Networking involves connecting computers together to share resources. There are different types of computer networks including LAN, MAN, and WAN. A LAN connects devices over a short distance like within an office building. A WAN spans large physical distances like the Internet. A MAN connects devices over an area larger than a LAN but smaller than a WAN, such as within a city. Network devices like hubs, switches, bridges and routers are used to connect different parts of a network.

1. NETWORKING BASICS BY MOHAN KUMAR.P B.E created

2. What is netwoking? Types of network Network topology Network components

3. What is Networking? A group of computers interconnected for the purpose of sharing information and resources among themselves.

4. Types of network: LAN(local area network) MAN(metropolitan area network) WAN(wide area network)

5. LAN: LAN connects network devices over a relatively short distance. A networked office building, school, or home. A network device called switch connects all the computer in LAN.

6. WAN: WAN is a large physical distance. The Internet is the largest WAN, spanning the Earth. A network device called a router connects LANs to a WAN.

7. MAN: A network spanning a physical area larger than a LAN but smaller than a WAN, such as a city.

8. Network Topology: Topology refers to the layout of connected devices.

9. Types of topology: Bus Ring Star Tree Mesh

10. Bus topology: Bus networks use a common backbone(cable) to connect all devices. A device wanting to communicate with another device on the network sends a broadcast message onto the wire that all other devices see, but only the intended recipient actually accepts and processes the message. This is done by matching the MAC address of destination. Ethernet bus topologies are relatively easy to install and bus networks work best with a limited number of devices. If the backbone cable fails, the entire network effectively becomes unusable.

11. Ring topology: In a ring network, every device has exactly two neighbors for communication purposes. All messages travel through a ring in the same direction (either "clockwise" or "counterclockwise"). A failure in any cable or device breaks the loop and can take down the entire network. Ring topologies are found in some office buildings or school campuses.

12. Star topology: A star network features a central connection point called a "network device" that may be a hub, switch or router. Compared to the bus topology, a star network generally requires more cable, but a failure in any star network cable will only take down one computer's network access and not the entire LAN.

13. Tree topology: Tree topologies integrate multiple star topologies together onto a bus. Only network devices connect directly to the tree bus, and each nework device functions as the "root" of a tree of devices.

14. Mesh topology: A mesh network in which every device connects to every other is called a full mesh. Partial mesh networks also exist in which some devices connect only indirectly to others.

15. OSI LAYERS What is osi layer? Types of osi layers Network components vs osi layer

16. OSI model: The Open Systems Interconnection (OSI) model is a reference tool for understanding data communications between any two networked systems. It divides the communications processes into seven layers. Each layer both performs specific functions to support the layers above it and offers services to the layers below it. The three lowest layers focus on passing traffic through the network to an end system. The top four layers come into play in the end system to complete the process.

17. Advantage of OSI: Easy troubleshooting. Prevent changes in one layer from affecting other layers. Divides network communication processor into smaller.

18. OSI LAYER: To creat interoperable network devices and software in the form of protocols, so that different vendor networks could work each other.

19. LAYERED APPOACH: It address all the process required for effective communication and divides these process into logical groupings called layers.

20. Types of layers: There are seven osi layers Application layer Presentation layer Session layer Transport layer Network layer Datalink layer Physical layer

21. Application layer: Where user actually communicate to computer. Acts as interface between actual application program and next layer down by providing ways for the application to send information down through the protocol stack. Responsible for identifying and establishing the availablity of intended communication partner and determine whether sufficient resources for intended communication exist. Examples message, file, database and application services .

22. Presentation layer: It present data to the application layer and is reponsible for data translation and code formating. Adapt the data into a standard format before transmission. In application layer, the data format will be computer language, when presentation layer receive, data format is converted to user understandable format(ASCII). Handles process such as data encryption, compression and translation services.

23. Session layer: Co-ordinates communication between systems and serves to organize their communication by offering three differnt modes: simplex, half duplex and full duplex. Keeps different application data seperate from other application data.

24. Transport layer: Segments and reassembles data into a data stream. Provide end-to-end data transport service. Establish logical connection between sender and destination. Provide multiplexing upper layer application. Provide virtual circuit. connection-oriented.

25. Flow control: A manageable data flow is maintained in order to avoid congestion, overloading and data loss. Data integrity is ensured. Prevents a sending host on one side of the connection from overflowing the buffer in the receiving host. The segments delivered are acknowledged back to the sender upon their receiption.

26. Windowing: The quantity of data segments(measured in bytes) that the transmitting machine is allowed to send without receiving any acknowledgement for them is called a window. Window size 2 Data 1 Data 2 Ack back Ack 3 sender receiver

27. Network layer: Performs network routing. Tracks the location of devices on the network. Determines the best way to move data. Uses local addressing scheme and perform error control funtion.

28. IP addressing: It is a numerical label that is assigned to devices participating in a computer network, that uses the Internet Protocol for communication between its nodes. Ip's are stored in hexadecimal format. Ipv4: It is 32 bit address seperated into 4 bytes by dot(192.168.10.0). Each octet extends from 0 to 255.

29. Ipv4 subnetting: class Range Network ID Host ID NO. of networks (subnet) NO. of address Class A 0 -- 127 N.H.H.H 0.H.H.H 2^7=128 2^24-2= 16,777,214 Class B 128 -- 191 N.N.H.H 0.0.H.H 2^14=16,384 2^16-2= 65,534 Class C 192 -- 223 N.N.N.H 0.0.0.H 2^21=2,097,152 2^8-2= 254

30. Ipv6 subnetting: The address size was increased from 32 to 128 bits or 16 octets. the new address space provides the maximum of 2^128, or about 3.403 × 10^38 unique addresses. By default Ipv6 not enabled in interface, we have to enable. It uses unicast,multicast and anycast. End-to-end security.

31. Data link layer: Allows a device to access the network to send and receive messages. Offers a physical address so a device’s data can be sent on the network. Performs error detection & correction.

32. Physical layer: Establishment & Termination of Connections. Connection Resolution & Flow Control of Communication Resources. Modulation & Conversion between Digital Data

33. TCP/IP Model: TCP/IP is a suite of protocols, also known as the Internet Protocol Suite. It was originally developed for the US Department of Defense Advanced Research Project Agency (DARPA) network.

34. TCP/IP network model layers:

35. TCP/IP Model: The network access layer is functionally equal to a combination of OSI physical and data link layers (1 and 2). The Internet layer performs the same functions as the OSI network layer (3). If the host-to-host protocol is TCP, the matching functionality is found in the OSI transport and session layers (4 and 5). Using UDP equates to the functions of only the transport layer of the OSI model. The TCP/IP process layer, when used with TCP, provides the functions of the OSI model’s presentation and application layers (6 and 7). When the TCP/IP transport layer protocol is UDP, the process layer’s functions are equivalent to OSI session, presentation, and application layers (5, 6, and 7).

36. Network devices: Hub Switch Bridge Router Modem

37. Hub: It is a device for connecting multiple twisted pair or fiber optic Ethernet devices together and making them act as a single network segment. Hubs work at the physical layer (layer 1) of the OSI model. The device is a form of multiport repeater. Repeater hubs also participate in collision detection, forwarding a jam signal to all ports if it detects a collision. It regenarates data and broadcast them to all ports. Run in Half duplex. Hub

38. LAN Hub device:

39. Limitations and features: Cannot link unlike segments. Donot isolate and filter packet. The most economic way of expanding the networks.

40. Bridge: bridge connects multiple network segments at the data link layer (Layer 2) of the OSI model. bridge and switch are very much alike; a switch being a bridge with numerous ports. Routing table is built to record the MAC address for easy transmission of data.

41. Creating a MAC table: Initialy no MAC address in table. Based on the address of the sending computer table created. New addresses are added if they are not in table. Add03 Add01 Add02 Switching Table Seg 1 Seg 2 S 01 D 02 Stop S 02 D 01

42. Remote bridge: Remote bridge can be used to connect remote segments via data-grade telephone line.

43. Features and limitations: Bridges can analyze incoming data packets to determine if the bridge is able to send the given packet to another segment of the network. Traffic from one network is managed . It cannot filter packet according to the protocol. Bridges are more complex than hubs or repeaters.

44. Hub vs bridge:

45. Switch: switch operates at the data-link layer(layer2) of the OSI model to create a different collision domain for each switch port. If you have 4 computers (e.g., A, B, C, and D) on 4 switch ports, then A and B can transfer data back and front, while C and D also do so simultaneously, and the two "conversations" will not interfere with one another. Dedicated bandwidth on point-to-point connections with every computer and to therefore run in Full duplex with no collisions.

46. LAN switch:

47. Ethernet Operations: CSMA/CD -Listen to wire before transmitting -Contention mention -Interframe gap . Also known as an interpacket gap .9.6 seconds -Collisions . Simultaneous frame transmission . 32-jam signal .Back-off period

48. Ethernet Operations (continued): CSMA/CD (continued) -Collision domain . Physical topology segment in which frames may collide - Layer 3, layer 2, and layer 1 . Repeaters and hubs do not microsegment .Switches and bridges microsegment at layer 2 .Routes and gateways segment at layer 2 and layer 3

49. Latency: Latency -Sometimes referred to as propagation delay -Length of time to forward, send, or propagate a data frame -Bit time -Slot time .5-4-3 rule -Latency differs depending on .Resistance of transmission medium .Number of nodes .Amount of processing of packet

50. Latency (continued):

51. Ethernet Errors: Most errors are caused by -Defective equipment -Incorrectly configured equipment Frame size errors Frame size minimum 64 bytes Frame size maximum 1518 bytes

52. Ethernet Errors (continued): Frame error classification - Short frame or runt -Long frame or giant -Jabber - Frame check sequence error -Alignment error

53. Collision Errors: As the number of devices increase so does the number of collisions Late collisions -Violate the 5-4-3 rule -Cable too long -Slot time exceeded Segment with a router Microsegment with a switch or bridge Transmitting station will attempt to retransmit 16 times -Additional collision detections will be considered a NIC error

54. Broadcasts: Sent to all nodes on a network Advertising a route service Broadcast storm -Network loop -126 or more broadcasts per second Possible solutions -Reduce the number of services on servers -Limit the number of protocols

55. Fast Ethernet: 100 Mbps 10/100 Autosense Full-duplex or half-duplex Category 5 or higher cable IEEE 802.3u implementations -100Base-TX -100Base-T4 -100Base-FX

56. Half- And Full-Duplex Communication: Half-duplex -Send and receive signals separately Full-duplex -Send and receive simultaneously -No collisions Benefits of full-duplex -No collisions .No retransmissions -Full bandwidth in both directions -No waiting for other transmissions

57. Half- And Full-Duplex Communication (continued): Four different duplex options on 2950 switch -Auto -Full -Full-flow control -Half

58. Half- And Full-Duplex Communication (continued):

59. LAN Segmentation: Segmenting with bridges Filter traffic at Data Link layer Segment LAN into 2 or 3 major segments Bridges build a MAC-to-segment table -Manual configuration -Learn from source MAC of arriving frame Bridges forward frames through the bridge when the destination of the frame is on a different segment

60. LAN Segmentation (continued): Points to remember about bridges -Reduce collisions -No effect on broadcasts or multicasts -Extend physical length of LAN -Efficient use of bandwidth

61. Segmenting With Routers: Points to remember when segmenting with routers -Forwards packets based on layer 3 addresses -Decrease collisions -Reduce broadcast and multicast traffic -Support multiple paths and routes between routers -Efficient use of bandwidth for the newly created segments

62. Segmenting With Routers (continued): -Increase security -Increase the physical distance of the network -Provide layer 3 routing, packet fragmentation and reassembly, and traffic flow control -Provide communications between different technologies such as Ethernet and Token Ring or Ethernet and Frame Relay -Higher latency than bridges

63. LAN Switching: Segmentation with switches -Switches are hardware controlled -Bridges are software controlled Microsegmentation -Switched bandwidth -Shared bandwidth Efficient use of bandwidth Able to connect segments of different speeds -10 Mbps to 100 Mbps -100 Mbps to 1Gbps

64. LAN Switching (continued):

65. LAN Switching (continued):

66. Switch Operations: MAC-to-switch port mapping Content-addressable memory (CAM) Learns MAC addresses automatically -Source address from arriving frame Two types of memory buffering -Port-based memory buffering --Shared-memory buffering Symmetric and asymmetric switching

67. Switching Methods: Four methods for processing and forwarding frames Store-and-forward -Read the entire frame Fragment-free -Reads first 64 bytes -Lower latency than store-and-forward -Also known as “modified cut-through” -Minor error detection

68. Switching Methods (continued): Cut-through -Forwards frame after destination MAC is read -First 14 bytes of frame -Lowest latency -No error detection Adaptive cut-through -Error sensing -Uses cut-through and store-and-forward

69. Switching Methods (continued):

70. Switching Methods (continued):

71. Spanning Tree Protocol: Spanning Tree Protocol (STP) -Physical loops -Logical loops Spanning Tree Algorithm (STA) IEEE 802.1d

72. Spanning Tree Protocol (continued):

73. Spanning Tree Protocol (continued): Build a logical path -Election process -Root bridge (root device) -Bridge protocol data units (BPDU) or -Configuration bridge protocol data units (CBPDU) -Root ports

74. Spanning Tree Protocol (continued): Port states -Stable states .Blocking: Send and receive BPDUs but no data frames .Forwarding: Send and receive all data frames and learn new MAC addresses .Disabled: No frames sent or received -Transitory states .Listening: Listening to election process only .Learning: Learning new MAC addresses

75. Spanning Tree Protocol (continued): STP switch port process -From bridge/switch bootup to blocking -From blocking to listening (or to disabled) -From listening to learning (or to disabled) -From learning to forwarding (or to disabled) -From forwarding to disabled (automatically or manually)

76. Virtual LANs: Logical grouping of network devices and nodes Broadcast domain Management VLAN -Also known as default VLAN -Cannot be deleted -Every port is on VLAN 1 by default Router are required to move traffic between VLANs

77. Virtual LANs (continued):

78. Virtual LANs (continued):

79. Benefits of VLANs: VLANS provide the following benefits -It is easier to add and move stations on the LAN -It is easier to reconfigure the LAN -There is better traffic control -There is increased security

80. Dynamic vs. Static VLANs: VLANs can be configured dynamically or statically Static VLANs are configured port-by-port Dynamic VLAN ports automatically learn their VLAN assignment -Software database of MAC address-to-VLAN mappings

81. VLAN Standardization: Frame filtering -Frames can be separated into VLANs -MAC addresses -Network-layer protocol type -Application type Frame tagging -IEEE 802.1q Also known as frame identification Adds a four-byte field to Ethernet frame -Inter-Switch Link (ISL) protocol Cisco proprietary frame-tagging method 26 byte header

82. Creating VLANs: VLAN configuration Rm410HL#vlan database Rm410(vlan)#vtp domain hudlogic Rm410(vlan)#vtp server Rm410(vlan)#vlan 2 name production Rm410(vlan)#vlan 3 name accounting Rm410(vlan)#vlan 4 name marketing

83. Creating VLANs (continued): Rm410#configure terminal Rm410(config)#interface f0/1 Rm410(config-if)#switchport mode trunk Rm410(config-if)#exit Rm410(config)#interface f0/2 Rm410(config-if)#switchport access vlan 1

84. Link Types And Configuration: Two types of links Trunk links -Switch-to-switch links -Switch-to-router links -100 Mbps links -1 Gbps links Access links -Non-VLAN aware devices

85. Link Types And Configuration (continued): Trunk links have five states -Auto -Desirable -Non-negotiate -Off -On Rm410(config)#interface f0/1 Rm410(config-if)#switchport mode trunk

86. Trunking Protocol: VLAN trunking protocol -Layer 2 messaging protocol -Manages all changes to the VLANs across networks VTP domains -VTP devices are organized in to domains -Switches can only belong to one domain Rm410HL#vlan database Rm410(vlan)#vtp domain hudlogic

87. Trunking Protocol (continued): VTP device modes -Server .Rm410(vlan)# vtp server -Client .Rm410(vlan)# vtp client -Transparent .Rm410(vlan)# vtp transparent Default to server mode VTP pruning -Reduces the number of VTP updates on trunk link .Rm410(vlan)# vtp pruning

88. Trunking Protocol (continued): Delete VLAN database -Rm410# delete flash:vlan.dat Switch interface descriptions -Rm410HL(config)#int f0/1 -Rm410HL(config-if)#description productionVLAN

89. Routers and VLANs: Increase security Manage traffic between VLANs Subinterfaces Access-lists Router-on-a-stick

90. Routers and VLANs (continued): Enable inter-VLAN communication between VLAN 1 and VLAN 2 Router(config)# interface e0.1 Router(config-subif)# ip address 164.106.1.1 255.255.255.0 Router(config-subif)# encapsulation isl 1 Router(config-if)# exit Router(config)# interface e0.2 Router(config-subif)# ip address 164.106.2.1 255.255.255.0 Router(config-subif)# encapsulation isl 2

91. Routers and VLANs (continued):

92. Advantages of switch: Reduce the possiblity of collision. Since isolated,hence secure. Collision occurs only when two devices try to get access to one channel. Can be solved by buffering one of them for later access. Data will only go to the destination, not to others.

93. Router: Router works in OSI layer3 (network layer). Device that interconnects two or more computer networks, and selectively interchanges packets of data between them. Router use the logical address of packets and routing tables to determine the best path for data delivery. Routers connect two or more logical subnets, which do not share a common network address. A router is a networking device whose software and hardware are customized to the tasks of routing and forwarding information.

94. Two primary functions: Routing table - a database keeps tracks of the routes to networks and the associated costs. Static routing - routes are manually configured by network admin. Dynamic routing - Dynamic routing allows routing tables in routers to change as the possible routes change. There are several protocols used to support dynamic routing including RIP and OSPF. A router will 'learn' routes to all directly connected networks first. It will then learn routes from other routers that run the same routing protocol. The router will then sort through it's list of routes and select one or more 'best' routes for each network destination it knows or has learned. Determines the best path. Shares detailes of routes with other routers.

95. Routing and forwarding: routing - refers to the overall, network-wide process that determines the end-to-end paths that datagrams will take from source to destination. forwarding -refers to the router-local action of transfering a datagram from an input link interface to the appropriate output link interface.

96. pc pc Router 1 Router 2 10.1.0.0/24 10.3.0.0/30 20.1.0.0/30 10.1.2.0/24 Destination 10.1.0.0/24 10.3.0.0/30 10.1.2.0/24 20.1.0.0/30 Next hop Direct Direct R2 R2

97. Wiring routers with network devices:

98. Router working: Only packets with known network address will be passed, hence traffic reduced. Router will select the most cost effective path for transmission of packet. Router can listen to the network and any changes occured in network means, router will update itself.

99. Routing table formation: Routing table is formed based on communication between routers using routing protocols. Routing protocols collect data about current network status and contribute to the selection of best path. The routing table stores the routes (and in some cases, metrics associated with those routes) to particular network destinations. This information contains the topology of the network immediately around it.

100. Routing table:

101. Hardware components of router: Network interfaces. Interconnection network. Processor with memory and CPU.

102. Router components:RAM Temporary storage of configuration files(running configuration). All datas will be lost on power-down. Hold routing tables. Stores ARP cache, fast switching cache, packet buffers, packet hold queues.

103. Router components:NVRAM Usually less than 100kbytes. Content is not lost on power cycle. Store the router and switch configuration(startup configuration). Configuration registery also stored.

104. Router components:Flash It is EEPROM created by intel. General storage and data transfer between the routers. Stores router IOS and allows for upgrading the operating system without replacing the chip. Multiple version of IOS can be stored in one flash(useful for testing and install a new version). Off board configuratio allows you to OS in your pocket.

105. Router components:ROM Generally installed in factory and never touched again. Stores POST and bootstrap program that calls IOS. Store mini IOS, in case of flash IOS failur, router will boot by mini IOS.

106. Cisco Internet operating system(IOS): A derivative of BDS UNIX. IOS is a package of routing, switching, internetworking and telecommunications functions tightly integrated with a multitasking operating system. Pre-packaged and static. GUI available but still 90% of users prefers command line configuration.

108. Cisco IOS command mode:

109. Router configuration states:

110. Cisco key function:

111. User executable mode: The user EXEC mode is entered when the router is accessed via a serial connection or when accessing the router via telnet The command prompt of the user EXEC mode is Configuration parameters cannot be read or modified in this mode. User EXEC commands allow you to connect to remote devices, change terminal settings on a temporary basis, perform basic tests, and list system information.

112. Privileged EXEC mode: To change or view configuration information of a router, user must enter system administrator mode called Privileged EXEC Mode. The privileged EXEC mode is used to read configuration files,reboot the router, and set operating parameters. Entering the privileged EXEC mode requires to type a password, called the enable secret. The privileged EXEC mode is entered by this commands; Typing the password displays the following command prompt:

113. Global configuration mode: The global configuration mode is used to modify system wide configuration parameters, such as routing algorithms and routing tables. This is done by typing The command prompt in the global configuration mode is Typing CTRL-z as in Changes from the global configuration to the privileged EXEC mode

114. Interface configuration mode: To modify the configuration parameters of a specific interface, for example, the IP address, a user must enter the interface configuration mode. In IOS, each network interface is associated with a name, Serial WAN interface (Serial) 100 Mbps Ethernet (FastEthernet) 10 Mbps Ethernet (Ethernet) FDDI Token Ring (FDDI) Asynchronous Transfer Mode (ATM) The slot number indicates the slot into which the interface card is inserted. (i.e. FastEthernet0/0, FastEthernet0/1).

115. Interface configuration mode: The interface configuration mode for the network interface on port 1 of a 10 Mbps Ethernet card inserted in slot 0 of the router is entered with the command. The command prompt of the interface configuration mode is To return to the global configuration mode type

116. Router configuration mode: The router configuration mode is used to configure the parameters for a specific routing protocol. Some of the routing protocols; Routing Information Protocol (RIP) Open Shortest Path First (OSPF) Border Gateway Protocol (BGP) The command to enter the routing router configuration mode for the routing protocol RIP from the global configuration mode is The command prompt for the router configuration protocol is

117. IOS command for interface config:

118. Commands ip routing- Enables IP forwarding. no ip routing- Disables IP forwarding.This command also deletes the content of the routing table. no shutdown- Enables a network interface. shutdown - Disables network interface. ip address IPaddress mask- Sets the IP address and netmask of an interface to IPaddress and netmask. bandwidth bw- Assigns the bandwidth bw to an interface. The bandwidth is used as a cost metric by some routing protocols. The bandwidth does not impose a limit on the transmission rate of a network interface.

119. show running-config- Displays the current configuration of the router. show startup-config- Displays the startup configuration of the router. Reload- Forces a reboot of IOS. This command discards the running configuration and reloads the startup configuration. Saves the current configuration as the startup configuration. The new startup configuration will be used the next time IOS is rebooted. show version- Displays the version of IOS. show protocols- Displays the IP configuration of the interfaces of the router. Also, indicates if IP forwarding is enabled or disabled. show ip route -Displays the routing table. show ip cache- Displays the routing cache show interfaces. show interfaces interfacename- Displays information about all network interfaces. When an interface name is given as argument, for example, Ethernet0/1, information is displayed only for the specified interface. show ip arp - Displays the contents of the ARP cache.

120. Typing a question mark (?) in a given command mode generates a list of all available commands in the current command mode This command helps to determine if a command can be executed in the current mode. When typing commands or the names of network interfaces, it is sufficient to type just enough characters so that IOS can interpret the input without ambiguity. • conf configure • w t write terminal • int e0/0 interface Ethernet0/0 When the Tab key (<Tab>) is typed in the command line interface, IOS attempts to complete the command • conf <Tab> configure • conf <Tab> t <Tab> configure terminal

121. Modem: Allows computers to communicate over a telephone line. Sending end:modulate the computer's digital signal into analog signal and transmits. Receiving end:demodulate the analog signal back into digital form.

122. Modem features: Data rates available from 4.8kbps to 52mbps. Modulation schemes includes BPSK, QPSK, OQPSK, 8PSK AND 16QAM. Built in Bit Error Rate(BER)tester- can insert or detect test patterns in data or overhead channels. Multiple-language menus.

123. Front panel indicator: Highly visible status indicator Receiver power display Graphical high-resolution LCD display 5 way cursor control and numerical entry

124. Rear panel view: Asynch ESC channel LVDS interface RJ 45 IP interface EIA530 interface(software selectable RS232,RS422,V.35,X.21

125. Modem standards: standards bps introduced remarks v.32bis 2,400 1984 v.32 9,600 1984 v.32bis 14,400 1991 v.32terbo 19,200 1993 Communicate only with another v.32terbo v.first class 28,800 1993 v.f.c v.34 28,800 1994 Improved v.f.c v.42bis 115,200 1995 With compression v.90 56,000 1998 Resolved comprtition between x2 and flex56

126. V.90: The actual data link is 64kbps. To prevent interference and allow some overhead data in communication, ITU recommands a lower rate of 56kbps. However, 56kbps be a therotical number. Depending on the quality and length of the analog link, the actual data rate can range from 30kbps to 53kbps.

127. Types of modem: Asynchronous modems No clocking devices. Commonly used in telephone networks. Data is transmitted in serial stream. Each character is turned into a string of 8 bits. Each of these character is seperated by one start bit and one or two stop bits.

128. Synchronous modems: Need clocking device. Data are transmitted in blocks. Used in digital networks.

129. Comparision: Asynchronous modems are relatively simple and economic. -can be up to 20 to 27% of data traffic. -error control is done by parity bit or higher layer protocols like MNP, V.42 Synchronoua modems are relatively complicated and expensive. -higher efficiency. -more sophisticated error control protocol is required.

130. Asymmetric digital subscriber line: ADSL Particularly suitable for high speed multimedia communications, general internet applications. Asymmetric-downstream 1.5 to 6.1mbps and upstream 16 to 640kbps. Digital- mainly for transmitting digital data. Subscriber line-analog connection between household and CO.

131. Other DSL technologies: HDSL-high speed DSL, 2 twisted-pair, 12,000 feet, 1.5 full-duplex symmetric. VDSL-very high bit rate DSL,downstream-52mbps, over 1000 feet, upstream-1.5 to 2.3mbps. RDSL-rate adaptive DSL, intelligent DSL to adjust data rate.

132. THANK YOU