing and discarding a frame requires computational resources Multicasting operates like broadcasting Single copy of the frame travels across the network All network interfaces receives a copy Interface hardware must be programmed with specifications Accepts or rejects frames according to the specifications

46. Multicast Addressing Some addresses reserved for multicast Interface is programmed to recognize only the computer�s address and the broadcast address Application wishing to receive multicast frames must inform interface Multicast address must be chosen for an application Application must be configured to use the address Passes multicast address to the interface Interface adds the address to the set it recognizes

47. Identifying Packets Contents The address does not specify what the packet contains Each frame contains additional information specifying the type of the contents Two methods used to identify contents of the frame Explicit frame type Network hardware designers specify how type information is included in the frame Different values used to identify various frame types Also called self identifying frame Implicit frame type Frame carries only data Sender and receiver must agree on the contents of the frame

48. Frame Headers & Format Frame format is defined by LAN technology Most LAN technologies define a frame consisting of two parts Frame header Contains information such as source and destination addresses Data area or payload Contains the information being sent All frames have same header size but different data area

49. Ethernet Frame Format Begins with a header with three fields 64-bit preamble contains alternating 1s and 0s for synchronization First two fields contains physical address Ethernet uses 48-bit static addressing scheme Third field contains16-bit frame type Ethernet types have been standardized

50. Ethernet Frame Format(Cont.) DIX standard specifies the values used in the header fields and their meanings

51. Networks Without Self Identifying Frames Some technologies do not include type field Type of data is specified by two approaches To use a single format of data To use first few octets of the data field to store type information

52. Type Information Standard IEEE standard includes a field to specify standards organization and individual field types Known as Logical Link Control(LLC) Sub Network Attachment Point(SNAP) LLS specifies that a type field follows SNAP contains two fields Organizationally Unique Identifier (OUI) identifying organization Second contains a type value defined by organization LLC/SNAP type field makes it possible to broadcast frames

53. Network Analyzer A device used to determine how well a network system is performing Most analyzers are portable & flexible Consists of a standard portable computer and LAN interface User configures parameters used by analyzers Network interface hardware is in promiscuous mode i.e. accepts all frames Can be used to debug problems on a network Network analyzers can be configured for specific analysis

54. Network Interface Hardware Networks operates at a much higher speed than a CPU Network adapter card/ network Interface Card (NIC) Connects computer to a network and handles all details of packet transmission and reception

55. NIC Operates independent of the CPU Handles the details of accessing the medium and transmitting bits Ex: Receiving a packet CPU allocates buffer space in memory Instructs NIC to read incoming packets NIC copies, verifies and checks the frame If address matches, NIC stores a copy Interrupts the CPU

56. Thick Ethernet Wiring Informally called thick wire Ethernet or Thicknet Consists of a large coaxial cable Digital hardware NIC handles digital aspects including error detection and address recognition Analog hardware Transceiver handles analog signals Must for each computer Attaches directly to the Ethernet cable A separate called Attachment Unit Interface(AUI) connects the transceiver and the NIC

57. Thick Ethernet Wiring (Cont.) AUI cables contains many wires Two for data One each for providing power to and controlling transceiver Cable terminated by a terminator It is a resistor connecting center wire in a cable to the shield Prevents reflection of the signal from the end

58. Connection Multiplexing Connection multiplexor allows multiple computers to attach to a single transceiver Provides exactly the same signal as a transceiver Cable from each computer connects to a port on multiplexor A single AUI cable connects the multiplexor to the Ethernet

59. Thin Ethernet Wiring Informally called thin wire Ethernet or Thinnet Uses a thinner, more flexible coaxial cable Advantages Costs less to install and operate No external transceivers are needed Uses BNC connectors instead of AUI cable Both thick and thin cables are coaxial, requires termination and use the bus topology

60. Twisted Pair Ethernet Formally called 10 Base �T Also twisted pair Ethernet or simply TP Ethernet An electronic device called an Ethernet hub serves as a center of the network Connection from NIC to the hub uses twisted pair wiring with RJ-45 connectors

61. Office Wiring Schemes

62. Topology Paradox Network technology can use a variety of wiring schemes Technology determines logical topology Wiring scheme determines the physical topology Physical topology can be different can be different from logical topology Ex: A twisted pair Ethernet forms a star but functions like a bus

63. NIC and Wiring Schemes Network interface supports multiple wiring schemes A single Ethernet NIC has three connectors Can use only one wiring scheme at a time Wiring can be changed without changing NIC

64. Other Network Technologies Different technologies accommodate a variety of wiring scheme Ex: The original Local Talk uses transceivers like thicknet Uses point-to-point connection between pairs of transceivers Although Local Talk is a bus technology it sometimes uses hub technology

65. LAN Design Distance limitation is a fundamental point LANs use a shared communication media CSMA/CD or Token passing is used to guarantee fair access to medium LAN is designed with a fixed maximum cable length to minimize delays An electrical signal gradually becomes weaker as it travels along a copper wire This puts a limitation on the maximum length of the wire allowed

66. Fiber Optic Extensions LAN extension mechanisms insert additional hardware components that can relay signals across longer distances Ex: Optical fibers and a pair of fiber modems Fiber has low density and high bandwidth Provides a connection between a computer and a distant Ethernet Inserted between the network interface on a computer and a remote transceiver

67. Repeaters An analog electronic device that continuously monitors signals on each cable Used to extend LAN Connects two Ethernet cables called segments When it senses a signal on one cable, it transmits an amplified copy on another A repeater can double the effective length Any pair of computers on the extended LAN can communicate

68. Repeaters(Cont.) Each repeater and segment along the path increase delay Ethernet standards limits that no more than four repeaters separate any pairs of stations The connection can be extended by using fiber modems and Fiber Optic Intra Repeater Link ( FOIRL) Along with valid transmissions, the repeaters propagates a collision or electrical interference

69. Bridges An electronic device that connects and extends two LAN segments Handles complete frames and uses same network interface as a conventional computer Helps isolate problems by forwarding only complete and correct frames Any pair of computers can communicate on extended LAN

70. Frame Filtering A typical bridge consists of a conventional computer with a CPU, memory and two network interfaces A bridge performs frame filtering Does not forward a frame unless necessary Uses physical address to determine whether to forward a frame Called adaptive or learning bridges because they learn the locations of computers automatically Uses source address to list computers

71. Bridged Networks Bridged networks running for a long time restricts frames to the fewest segments necessary Propagation principle In the steady state, a bridge forwards each frame only as far as necessary Permits communication on separate segments at the same time(parallelism) To optimize performance, a set of computers that interact frequently should be attached to the same segment

72. Bridging Between Buildings An optical fiber and pair of fiber modems are used to extend one of the connections between a bridge and a LAN segment The use of a bridge has following advantages Single fiber connection makes it less expensive Individual computer can be added or removed without installing or changing the wiring Communication in buildings is independent

73. Bridges Across Longer Distances Involves a long distance point-to-point connection and special bridge hardware Leased serial line used because it is less expensive Leased satellite channel used for communication across an arbitrary distance Bridge hardware has two main functions Filtering frames Buffering

74. Cycle Of Bridges A bridge network can span many segments Not all bridges allowed to broadcast frames A cycle of bridges causes infinite number of frames

75. Distributed Spanning Tree(DST) To prevent infinite loops, a bridged network cannot allow All bridges forwarding all frames A cycle of bridged segments To prevent loops, bridges configure themselves automatically When a bridge first boots, it communicates with other bridges Computes Distributed Spanning Tree algorithm To decide which bridges will not forward frames DST prevents bridges from introducing a cycle After DST completes, bridges are arranged in a form of a tree

76. Switching A switched LAN consists of a single electronic device that transfers frame among many computers A switch simulates a bridged LAN with one computer per segment Consists of multiple ports each attached to a computer One-half of the computers can send data at the same time

77. Switches And Hubs Switches cost more per connection than a hub because it provides higher aggregate data rates Combination is used to reduce cost A hub connects to each port on switch Each computer connects to one of the hub Each hub appears to be a single LAN segment Switch makes it appears that bridges connect all segments Communication can occur in parallel

78. Digital Telephony Digitization is performed by an analog-to-digital converter(A-to-D converter Takes analog input a signal Samples the signal regularly Computes a corresponding value at time of the sample Known as Pulse Code Modulation (PCM) Samples once every 125 � sec and converts into an integer between 0 and 255

79. Synchronous Communication Telephone industry have devise complex digital communication systems Voice system use synchronous or clocked technology Most data networks use asynchronous technology Data moves at a precise rate in synchronous network Network does not slow down as traffic increases Telephone systems transmits additional information along with digitized data to ensure continuous transmission

80. Digital Circuits and DSU/CSU leased digital circuits from common carriers form the fundamental building blocks for long distance computer networks Standards differ between computer and telephone industry Data Service Unit/Channel Service Unit (DSU/CSU) Hardware needed to interface a computer to a digital circuit CSU portion Handles line termination and diagnostics Helps in installing and testing circuits Uses bit stuffing DSU portion Translates data between two digital formats

81. Telephone Standards

82. DS Standards A single voice channel requires 64 Kbps(8000 8 bit samples/sec) Digital circuits are classified according to a set of telephone standards Most popular circuit types in North America T1 and T3 Digital signal level standards or DS standards Specify how to multiplex phone calls onto a single connection 28 T1 circuits can be multiplexed over single T3 circuit

83. Lower Capacity Circuits T1 circuit is too expensive Fractional T1 circuits Capacity much less than 1.544 Mbps Most popular fractional T1 rate is 56 Kbps Time Division Multiplexing (TDM) Concept of subdividing T1 circuits

84. Intermediate Capacity Digital Circuits Slightly more than T1 and less than T3 Inverse multiplexing is used Allows one to lease multiple T1 circuits Multiple circuits acts like a single higher capacity circuit Inverse multiplexor is needed at each end of line DSU/CSU may be required if not built in inverse mux

85. Highest Capacity Circuits Also termed as trunk Synchronous Transport Signal (STS) standards Specifies details of high speed connections Serves connections across country or between countries

86. Optical Carrier Higher data rates associated with the STS standards require optical fiber STS referred to electrical signals OC refers to optical signals Both can be concatenated (suffix C) C denotes a circuit with no inverse multiplexing OC-3 consists of 3 OC-1 operating at 51.840 Mbps each OC-3C (STS-3C) is a single circuit operating at 155.520 Mbps Single circuit is more flexible

87. Synchronous Optical NETwork (SONET) Used in North America Known as Synchronous Digital Hierarchy(SDH) in Europe Specifies details about framing, multiplexing and synchronization Size of the SONET frame depends on the bit rate Can be used to build a high capacity ring network with multiple data circuits Mostly used to define framing and encoding

88. Local Subscriber Loop Termed local loop or local subscriber line Connection between the phone company Central Office and individual subscriber residence Uses analog signals Most subscribers use a telephone to dial a local service provider Voice bandwidth and signal-to-noise ratio of telephone lines limit the rate at which bits are sent

89. ISDN Integrated Services Digital Network Provides digitized voice and data over local loop wiring Uses twisted pair copper wiring Offers three separate digital channels B, B and D (2B + D) The two B channels Operate at 64 Kbps each Carries digitized voice, data or compressed video The D channel Operates at 16 Kbps Intended as a control channel Manages or terminates a session Both B channels bonded as a single channel

90. Asymmetric Digital Subscriber Line Asymmetric service, termed ADSL Bit rate in one direction is much higher Typical users receive more information than they send ADSL provides higher bit rate downstream (to the subscriber) than upstream (from subscriber to the provider) Maximum downstream rate is 6.144 Mbps Maximum upstream rate is 640 Kbps Operates on local loop wiring

91. ADSL (Cont.) ADSL is adaptive Modems probe the line and agree to communicate using techniques to optimize line Uses Discrete Multitone Modulation (DMT) Combination of frequency division and inverse multiplexing Divides bandwidth in 286 separate frequencies or sub channels Selects the best frequencies and modulation techniques

92. Other DSL Technologies Symmetric Digital Subscriber Line (SDSL) Provides symmetric rates in both direction Businesses prefer SDSL Can operate over local loops High-rate Digital Subscriber Line (HDSL) Provides DS-1 (1.544 Mbps) in two directions Requires two independent twisted pairs Able to tolerate failure Very-high bit rate Digital Subscriber Line(VDSL) Data rate of up to 52 Mbps Requires intermediate concentration parts

93. Cable Modem Technology Uses cable TV wiring Offers higher speed and less susceptibility to electromagnetic interference Consists of high capacity coaxial cable Uses broadband signaling (frequency division multiplexing) One pair of cable modems is required for each subscriber When subscribers are more, Time Division Multiplexing is used One frequency for a set of subscribers

94. Upstream Communication CATV is designed for downstream direction only Dual path approach Cable system handles only downstream traffic Upstream traffic travels across a dial-up telephone connection Needs hardware interface device to connect cable modem and dial-up modem Hybrid Fiber Coax (HFC) Combination of optical fibers and coaxial cables HFC can only be used with modified infrastructure Trunk lines replaced by optical fibers All amplifier modified to be bi-directional

95. Large Networks Local Area Networks (LAN) Spans a single building or campus Metropolitan Area Networks (MAN) Spans a single city Wide Area Networks (WAN) Spans sites in multiple cities, countries or continents WAN differs from LAN Must be able to grow (scalability) Must deliver reasonable performance to large sized networks Must provide capacity for simultaneous communication

96. Packet Switches WAN is constructed from many switches to which individual computers connect Called a packet switch Moves complete packet from one connection to another Consists of a small computer with processor, memory and I/O devices Two types of I/O connectors One operates at high speed and connects to other packet switches Second operates at low speed and connects switch to computers

97. Forming a WAN A set of packet switches are interconnected A switch has multiple I/O connectors Forms many different topologies Can connect multiple computers

98. Store And Forward WAN permits many computers to send packets simultaneously Uses store and forward switching A packet switch must buffer packets in memory The store operation Occurs when a packet arrives Copies the packet in memory Informs the processor The forward operation Processor examines the packet Determines the destination path Start the output device Buffers a short burst of packets that arrives simultaneously

99. Physical Addressing Each computer assigned a physical address For efficient forwarding hierarchical addressing scheme is used Divides an address into multiple parts First part indicates a packet switch Second part identifies computer attached to that packet switch An address is represented as a single binary value

100. Next-hop Forwarding A packet switch uses destination address to forward each packet Next-hop forwarding Switch contains information about the next place (hop) Depends on the packet�s destination and not on the source Called source independent

101. Hierarchical Addresses To Routing Routing Process of forwarding a packet to its next hop Routing table Table used to store next-hop information All destination addresses have an identical first part Using only the first part helps in Reducing computation time Shortening routing table The final packet switch uses the second part

102. Routing in a WAN A WAN with large capacity can be build by increasing switching capacity Interior switches Handles load, but need not have computers attached Exterior switches Packet switches to which computers attach Both switches have routing tables Universal routing Routing table contains next-hop route for each possible destination Optimal routes The next-hop value points to the shortest path to the destination

103. Routing in a WAN (Cont.) A graph can model a network Each node corresponds to a packet switch Each link corresponds to a direct connection

104. Default Routes A graph representing a large WAN may contain many duplicate entries Default route or Default routing table A long list of entries having same next-hop value is replaced by a single entry Only one default entry is allowed in any routing table A default entry is present only if more than one destination has the same next-hop value

105. Routing Table Computation Static routing A program computes and installs routes when a packet switch boots, the routes do not change Dynamic routing A program builds an initial routing table and then alters the table as condition changes Static routing is simple and has low overhead Most networks use dynamic routing because Handles problems automatically Modifies routes to accommodate failures

106. Shortest Path Computation Dijkstra�s algorithm Finds the distance along a shortest path from a single source node to each of the other nodes in the graph A next-hop routing table is constructed during the computation of shortest path Uses weights on edges as a measure of distance A path with fewest number of edges may not be the path with least weight

107. Distributed Route Computation Each packet switch computes its routing table locally Informs the network of the result Sends routing information to neighbors periodically Each packet switch learns the shortest path to all destinations Produces the same next-hop routing table as Dijkstra�s algorithm Allows the network to adapt to a failure

108. Distance Vector Routing Distance-vector algorithm uses distributed route computation Each link in network is assigned a weight Distance to a destination is defined to be the sum of weights along the paths A packet switch periodically updates the network Each message contains pairs of (destination,distance)

109. Link-State Routing (SPF) Also called shortest path first or SPF routing Packet switches sends messages with status of the link Message broadcast to all switches Each switch collects information and builds the graph of the network Switches use Dijkstra�a algorithm to produce routing table SPF algorithm can adapt to hardware failures

110. WAN Technologies ARPANET One of the first packet switched WANs Fast when invented, slow by current standards X.25 Developed an early standard for WAN technology More popular in Europe Frame relay Accepts and delivers blocks of data Must operate at high data rates Switched Multi-megabit Data Service( SMDS) Offered by long-distance carriers Operates at speed faster than frame relay

111. Asynchronous Transfer Mode (ATM) Provides voice, video and data services across a wide area Has high data rates, low delay and low jitter (low variance in delay) Data divided into fixed sized packets called cells Each ATM cell has 53 octets 5 for header information and 48 for data A Constant Bit Rate (CBR) is specified for voice or video Uses switches as primary building blocks Uses optical fiber as interconnection media

112. Network Ownership Private networks Owned and used by a single company or an individual Public networks Owned by common carriers such as telephone networks Anyone can subscribe to the service and connect a computer LAN technology is most often used for public networks Almost all public networks are WANs The chief advantage with private networks is complete control Public networks are flexible and able to use state-of-the-art networking without maintaining technical expertise

113. Virtual Private Networks (VPN) Combines advantages of both private and public networks Allows a company with multiple sites to have private network Uses a public network as a carrier VPN technology restricts traffic only between the company�s sites A special hardware and software system is placed between company�s and public network VPN encrypts each packet before transmission Network manager must also configure routing

114. Service Paradigm Connection-oriented service Operates analogous to a telephone system Requires a pair of computers to establish a connection before sending data Either computer can choose to terminate the connection Connection-less service Operates analogous to a postal mail system Computers do not need to establish a connection before they can communicate Accepts and delivers individual frames that each specify a destination Less initial overhead

115. Connection Types Permanent connection Dedicated wires between a pair of computers Is persistent and always available Does not require maintenance Always ready to accept data Switched connection Must establish a connection to communicate Each computer maintains physical connection to network Is flexible and general Permanent connections survive either a computer or a network reboot

116. Examples of Service Paradigms

117. Connection Identifiers Connection oriented service uses abbreviated addresses A small integer used to communicate after a connection is established Ex: ATM network uses 28-bit connection identifiers The computer places the identifier in each outgoing cell ATM divides connection identifiers into two parts 12-bit virtual path identifier ( VPI ) 16-bit virtual circuit identifier ( VCI)

118. Network Performance Characteristics Delay Specifies how long it takes for a bit of data to travel across the network ( in seconds) Propagation delay Time a signal requires to travel across a wire or optical fiber Switching delay Delay introduced by electronic devices in network Access delay Delays caused when waiting to access a shared media Queuing delay Occurs in packet switched WAN because it enqueues packets

119. Network Performance Characteristics (Cont.) Throughput Measure of the rate at which data can be sent through a network Specified in bits per second, bps Throughput is measure of capacity, not speed Throughput and delay are related by D = D0 / ( 1 � U) D0 = idle network delay U = current utilization between 0 and 1 D = Effective delay Volume of data present on the network Product of delay and throughput ( T * D )

120. Protocols Protocol A set of rules that specify the format of messages and the appropriate action required for each message Protocol software The software that implements such rules Application programs do not interact with network hardware Communication software is divided into multiple protocols Protocols are designed and developed in complete, cooperative sets called suites or families

121. Protocol Design Layering model Describes one way a communication problem can be divided into sub-pieces called layers ISO defined a 7-layer model

122. The Seven Layers Layer 1 : Physical Corresponds to basic network hardware Ex: RS 232 Layer 2: Data Link Specifies how to organize data into frames and transmit over a network Ex: Frame format and CRC Layer 3: Network Specifies how addresses are assigned and how packets are forwarded Layer 4: Transport Specifies how to handle details of reliable transfer

123. The Seven Layers (Cont.) Layer 5: Session Specifies how to establish a communication session with a remote system Ex: Security details Layer 6: Presentation Specifies how to represent data Needed to translate from the representation on one computer to another Layer 7: Application Specifies how one particular application uses the network Ex: specifications for an application that transfers files

124. Stacks : Layered Software When protocol software sends or receives data, each module only communicates with the next highest and lowest level Incoming and outgoing data passes through each layer

125. Stacks : Layered Software (Cont.) Vendors use the word stack to refer to protocol software Software in the given layer on the sending computer adds information to outgoing data Software in the same layer on receiving computer uses the additional information to process incoming data

126. Multiple, Nested Headers Each layer places additional information in a header before sending data to a lower layer The header corresponding to the lowest-level protocol occurs first

127. Scientific Basis for Layering Layering principle Layer N software on the destination computer must receive the exact message sent by layer N software on the sending computer Whatever transformation a protocol applies before sending a frame must be completely reversed when the frame is received

128. Techniques Protocols Use Sequencing for Out-of-order Delivery Connectionless networks often deliver packets out of order To handle this transfer protocol use sequencing Each packet has a sequence number Sequencing to Eliminate Duplicate Packets Malfunctioning hardware causes packet duplication Ex: a transceiver using CSMA/CD Sequencing solves the problem of duplication

129. Techniques Protocols Use (Cont.) Retransmit ting Lost Packets Protocols use positive acknowledgement with retransmission Protocol software uses a timer Protocols bound the maximum number of retransmissions Avoiding Replay Caused by Excessive Delay Replay means that an old, delayed packet affects later communication A correct packet may be discarded as a duplicate Protocols mark each session with a unique ID

130. Techniques Protocols Use (Cont.) Flow Control to Prevent Data Overrun Data overrun : A computer sends data faster than the destination can absorb Flow control techniques

131. Techniques Protocols Use (Cont.) Mechanisms to Avoid Net Congestion Congestion : More packets arrive than can be send The queue grows and the effective delay increases Congestion collapse Persistent congestion causes the entire network to become unusable Protocols avoid congestion collapse by Arranging for packet switches to inform senders when congestion occurs Use packet loss as as estimate of congestion