1. Multiple Access

2. INTRODUCTION The medium access sub layer is the bottom part of data link layer. The medium access sub layer is known as MAC(Medium access control) sub layer. The medium access sub layer is the bottom part of data link layer. The medium access sub layer is known as MAC(Medium access control) sub layer. When common medium shared by many stations MAC layer plays very important role. Without MAC Control several station transmits simultaneously could produce garbled message. When common medium shared by many stations MAC layer plays very important role. Without MAC Control several station transmits simultaneously could produce garbled message. The basic function of MAC sublayer is the media access control,error detection and station addressing. The basic function of MAC sublayer is the media access control,error detection and station addressing. Media access control procedure are ensure that every station get a fair chance to transmit and avoid the collision Media access control procedure are ensure that every station get a fair chance to transmit and avoid the collision When a number of user station share a single transmission medium. this is called as Multiple access communication. When a number of user station share a single transmission medium. this is called as Multiple access communication. 2

3. 3 Outline Multiple access mechanisms Multiple access mechanisms Random access Random access Controlled access Controlled access Channelization Channelization

4. 4 Sublayers of Data Link Layer

5. 5 Multiple Access Mechanisms

6. Random Access

7. 7 Also called contention-based access Also called contention-based access No station is assigned to control another No station is assigned to control another

8. ALOHA The ALOHA protocol was developed at University of Hawaii in the early 1970s.ALOHA was developed for packet radio network. ALOHA is applicable to any shared transmission medium. The ALOHA protocol was developed at University of Hawaii in the early 1970s.ALOHA was developed for packet radio network. ALOHA is applicable to any shared transmission medium. In a system when multiple users try to send a message to other station through a common broadcast channel random access technique are used. In a system when multiple users try to send a message to other station through a common broadcast channel random access technique are used. Random access means there is no scheduled time for any station to transmitt. Random access means there is no scheduled time for any station to transmitt. The basic idea of ALOHA system is applicable to any system in which uncoordinated users competing for the use of a single shared channel. The basic idea of ALOHA system is applicable to any system in which uncoordinated users competing for the use of a single shared channel. When a station send a data another station may attempt to do so at same time so the data from two station are collide. to avoid collision each station simply wait a random time and try it again When a station send a data another station may attempt to do so at same time so the data from two station are collide. to avoid collision each station simply wait a random time and try it again 8

9. 9 ALOHA Network

10. Pure ALOHA 10 The original ALOHA protocol is called pure ALOHA. In pure ALOHA each station send a frame whenever it has a frame to send. since there is the chance of collision between frame from different station The original ALOHA protocol is called pure ALOHA. In pure ALOHA each station send a frame whenever it has a frame to send. since there is the chance of collision between frame from different station The below figure in next slide shows the pure aloha The below figure in next slide shows the pure aloha The pure ALOHA Protocol relies on acknowledgement from the receiver. When user send a frame it except the receiver to send an acknowledgement. if the acknowledgement does not arrive after a time out period the station assume that the frame has been destroyed and resend the frame. The pure ALOHA Protocol relies on acknowledgement from the receiver. When user send a frame it except the receiver to send an acknowledgement. if the acknowledgement does not arrive after a time out period the station assume that the frame has been destroyed and resend the frame. Whenever two frames try to occupy the channel at same time there is the chance of collision and both will be garbled.if the first bit of new frame overlaps with last bit of a frame almost finished both frame will be destroyed and both will be retransmit later. Whenever two frames try to occupy the channel at same time there is the chance of collision and both will be garbled.if the first bit of new frame overlaps with last bit of a frame almost finished both frame will be destroyed and both will be retransmit later.

11. 11 Pure ALOHA dictate that when the time out period passes,each user wait random amount of time before resending the frame.the randomness will help to avoid more collision.the time is called back- off time (TB) Pure ALOHA dictate that when the time out period passes,each user wait random amount of time before resending the frame.the randomness will help to avoid more collision.the time is called back- off time (TB)

12. 12 Frames in Pure ALOHA

13. 13 ALOHA Protocol

14. 14 ALOHA: Vulnerable Time

15. 15 ALOHA: Throughput Assume number of stations trying to transmit follow Poisson Distribution Assume number of stations trying to transmit follow Poisson Distribution The throughput for pure ALOHA is The throughput for pure ALOHA is S = G × e −2G where G is the average number of frames requested per frame-time The maximum throughput The maximum throughput S max = 0.184 when G= 1/2 S max = 0.184 when G= 1/2

16. 16 Slotted ALOHA

17. 17 Slotted ALOHA: Vulnerable Time

18. 18 Slotted ALOHA: Throughput The throughput for Slotted ALOHA is The throughput for Slotted ALOHA is S = G × e −G where G is the average number of frames requested per frame-time The maximum throughput The maximum throughput S max = 0.368 when G= 1 S max = 0.368 when G= 1

19. 19 Example A Slotted ALOHA network transmits 200- bit frames on a shared channel of 200 kbps. What is the throughput if the system (all stations together) produces A Slotted ALOHA network transmits 200- bit frames on a shared channel of 200 kbps. What is the throughput if the system (all stations together) produces 1000 frames per second 1000 frames per second 500 frames per second 500 frames per second 250 frames per second 250 frames per second

20. 20 CSMA Carrier Sense Multiple Access Carrier Sense Multiple Access "Listen before talk" "Listen before talk" Reduce the possibility of collision Reduce the possibility of collision But cannot completely eliminate it But cannot completely eliminate it

21. 21 Collision in CSMA B C

22. 22 CSMA: Vulnerable Time

23. 23 Persistence Methods What a station does when channel is idle or busy What a station does when channel is idle or busy

24. Non-persistent CSMA In non-persistent CSMA when a station having a packet(frame)to transmit and find that channel is busy it back off for a fixed interval of time.it then check it if channel is free then it transmitts In non-persistent CSMA when a station having a packet(frame)to transmit and find that channel is busy it back off for a fixed interval of time.it then check it if channel is free then it transmitts 24

25. 1-Persistence CSMA Any station wishing to transmit monitor the channel continuously until the channel is idle and then transmit immediately with probability one hence the name 1-persistent Any station wishing to transmit monitor the channel continuously until the channel is idle and then transmit immediately with probability one hence the name 1-persistent When two or more station are waiting to transmitt a collision is guaranteed.since each station will transmitt immediately at the end of busy period.in this case each will wait random amount of time and then reattempt to transmit. When two or more station are waiting to transmitt a collision is guaranteed.since each station will transmitt immediately at the end of busy period.in this case each will wait random amount of time and then reattempt to transmit. 25

26. P-persistence CSMA To reduce the probability of collision in 1-persistent CSMA not all waiting station are allowed to transmit immediately after the channel is idle. To reduce the probability of collision in 1-persistent CSMA not all waiting station are allowed to transmit immediately after the channel is idle. When a station become ready to send and it senses the channel to be idle it either to transmit with a probability p or it defer transmission by one time slot with a probability q=1-p if deferred slot is idle the station either transmit with probability p or defer again with a probability q this process is repeated until either packet are transmitted or channel become busy When a station become ready to send and it senses the channel to be idle it either to transmit with a probability p or it defer transmission by one time slot with a probability q=1-p if deferred slot is idle the station either transmit with probability p or defer again with a probability q this process is repeated until either packet are transmitted or channel become busy 26

27. 27 Persistence Methods

28. 28 CSMA/CD Carrier Sense Multiple Access with Collision Detection Carrier Sense Multiple Access with Collision Detection Station monitors channel while sending a frame Station monitors channel while sending a frame

29. 29 Energy Levels

30. 30 CSMA/CD: Flow Diagram

31. 31 CSMA/CA Carrier Sense Multiple Access with Collision Avoidance Carrier Sense Multiple Access with Collision Avoidance Used in a network where collision cannot be detected Used in a network where collision cannot be detected E.g., wireless LAN E.g., wireless LAN IFS – Interframe Space

32. 32 CSMA/CA: Flow Diagram contention window size is 2 K -1 After each slot: - If idle, continue counting - If busy, stop counting

33. Controlled Access

34. 34 Control Access A station must be authorized by someone (e.g., other stations) before transmitting A station must be authorized by someone (e.g., other stations) before transmitting Three common methods: Three common methods: Reservation Reservation Polling Polling Token passing Token passing

35. 35 Reservation Method

36. 36 Polling Method

37. 37 Token Passing

38. Channelization

39. 39 Channelization Similar to multiplexing Similar to multiplexing Three schemes Three schemes Frequency-Division Multiple Access (FDMA) Frequency-Division Multiple Access (FDMA) Time-Division Multiple Access (TDMA) Time-Division Multiple Access (TDMA) Code-Division Multiple Access (CDMA) Code-Division Multiple Access (CDMA)

40. 40 FDMA

41. 41 TDMA

42. 42 CDMA One channel carries all transmissions at the same time One channel carries all transmissions at the same time Each channel is separated by code Each channel is separated by code

43. 43 CDMA: Chip Sequences Each station is assigned a unique chip sequence Each station is assigned a unique chip sequence Chip sequences are orthogonal vectors Chip sequences are orthogonal vectors Inner product of any pair must be zero Inner product of any pair must be zero With N stations, sequences must have the following properties: With N stations, sequences must have the following properties: They are of length N They are of length N Their self inner product is always N Their self inner product is always N

44. 44 CDMA: Bit Representation

45. 45 Transmission in CDMA

46. 46 CDMA Encoding

47. 47 Signal Created by CDMA

48. 48 CDMA Decoding

49. 49 Sequence Generation Common method: Walsh Table Common method: Walsh Table Number of sequences is always a power of two Number of sequences is always a power of two

50. 50 Example: Walsh Table Find chip sequences for eight stations Find chip sequences for eight stations

51. 51 Example: Walsh Table There are 80 stations in a CDMA network. What is the length of the sequences generated by Walsh Table? There are 80 stations in a CDMA network. What is the length of the sequences generated by Walsh Table?

52. WIRED LAN ETHERNET WIRED LAN ETHERNET 52

53. IEEE STANDARDS IEEE STANDARDS In 1985, the Computer Society of the IEEE started a project, called Project 802, to set standards to enable intercommunication among equipment from a variety of manufacturers. Project 802 is a way of specifying functions of the physical layer and the data link layer of major LAN protocols. Data Link Layer Physical Layer Topics discussed in this section:

54. Figure 13.1 IEEE standard for LANs

55. 13-2 STANDARD ETHERNET The original Ethernet was created in 1976 at Xerox’s Palo Alto Research Center (PARC). Since then, it has gone through four generations. We briefly discuss the Standard (or traditional) Ethernet in this section. MAC Sublayer Physical Layer Topics discussed in this section:

56. Figure 13.3 Ethernet evolution through four generations

57. 13.57 Figure 13.4 802.3 MAC frame

58. 13.58 Figure 13.5 Minimum and maximum lengths

59. 13.59 Frame length: Minimum: 64 bytes (512 bits) Maximum: 1518 bytes (12,144 bits) Note

60. 13.60 Figure 13.6 Example of an Ethernet address in hexadecimal notation

61. 13.61 Figure 13.7 Unicast and multicast addresses

62. 13.62 The least significant bit of the first byte defines the type of address. If the bit is 0, the address is unicast; otherwise, it is multicast. Note

63. 13.63 The broadcast destination address is a special case of the multicast address in which all bits are 1s. Note

64. 13.64 Define the type of the following destination addresses: a. 4A:30:10:21:10:1A b. 47:20:1B:2E:08:EE c. FF:FF:FF:FF:FF:FF Solution To find the type of the address, we need to look at the second hexadecimal digit from the left. If it is even, the address is unicast. If it is odd, the address is multicast. If all digits are F’s, the address is broadcast. Therefore, we have the following: a. This is a unicast address because A in binary is 1010. b. This is a multicast address because 7 in binary is 0111. c. This is a broadcast address because all digits are F’s. Example 13.1

65. 13.65 Figure 13.8 Categories of Standard Ethernet

66. 13.66 Figure 13.9 Encoding in a Standard Ethernet implementation

67. 13.67 Figure 13.10 10Base5 implementation

68. 13.68 Figure 13.11 10Base2 implementation

69. 13.69 Figure 13.12 10Base-T implementation

70. 13.70 Figure 13.13 10Base-F implementation

71. 13.71 Table 13.1 Summary of Standard Ethernet implementations

72. 13.72 13-3 CHANGES IN THE STANDARD The 10-Mbps Standard Ethernet has gone through several changes before moving to the higher data rates. These changes actually opened the road to the evolution of the Ethernet to become compatible with other high- data-rate LANs. Bridged Ethernet Switched Ethernet Full-Duplex Ethernet Topics discussed in this section:

73. 13.73 Figure 13.14 Sharing bandwidth

74. 13.74 Figure 13.15 A network with and without a bridge

75. 13.75 Figure 13.16 Collision domains in an unbridged network and a bridged network

76. 13.76 Figure 13.17 Switched Ethernet

77. 13.77 Figure 13.18 Full-duplex switched Ethernet

78. 13.78 13-4 FAST ETHERNET Fast Ethernet was designed to compete with LAN protocols such as FDDI or Fiber Channel. IEEE created Fast Ethernet under the name 802.3u. Fast Ethernet is backward-compatible with Standard Ethernet, but it can transmit data 10 times faster at a rate of 100 Mbps. MAC Sublayer Physical Layer Topics discussed in this section:

79. 13.79 Figure 13.19 Fast Ethernet topology

80. 13.80 Figure 13.20 Fast Ethernet implementations

81. 13.81 Figure 13.21 Encoding for Fast Ethernet implementation

82. 13.82 Table 13.2 Summary of Fast Ethernet implementations

83. 13.83 13-5 GIGABIT ETHERNET The need for an even higher data rate resulted in the design of the Gigabit Ethernet protocol (1000 Mbps). The IEEE committee calls the standard 802.3z. MAC Sublayer Physical Layer Ten-Gigabit Ethernet Topics discussed in this section:

84. 13.84 In the full-duplex mode of Gigabit Ethernet, there is no collision; the maximum length of the cable is determined by the signal attenuation in the cable. Note

85. 13.85 Figure 13.22 Topologies of Gigabit Ethernet

86. 13.86 Figure 13.23 Gigabit Ethernet implementations

87. 13.87 Figure 13.24 Encoding in Gigabit Ethernet implementations

88. 13.88 Table 13.3 Summary of Gigabit Ethernet implementations

89. 13.89 Table 13.4 Summary of Ten-Gigabit Ethernet implementations

90. Figure 11-13 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998

91. Figure 11-14 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 HDLC Configuration

92. Figure 11-14-continued WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 HDLC Configuration

93. Figure 11-14-continued WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 HDLC Configuration

94. Figure 11-15 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 HDLC Modes

95. Figure 11-16 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 HDLC Frame Types

96. Figure 11-16-continued WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 HDLC Frame Types

97. Figure 11-16-continued WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 HDLC Frame Types

98. Figure 11-17 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 HDLC Flag Field

99. Figure 11-18 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 Bit Stuffing

100. Figure 11-19 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 HDLC Address Field

101. Figure 11-20 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 HDLC Control Field

102. Figure 11-21 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 Poll/Final

103. Figure 11-22 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 HDLC Information Field

104. Figure 11-23 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 HDLC FCS Field

105. Figure 11-24 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998

106. Figure 11-25 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 Use of P/F Field

107. Figure 11-25-continued WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 Use of P/F Field

108. Figure 11-25-continued WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 Use of P/F Field

109. Figure 11-25-continued WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 Use of P/F Field

110. Figure 11-25-continued WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 Use of P/F Field

111. Figure 11-26 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 U-Frame Control Field

112. Figure 11-26-continued WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 U-Frame Control Field

113. Figure 11-27 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 Polling Example

114. Figure 11-28 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 Selecting Example

115. Figure 11-29 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 Peer-to-Peer Example

116. Figure 11-29-continued WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 Peer-to-Peer Example

117. Point-to-Point Access: PPP

118. PPP In a network, two devices can be connected by a dedicated link or a shared link. In the first case, the link can be used by the two devices at any time. We refer to this type of access as point-to-point access. In the second case, the link is shared between pairs of devices that need to use the link. We refer to this type of access as multiple access. In a network, two devices can be connected by a dedicated link or a shared link. In the first case, the link can be used by the two devices at any time. We refer to this type of access as point-to-point access. In the second case, the link is shared between pairs of devices that need to use the link. We refer to this type of access as multiple access. One of the most common protocols for point-to-point access is the Point-to-Point Protocol (PPP). One of the most common protocols for point-to-point access is the Point-to-Point Protocol (PPP).

119. PPP services It defines the format of the frame to be exchanged between devices. It defines the format of the frame to be exchanged between devices. It defines how two devices can negotiate the establishment of the link and the exchanged of data. It defines how two devices can negotiate the establishment of the link and the exchanged of data. It defines how network layer data are encapsulated in the data link frame. It defines how network layer data are encapsulated in the data link frame. It defines how two devices can authenticate each other. It defines how two devices can authenticate each other.

120. PPP FRAME

121. Flag field. The flag fields identify the boundaries of a PPP frame. Its value is 01111110. Flag field. The flag fields identify the boundaries of a PPP frame. Its value is 01111110. Address field. Because PPP is used for a point-to-point connection, it uses the broadcast address of HDCL, 11111111, to avoid a data link address in the protocol. Address field. Because PPP is used for a point-to-point connection, it uses the broadcast address of HDCL, 11111111, to avoid a data link address in the protocol. Control field. The control field uses the format of the U- frame in HDCL. See pages 285-286. Control field. The control field uses the format of the U- frame in HDCL. See pages 285-286. Protocol field. The protocol field defines what is being carried in the data field: user data or other information. Protocol field. The protocol field defines what is being carried in the data field: user data or other information. Data field. This field carries either the user data or other information. Data field. This field carries either the user data or other information. Frame check sequence (FCS) field. This field is used for error detection. Frame check sequence (FCS) field. This field is used for error detection.

122. Transition states A PPP connection goes through different phases called transition sates.

123. Transition States Idle state. The idle state means that the link is not being used. There is no active carrier, and the line is quiet. Idle state. The idle state means that the link is not being used. There is no active carrier, and the line is quiet. Establishing link. When one of the end point starts the communication, the connection goes into the establishing state. In this state, options are negotiated between the two parties. If the negotiation is successful, the system goes to the authenticating state (if authentication is required) or directly to the networking state. Establishing link. When one of the end point starts the communication, the connection goes into the establishing state. In this state, options are negotiated between the two parties. If the negotiation is successful, the system goes to the authenticating state (if authentication is required) or directly to the networking state. Authenticating state. The authenticating state is optional. If the result is successful, the connection goes to the networking state; otherwise, it goes to the terminating state. Authenticating state. The authenticating state is optional. If the result is successful, the connection goes to the networking state; otherwise, it goes to the terminating state.

124. Transition States Networking State. When a connection reaches this state, the exchange of user control and data packets can be started. The connection remains in this state until one of the endpoints wants to terminate the connection. Networking State. When a connection reaches this state, the exchange of user control and data packets can be started. The connection remains in this state until one of the endpoints wants to terminate the connection. Terminating state. When the connection is in the terminating state, several packets are exchanged between the two ends for house cleaning and closing the link. Terminating state. When the connection is in the terminating state, several packets are exchanged between the two ends for house cleaning and closing the link.

125. PPP Stack PPP is a data-link layer protocol, PPP uses a stack of other protocols to establish the link, to authenticate the parties involved, and to carry the network layer data. PPP is a data-link layer protocol, PPP uses a stack of other protocols to establish the link, to authenticate the parties involved, and to carry the network layer data. Three sets of protocols are used by PPP: Link control protocol, authentication protocols, and network control protocol. Three sets of protocols are used by PPP: Link control protocol, authentication protocols, and network control protocol.

126. Protocol stack

127. Link Control Protocol (LCP) It is responsible for establishing, maintaining, configuring, and terminating links. It is responsible for establishing, maintaining, configuring, and terminating links. It also provides negotiation mechanisms to set options between endpoints. Both endpoints of the link must reach an agreement about the options before the link can be established. It also provides negotiation mechanisms to set options between endpoints. Both endpoints of the link must reach an agreement about the options before the link can be established. When PPP is carrying an LCP packet, it is either in the establishing state or in the terminating state. When PPP is carrying an LCP packet, it is either in the establishing state or in the terminating state. All LCP packets are carried in the data field of the PPP frame. What defines the frame as one carrying an LCP packet is the value of the protocol field, which is set to C021 (base 16). All LCP packets are carried in the data field of the PPP frame. What defines the frame as one carrying an LCP packet is the value of the protocol field, which is set to C021 (base 16).

128. LCP packet encapsulated in a frame

129. Link Control Protocol (LCP) Code. This field defines the type of LCP packet. Code. This field defines the type of LCP packet. ID. This field holds a value used to match a request with reply. One endpoint inserts a value in this field, which will be copied in the reply packet. ID. This field holds a value used to match a request with reply. One endpoint inserts a value in this field, which will be copied in the reply packet. Length. This field defines the length of the entire LCP packet. Length. This field defines the length of the entire LCP packet. Information. This field contains extra information needed for some LCP packets. Information. This field contains extra information needed for some LCP packets.

130. Link Control Protocol (LCP) Configuration packets are used to negotiate the options between the two ends. There are four different types of packets for this purpose: configure-request, configure- ack, configure-nak, and configure-reject. Configuration packets are used to negotiate the options between the two ends. There are four different types of packets for this purpose: configure-request, configure- ack, configure-nak, and configure-reject. Link termination packets. The link termination packets are used to disconnect the link between two endpoints. Link termination packets. The link termination packets are used to disconnect the link between two endpoints. There are two types: terminate-request and terminate- ack. Link monitoring and debugging packets. These packets are used for monitoring and debugging the link. There are five types: code-reject, protocol-reject, echo- reply, discard-request. Link monitoring and debugging packets. These packets are used for monitoring and debugging the link. There are five types: code-reject, protocol-reject, echo- reply, discard-request.

131. LCP packets and their codes CodePacket TypeDescription 01 16 Configure-request Contains the list of proposed options and their values 02 16 Configure-ack Accepts all options proposed 03 16 Configure-nak Announces that some options are not acceptable 04 16 Configure-reject Announces that some options are not recognized 05 16 Terminate-request Requests to shut down the line 06 16 Terminate-ack Accepts the shut down request 07 16 Code-reject Announces an unknown code 08 16 Protocol-reject Announces an unknown protocol 09 16 Echo-request A type of hello message to check if the other end is alive 0A 16 Echo-reply The response to the echo-request message 0B 16 Discard-request A request to discard the packet

132. Authentication Protocols Authentication plays a very important role in PPP because PPP is designed for use over dial-up links where verification of user identity is necessary. Authentication plays a very important role in PPP because PPP is designed for use over dial-up links where verification of user identity is necessary. Authentication means validating the identity of a user who needs to access a set of resources. Authentication means validating the identity of a user who needs to access a set of resources. PPP uses two protocols for authentication: Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP) PPP uses two protocols for authentication: Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP)

133. PAP The PAP is a simple authentication procedure with two steps: The PAP is a simple authentication procedure with two steps: 1. The user who wants to access a system sends an ID (identification) and a password. 2. The system checks the validity of the identification and password and either accepts or denies a connection. For those systems that require greater security, PAP is not enough. A third party with access to the link can easily pick up the password and access the system resources. For those systems that require greater security, PAP is not enough. A third party with access to the link can easily pick up the password and access the system resources.

134. PAP

135. PAP packets

136. CHAP The CHAP protocol is a three-way handshaking authentication protocol that provides greater security than PAP. The CHAP protocol is a three-way handshaking authentication protocol that provides greater security than PAP. In this method, the password is kept secret; it is never sent on-line. In this method, the password is kept secret; it is never sent on-line.Steps The system sends to the user a challenge packet containing a challenge value, usually a few bytes. The system sends to the user a challenge packet containing a challenge value, usually a few bytes. The user applies a predefined function that takes the challenge value and the user’s own password and creates a result. The user sends the result in the response packet to the system. The user applies a predefined function that takes the challenge value and the user’s own password and creates a result. The user sends the result in the response packet to the system.

137. CHAP The system does the same. It applies the same function to the password of the user and the challenge value to create a result. If the result created is the same as the result sent in the response packet, access is granted; otherwise, it is denied. The system does the same. It applies the same function to the password of the user and the challenge value to create a result. If the result created is the same as the result sent in the response packet, access is granted; otherwise, it is denied.

138. CHAP

139. CHAP packets

140. Network Control Protocol (NCP) After the link is established and authentication (if any) is successful, the connection goes on the networking state. After the link is established and authentication (if any) is successful, the connection goes on the networking state. NCP is a set of control protocols to allow the encapsulation of data coming from network layer protocols into the PPP frame. NCP is a set of control protocols to allow the encapsulation of data coming from network layer protocols into the PPP frame. The set of packets that establish and terminate a network layer connection is called Internetwork Protocol Control Protocol (IPCP). The set of packets that establish and terminate a network layer connection is called Internetwork Protocol Control Protocol (IPCP).

141. IPCP packet encapsulated in PPP frame

142. Table 12.3 Code value for IPCP packets CodeIPCP Packet 01Configure-request 02Configure-ack 03Configure-nak 04Configure-reject 05Terminate-request 06Terminate-ack 07Code-reject

143. An example