Multiple Access

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Multiple Access

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.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.Media access control procedure are ensure that every station get a fair chance to transmit and avoid the collisionWhen a number of user station share a single transmission medium. this is called as Multiple access communication.

2

3

Outline

Multiple access mechanisms

Random access

Controlled access

Channelization

4

Sublayers of Data Link Layer

5

Multiple Access Mechanisms

Random Access

7

Random Access

Also called

contention-based

access

No station is assigned to control another

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.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.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

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9

ALOHA Network

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 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.

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.

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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)

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Frames in Pure ALOHA

13

ALOHA Protocol

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ALOHA: Vulnerable Time

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ALOHA: Throughput

Assume number of stations trying to transmit follow Poisson Distribution

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

S

max

= 0.184 when G= 1/2

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Slotted ALOHA

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Slotted ALOHA: Vulnerable Time

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Slotted ALOHA: Throughput

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

S

max

= 0.368 when G= 1

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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

1000 frames per second

500 frames per second

250 frames per second

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CSMA

C

arrier

S

ense

M

ultiple

A

ccess

"Listen before talk"

Reduce the possibility of collision

But cannot completely eliminate it

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Collision in CSMA

B

C

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CSMA: Vulnerable Time

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Persistence Methods

What a station does when channel is idle or busy

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

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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-persistentWhen 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.

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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.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

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Persistence Methods

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CSMA/CD

Carrier Sense Multiple Access with Collision DetectionStation monitors channel while sending a frame

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Energy Levels

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CSMA/CD: Flow Diagram

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CSMA/CA

Carrier Sense Multiple Access with Collision AvoidanceUsed in a network where collision cannot be detectedE.g., wireless LAN

IFS – Interframe Space

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CSMA/CA: Flow Diagram

contention window size is 2

K-1

After each slot:

- If idle, continue counting

- If busy, stop counting

Controlled Access

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Control Access

A station must be authorized by someone (e.g., other stations) before transmitting

Three common methods:

Reservation

Polling

Token passing

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Reservation Method

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Polling Method

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Token Passing

Channelization

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Channelization

Similar to

multiplexing

Three schemes

Frequency-Division Multiple Access (FDMA)

Time-Division Multiple Access (TDMA)

Code-Division Multiple Access (CDMA)

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FDMA

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TDMA

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CDMA

One channel carries all transmissions at the same timeEach channel is separated by code

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CDMA: Chip Sequences

Each station is assigned a unique chip sequenceChip sequences are orthogonal vectorsInner product of any pair must be zeroWith N stations, sequences must have the following properties:They are of length NTheir self inner product is always N

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CDMA: Bit Representation

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Transmission in CDMA

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CDMA Encoding

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Signal Created by CDMA

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CDMA Decoding

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Sequence Generation

Common method: Walsh TableNumber of sequences is always a power of two

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Example: Walsh Table

Find chip sequences for eight stations

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Example: Walsh Table

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

WIRED LAN ETHERNET

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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:

Figure 13.1

IEEE standard for LANs

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:

Figure 13.3

Ethernet evolution through four generations

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Figure 13.4

802.3 MAC frame

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Figure 13.5

Minimum and maximum lengths

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Frame length:

Minimum: 64 bytes (512 bits)

Maximum: 1518 bytes (12,144 bits)

Note

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Figure 13.6

Example of an Ethernet address in hexadecimal notation

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Figure 13.7

Unicast and multicast addresses

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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

13.63

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

Note

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

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Figure 13.8

Categories of Standard Ethernet

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Figure 13.9

Encoding in a Standard Ethernet implementation

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Figure 13.10

10Base5 implementation

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Figure 13.11

10Base2 implementation

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Figure 13.12

10Base-T implementation

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Figure 13.13

10Base-F implementation

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Table 13.1 Summary of Standard Ethernet implementations

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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:

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Figure 13.14

Sharing bandwidth

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Figure 13.15

A network with and without a bridge

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Figure 13.16

Collision domains in an unbridged network and a bridged network

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Figure 13.17

Switched Ethernet

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Figure 13.18

Full-duplex switched Ethernet

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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:

13.79

Figure 13.19

Fast Ethernet topology

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Figure 13.20

Fast Ethernet implementations

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Figure 13.21

Encoding for Fast Ethernet implementation

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Table 13.2 Summary of Fast Ethernet implementations

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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:

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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

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Figure 13.22

Topologies of Gigabit Ethernet

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Figure 13.23

Gigabit Ethernet implementations

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Figure 13.24

Encoding in Gigabit Ethernet implementations

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Table 13.3 Summary of Gigabit Ethernet implementations

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Table 13.4 Summary of Ten-Gigabit Ethernet implementations

Figure 11-13

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Figure 11-14

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HDLC Configuration

Figure 11-14-continued

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HDLC Configuration

Figure 11-14-continued

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HDLC Configuration

Figure 11-15

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HDLC Modes

Figure 11-16

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HDLC Frame Types

Figure 11-16-continued

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HDLC Frame Types

Figure 11-16-continued

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HDLC Frame Types

Figure 11-17

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HDLC Flag Field

Figure 11-18

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Bit Stuffing

Figure 11-19

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HDLC Address Field

Figure 11-20

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HDLC Control Field

Figure 11-21

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Poll/Final

Figure 11-22

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HDLC Information Field

Figure 11-23

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HDLC FCS Field

Figure 11-24

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Figure 11-25

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Use of P/F Field

Figure 11-25-continued

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Use of P/F Field

Figure 11-25-continued

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Use of P/F Field

Figure 11-25-continued

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Use of P/F Field

Figure 11-25-continued

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Use of P/F Field

Figure 11-26

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U-Frame Control Field

Figure 11-26-continued

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U-Frame Control Field

Figure 11-27

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Polling Example

Figure 11-28

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Selecting Example

Figure 11-29

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Peer-to-Peer Example

Figure 11-29-continued

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Peer-to-Peer Example

Point-to-Point

Access:

PPP

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

.

One of the most common protocols for point-to-point access is the

Point-to-Point Protocol

(PPP).

PPP services

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 network layer data are encapsulated in the data link frame.

It defines how two devices can authenticate each other.

PPP FRAME

PPP FRAME

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.

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.

Data field

. This field carries either the user data or other information.

Frame check sequence (FCS) field

. This field is used for error detection.

Transition states

A PPP connection goes through different phases called transition sates.

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.

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.

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.

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.

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.

Three sets of protocols are used by PPP: Link control protocol, authentication protocols, and network control protocol.

Protocol stack

Link Control Protocol (LCP)

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.

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).

LCP packet encapsulated in a frame

Link Control Protocol (LCP)

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.

Length

. This field defines the length of the entire LCP packet.

Information

. This field contains extra information needed for some LCP packets.

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.

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

.

LCP packets and their codes

Code

Packet Type

Description

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

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 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)

PAP

The PAP is a simple authentication procedure with two steps:

The user who wants to access a system sends an ID (identification) and a password.

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.

PAP

PAP packets

CHAP

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.

Steps

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.

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.

CHAP

CHAP packets

Network Control Protocol (NCP)

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.

The set of packets that establish and terminate a network layer connection is called Internetwork Protocol Control Protocol (IPCP).

IPCP packet encapsulated in PPP frame

Table 12.3 Code value for IPCP packets

Code

IPCP Packet

01

Configure-request

02

Configure-ack

03

Configure-nak

04

Configure-reject

05

Terminate-request

06

Terminate-ack

07

Code-reject

An example