Accessing the WAN – Chapter 2
ITE I Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
1

 Describe the fundamental concepts of point-to-point serial communication including TDM, demarcation point, DTE-DCE functions, HDLC encapsulation, and serial interface troubleshooting.
 Describe PPP concepts including PPP layered architecture, PPP frame structure, PPP session establishment, multiprotocol encapsulation support, link control protocol (LCP), network control protocol (NCP), and Internet Protocol Control Protocol
(IPCP).
 Configure PPP on a serial interface including enabling PPP encapsulation, verifying the PPP connection and troubleshooting encapsulation problems.
 Configure PPP authentication including explaining PAP and
CHAP authentication protocols, configuring PPP authentication using PAP and CHAP, and troubleshooting PPP authentication problems.
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
2

 Concept of serial communication as the basis of WAN technologies
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
3

 All long-haul communications and most computer networks use serial connections, because
 the cost of cable and synchronization difficulties make parallel connections impractical.
 The most significant advantage is simpler wiring.
 Also, serial cables can be longer than parallel cables, because there is much less interaction (crosstalk) among the conductors in the cable.
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
4

 Time Division Multiplexing
 TDM is a Physical layer concept. It has no regard for the nature of the information that is being multiplexed onto the output channel.
 Statistical Time Division Multiplexing
 To overcome the inefficiency of TDM, STDM was introduced as in TDM the slot remains empty if no data to transmit at the sender side.
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
5

 How two or more data streams are transported across a single physical connection using TDM
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
6

–A serial connection has a DTE device at one end of the connection and a DCE device at the other end. The connection between the two DCE devices is the WAN service provider transmission network. In this case:
–The CPE, which is generally a router, is the DTE. The DTE could also be a terminal, computer, printer, or fax machine if they connect directly to the service provider network.
–The DCE, commonly a modem or CSU/DSU, is the device used to convert the user data from the DTE into a form acceptable to the WAN service provider transmission link. This signal is received at the remote
DCE, which decodes the signal back into a sequence of bits. The remote DCE then signals this sequence to the remote DTE.
7
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public

 WAN Encapsulation Protocols
 To ensure that the correct protocol is used, you need to configure the appropriate Layer 2 encapsulation type. The choice of protocol depends on the WAN technology and the communicating equipment.
 HDLC The default encapsulation type on point-to-point connections, dedicated links, and circuit-switched connections when the link uses two
Cisco devices.
 PPP - Provides router-to-router and host-to-network connections
 Serial Line Internet Protocol (SLIP) - A standard protocol for point-topoint serial connections using TCP/IP. SLIP has been largely displaced by
PPP.
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
8

 Frame Relay - Industry standard, switched, Data Link layer protocol that handles multiple virtual circuits. Frame Relay is a next generation protocol after X.25.
 ATM - The international standard for cell relay in which devices send multiple service types (such as voice, video, or data) in fixed-length (53byte) cells. Fixed-length cells allow processing to occur in hardware, thereby reducing transit delays.
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
9

 Flag - The flag field initiates and terminates error checking. The frame always starts and ends with an 8-bit flag field. The bit pattern is 01111110.
 Address - The address field contains the HDLC address of the secondary station. This address can contain a specific address, a group address, or a broadcast address. A primary address is either a communication source or a destination, which eliminates the need to include the address of the primary.
 Control - The control field uses three different formats,
– Information (I) frame : I-frames carry upper layer information and some control information.
– Supervisory (S) frame: S-frames provide control information. An S-frame can request and suspend transmission, report on status, and acknowledge receipt of I-frames.
– Unnumbered (U) frame: U-frames support control purposes
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
10

 Protocol(only used in Cisco HDLC) This field specifies the protocol type encapsulated within the frame
 Data -The data field contains a path information unit (PIU) or exchange identification (XID) information.
 Frame check sequence (FCS)The FCS precedes the ending flag delimiter and is usually a cyclic redundancy check (CRC) calculation remainder.
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
11

 Router (config-if)# encapsulation hdlc
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
12

 show interfaces serial
 show controllers
 Cisco 7000 series routers use a cBus controller card for connecting serial links. With these routers, use the
 show controllers cbus command.
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
13

 What is PPP?
 Recall that HDLC is the default serial encapsulation method when you connect two Cisco routers.
 With an added protocol type field, the Cisco version of HDLC is proprietary.
 Thus, Cisco HDLC can only work with other Cisco devices.
 However, when you need to connect to a non-Cisco router, you should use PPP encapsulation.
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
14

 PPP encapsulates data frames for transmission over Layer 2 physical links.
 PPP establishes a direct connection using serial cables, phone lines, trunk lines, cellular telephones, specialized radio links, or fiber-optic links.
 There are many advantages to using PPP, including the fact that it is not proprietary.
 The link quality management feature monitors the quality of the link. If too many errors are detected, PPP takes the link down.
 PPP supports PAP and CHAP authentication.
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
15

 Purpose and format of each of the fields in a PPP frame
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
16

 Three phases of PPP session establishment
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
17

 PPP can be configured to support various functions including:
 Authentication using either PAP or CHAP
 Compression using either Stacker or Predictor
 Error Detection
 Multilink which combines two or more channels to increase the WAN bandwidth
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
18

 R3#configure terminal
 R3(config)#interface serial 0/0/0
 R3(config-if)#encapsulation ppp
 Compression
 R3(config-if)#compress [predictor | stac]
 Quality
 R3(config-if)#ppp quality 80
 Multilink
 Router(config-if)#ppp multilink –this command perform load balancing
 Verification-- show interfaces serial
 Debug-Packet, negotiation, authentication, compression, etc
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
19

 Password Authentication Protocol (PAP)
 Initiating PAP
PAP provides a simple method for a remote node to establish its identity using a two-way handshake.
 Command is ppp authentication pap
 username name password password
 It must match the user name and password of the other router.
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
20

 Challenge Handshake Authentication Protocol (CHAP)
 Unlike PAP, which only authenticates once, CHAP conducts periodic challenges to make sure that the remote node still has a valid password value.
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
21

 Differentiate between PAP and CHAP
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
22

 Describe how to use PAP to authenticate a PPP connection
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
23

 Describe how to use CHAP to authenticate a PPP connection
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
24

 Explain how to configure a PPP connection with authentication
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
25

 Explain the output of the debug ppp authentication command
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
26

 PPP is a widely used WAN protocol
 PPP provides multi-protocol LAN to WAN connections
 PPP session establishment – 4 phases
Link establishment
Link quality determination
Network layer protocol configuration negotiation
Link termination
 WAN Encapsulation
–HDLC default encapsulation
–PPP
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
27

 PPP authentication
–PAP
•2 way handshake
–CHAP
•3 way handshake
–Use debug ppp authentication to confirm authentication configuration
 PPP configuration
–Done on a serial interface
 After PPP configuration, use show interfaces command to display:
–LCP state
–NCP state
ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
28

ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
29