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Point-to-Point Protocol
(PPP)
點對點網路協定
CCNA Exploration
Accessing the WAN – Chapter 2
Introduction to Serial Communication


With a serial connection (串列連線) ,
information is sent across one wire, one data
bit at a time.
In reality, it is often the case that serial links
can be clocked considerably faster than
parallel links, and they achieve a higher data
rate.
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Serial Communication Standards

There are three key serial communication
standards affecting LAN-to-WAN
connections:

RS-232
• A serial port is a general-purpose interface that can be
used for almost any type of device including modems.

V.35
• V.35 is the interface standard used by most routers and
DSUs that connect to T1 carriers.

HSSI
• It supports transmission rates up to 52 Mb/s.
• Engineers use HSSI to connect routers on LANs with
WANs over high-speed lines such as T3 lines.
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Time Division Multiplexing (分時多工)


Bell Laboratories invented time-division
multiplexing (TDM) to maximize the amount
of voice traffic carried over a medium.
TDM divides the bandwidth of a single link
into separate channels or time slots.

The channels take turns using the link.
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Time Division Multiplexing

At sending end:


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
A multiplexer (MUX,多工器) at the transmitter
accepts different signals.
The MUX breaks each signal into segments.
The MUX puts each segment into a single channel
by inserting each segment into a timeslot.
At receiving end:

A MUX at the receiving end reassembles the TDM
stream into the different data streams based only
on the timing of the arrival of each bit.
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Statistical Time Division Multiplexing

Problem of TDM:


TDM shares this inefficiency when traffic is intermittent,
because the time slot is still allocated even when the
channel has no data to transmit.
Statistical time-division multiplexing (STDM) uses a
variable time slot length allowing channels to
compete for any free slot space.

STDM requires each transmission to carry identification
information (a channel identifier).
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TDM Examples - ISDN and SONET

An example of a technology that uses
synchronous TDM is ISDN.


ISDN basic rate (BRI) has three channels
consisting of two 64 kb/s B-channels (B1 and B2),
and a 16 kb/s D-channel.
The TDM has nine timeslots, which are repeated
in the sequence shown in the following figure.
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TDM Examples - ISDN and SONET

On a larger scale, the telecommunications
industry uses the SONET or SDH standard
for optical transport of TDM data.

SONET/SDH takes n bit streams, multiplexes them,
and optically modulates the signal, sending it out
using a light emitting device over fiber with a bit
rate equal to (incoming bit rate) x n.
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TDM Examples - ISDN and SONET

The original unit used in multiplexing
telephone calls is 64 kb/s for one phone call.


It is referred to as a DS-0 or DS0 (digital signal
level zero).
In North America, 24 DS0 units, outside North
America, 32 DS0 units.
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TDM Examples - ISDN and SONET

T-carrier refers to the bundling of DS0s and
E-carrier Hierarchy is similar to T-carrier.
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Demarcation Point
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DTE-DCE

From the point of view of connecting to the
WAN, a serial connection has a DTE device at
one end of the connection and a DCE device
at the other end.
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DTE-DCE

The router end of the shielded serial transition cable
may be a DB-60 connector, which connects to the
DB-60 port on a serial WAN interface card.

The router interface end of the Smart Serial cable is
a 26-pin connector.
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DTE-DCE

Router-to-router connection
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DTE-DCE

Parallel to Serial Conversion

Your PC also has a Universal Asynchronous
Receiver/Transmitter (UART) chip on the
motherboard.
• The UART is the DTE agent of your PC and
communicates with the modem or other serial device.
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WAN Encapsulation Protocols

To ensure that the correct protocol is used
on the WAN link, you need to configure the
appropriate Layer 2 encapsulation type.
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HDLC Encapsulation



HDLC is a synchronous Data Link layer bit-oriented
protocol developed by ISO.
HDLC provides both connection-oriented and
connectionless service.
HDLC defines a Layer 2 framing structure that allows
for flow control and error control through the use of
acknowledgments.


Each frame has the same format, whether it is a data frame
or a control frame.
HDLC uses a frame delimiter, or flag, to mark the beginning
and the end of each frame.
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HDLC Encapsulation


Cisco has developed an extension to the
HDLC protocol to solve the inability to
provide multiprotocol support.
Cisco HDLC frames contain a field for
identifying the network protocol being
encapsulated.
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HDLC Frame Field
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HDLC Frame Field

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.
Protocol (only used in Cisco HDLC)

This field specifies the protocol type
encapsulated within the frame (e.g. 0x0800 for IP).
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HDLC Frame Field

Control

The control field uses three different formats,
depending on the type of HDLC frame used:
• Information (I) frame:
– I-frames carry upper layer information and some control
information.
– This frame sends and receives sequence numbers, and the
poll final (P/F) bit performs flow and error control.
• 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 and are not sequenced.
– A U-frame can be used to initialize secondaries.
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HDLC Frame Field

Data



The data field contains a path information unit
(PIU) or exchange identification (XID) information.
Variable length field (L3 packets).
Frame check sequence (FCS)


The FCS is usually a cyclic redundancy check
(CRC) calculation remainder.
The CRC calculation is redone in the receiver.
• If the result differs from the value in the original frame,
an error is assumed.
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Configuring HDLC Encapsulation



Cisco HDLC is the default encapsulation method
used by Cisco devices on synchronous serial lines.
If the default encapsulation method has been
changed, use the encapsulation hdlc command to reenable HDLC.
Two steps :
 Enter the interface configuration mode of the
serial interface.
 Enter the encapsulation hdlc command to specify
the encapsulation protocol on the interface.
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Troubleshooting a Serial Interface

The show interface serial command returns
any of the following five possible problem
states in the interface status line:

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Serial x is down, line protocol is down
Serial x is up, line protocol is down
Serial x is up, line protocol is up (looped)
Serial x is up, line protocol is down (disabled)
Serial x is administratively down, line protocol is
down
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Troubleshooting a Serial Interface

The show controllers command is used to
indicate the state of the interface channels
and whether a cable is attached to the
interface.
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Point-to-Point Protocol (PPP)
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

PPP establishes a direct connection using
serial cables, phone lines, trunk lines,
cellular telephones, specialized radio links,
or fiber-optic links.
When you need to connect to a non-Cisco
router, you should use PPP encapsulation.
Advantages of using PPP

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.
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Point-to-Point Protocol (PPP)

PPP contains three main components:



HDLC protocol for encapsulating datagrams over
point-to-point links.
Extensible Link Control Protocol (LCP) to
establish, configure, and test the data link
connection.
Family of Network Control Protocols (NCPs) for
establishing and configuring different Network
layer protocols.
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PPP Architecture

PPP and OSI share the same Physical layer,
but PPP distributes the functions of LCP and
NCP differently.
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PPP Architecture

At the physical layer, you can configure PPP
on a range of interfaces, including:




Asynchronous serial
Synchronous serial
HSSI
ISDN
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PPP Architecture

Link Control Protocol Layer


LCP has a role in establishing, configuring, and
testing the data-link connection.
The LCP provides automatic configuration of the
interfaces at each end, including:
•
•
•
•
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Handling varying limits on packet size
Detecting common misconfiguration errors
Terminating the link
Determining when a link is functioning properly or when
it is failing
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PPP Architecture

Network Control Protocol Layer

PPP permits multiple network layer
protocols to operate on the same
communications link.
• For every network layer protocol used, a
separate Network Control Protocol (NCP) is
provided.
• For example, Internet Protocol (IP) uses the IP
Control Protocol (IPCP), and Internetwork
Packet Exchange (IPX) uses the Novell IPX
Control Protocol (IPXCP).
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PPP Frame

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Flag - Indicates the beginning or end of a frame (01111110).
Address - Consists of the standard broadcast address,
11111111.
Control - A connectionless link service is provided.
Protocol - identify the protocol encapsulated in the data field of
the frame.
Data - 0 or more bytes that contain the datagram for the
protocol specified in the protocol field. The default maximum
length of the data field is 1500 bytes.
FCS - for error control purposes.
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Establishing a PPP Session
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Establishing a PPP Session

Phase 1:

Link establishment and configuration negotiation
• Before PPP exchanges any Network layer datagrams (for
example, IP), the LCP must first open the connection and
negotiate configuration options.
• This phase is complete when the receiving router sends
a configuration-acknowledgment frame back to the
router initiating the connection.

Phase 2:

Link quality determination (optional)
• The LCP tests the link to determine whether the link
quality is sufficient to bring up Network layer protocols.
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Establishing a PPP Session

Phase 3:

Network layer protocol configuration
negotiation
• After the LCP has finished the link quality
determination phase, the appropriate NCP can
separately configure the Network layer
protocols, and bring them up and take them
down at any time.
• If the LCP closes the link, it informs the
Network layer protocols so that they can take
appropriate action.
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Establishing a PPP Session
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LCP Operation

LCP operation uses three classes of LCP
frames to accomplish the work of each of the
LCP phases:



Link-establishment frames establish and
configure a link (Configure-Request, ConfigureAck, Configure-Nak, and Configure-Reject)
Link-maintenance frames manage and debug a
link (Code-Reject, Protocol-Reject, Echo-Request,
Echo-Reply, and Discard-Request)
Link-termination frames terminate a link
(Terminate-Request and Terminate-Ack)
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PPP Configuration Options

PPP can be configured to support various functions
including:

Authentication (認證) using either PAP or CHAP


Compression using either Stacker or Predictor
Multilink (多連結) which combines two or more channels to
increase the WAN bandwidth
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NCP Process



After the LCP has configured and
authenticated the basic link, the appropriate
NCP is invoked to complete the specific
configuration of the Network layer protocol
being used.
When the NCP has successfully configured
the Network layer protocol, the network
protocol is in the open state on the
established LCP link.
Reference
 http://www.netadmin.com.tw/article_conte
nt.asp?sn=0805270013
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PPP Configuration Commands

Enabling PPP on an Interface

The following example enables PPP
encapsulation on serial interface 0/0/0:
• R3#configure terminal
• R3(config)#interface serial 0/0/0
• R3(config-if)#encapsulation ppp

Configuring PPP compression

To configure compression over PPP, enter the
following commands:
• R3(config)#interface serial 0/0/0
• R3(config-if)#encapsulation ppp
• R3(config-if)#compress [predictor | stac]
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Verifying and debugging
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Verifying and debugging
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Password Authentication Protocol (PAP)

Characteristics of PAP


Two-way process (2-way handsake,雙向握手)
PAP is not a strong authentication
protocol.
• Passwords are sent across the link in clear text
and there is no protection from playback or
repeated trial-and-error attacks.


The remote node is in control of the
frequency and timing of the login attempts.
Authentication process performs once.
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Password Authentication Protocol (PAP)

When to use PAP

PAP may be used in the following
environments:
• A large installed base of client applications that
do not support CHAP
• Incompatibilities between different vendor
implementations of CHAP
• Situations where a plaintext password must be
available to simulate a login at the remote host
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Password Authentication Protocol (PAP)

Initiating PAP

Completing PAP
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Challenge Handshake Authentication Protocol (CHAP)

Characteristics of CHAP



CHAP is used at the startup of a link and
periodically verifies the identity of the
remote node.
The local router or a third-party
authentication server is in control of the
frequency and timing of the challenges.
CHAP provides protection against
playback attack through the use of a
variable challenge value that is unique and
unpredictable.
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Challenge Handshake Authentication Protocol (CHAP)

Initiating CHAP (By remote router)

Responding CHAP (By local router)
Hash value
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Challenge Handshake Authentication Protocol (CHAP)

Completing CHAP (By remote router)
Reference:
http://en.wikipedia.org/wiki/Challenge-handshake_authentication_protocol
http://www.cisco.com/en/US/tech/tk713/tk507/technologies_tech_note09186a00800b4131.shtml
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PPP Encapsulation and Authentication Process
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Configuring PPP Authentication

Configuring PAP authentication
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Configuring PPP Authentication

Configuring CHAP authentication
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End
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