Download 7.1 WAN Devices and Technology

Implementing
Enterprise WAN
Links
Introducing Routing and Switching in the Enterprise –
Chapter 7
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Objectives

Describe the features and benefits of common WAN
connectivity options.

Compare and configure common WAN
encapsulations.

Describe Frame Relay
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7.1 WAN Devices and Technology
 WAN Services purchased
 Serial transmissions verses Ethernet in LANS
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7.1 WAN Devices and Technology
 Translation device – prepare data for transmission: modem
(analog) or CSU/DSU (digital)
 Central Office (CO) and Customer Premises Equipment
(CPE)
 Demarc - point at which responsibility of the customer ends
and the service provider begins
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7.1 WAN Devices and Technology
 Local loop – last mile (first mile for customer)
 CSU/DSU or modem – controls the rate data moves onto
the loop (DCE), also provides clocking signal to router
(DTE)
 Various physical layer protocols used
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7.1 WAN Devices and Technology
Note: DS0 – Digital Signal 0, T1 (DS1) = 24 DS0s
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Activity 7.1.1.5
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Activity 7.1.1.5
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7.1.2 WAN Standards
Layer 2 WAN protocols:
 LAPF
 HDLC
 PPP
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7.1.2 WAN Standards
Activity 7.1.2.2
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7.1.2 WAN Standards
Activity 7.1.2.2
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7.1.3 Accessing the WAN
 Modems enable POTS to be used for WAN connections
 Modems are also used for DSL and cable connections to ISP
 A modem encodes the information onto that carrier wave before
transmission and then decodes it at the receiving end
 The modulated carrier wave carries information to destination
across the telephone network
 Destination demodulates the carrier and extracts information
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7.1.3 Accessing the WAN
 DSO channel = time slice of the physical bandwidth
 Fractional connections – part of a T1
 Two techniques which information from multiple channels can be
allocated bandwidth on a single cable based on time:
Time Division Multiplexing (TDM) - if a sender has nothing to say, its time
slice goes unused, wasting valuable bandwidth.
Statistical-Time Division Multiplexing (STDM) - dynamically reassigns
unused time slices on an as-needed basis, minimizes wasted bandwidth
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Activity 7.1.3.4
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Activity 7.1.3.4
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7.1.4 Packet and Circuit Switching
 Dedicated Leased Line – point-to-point serial
 Circuit switching:
Dedicated bandwidth
Higher cost
Higher level of security
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7.1.4 Packet and Circuit Switching
 Packet switching:
Bandwidth efficiency
Identifier on each packet
Preconfigured, but non-exclusive, link
 Cell switching:
high-speed packet switching
ATM (fixed length cells)
Large amount of overhead
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7.1.4 Packet and Circuit Switching
 Virtual circuits
Switched virtual circuits - dynamically established between two
points when a router requests a transmission
Permanent virtual circuits - provides a permanent path to
forward data between two points (Frame Relay)
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7.1.5 Last mile and long range WAN
technologies
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7.1.5 Last mile and long range WAN
technologies
 Elements of Layer 2 encapsulations
Flag
Address
Control
Protocol
Data
FCS
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7.1.5 Last mile and long range WAN
technologies
 Great Distances with Fiber
Synchronous Optical Network (SONET)
Synchronous Digital Hierarchy (SDH)
SDH and SONET are used for moving both voice and data.
New developments for extremely long - dense wavelength
division multiplexing (DWDM) - can carry IP, SONET, and ATM
data concurrently
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Activity 7.1.5.3
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Activity 7.1.5.3
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7.2.1 Ethernet and WAN Encapsulations
 Encapsulation occurs before data travels across the
WAN
 Layer 2 adds header information specific to the type of
physical network transmission
LAN – Ethernet
WAN – depends on link
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7.2.2 Comparing Common WAN Encapsulations
 Data Link Layer encapsulation may change continuously
to match the technology in use
 Network Layer encapsulation will not change
 Packets exit the LAN by way of the default gateway
router
 Router strips off the Ethernet frame and then reencapsulates that data into the correct frame type for the
WAN - acts as a media converter, by adapting the Data
Link Layer frame format to a format that is appropriate to
the interface
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7.2.2 Comparing Common WAN Encapsulations
 Standard bit-oriented Layer 2 encapsulation: HDLC
 Cisco HDLC: additional Type field
Default WAN encapsulation on Cisco devices
Allows multiple Network Layer protocols to share a link
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7.2.2 HDLC and PPP
Point-to-Point Protocol (PPP):
 Data Link Layer
encapsulation for serial links
 Uses a layered architecture
to encapsulate & carry multiprotocol datagrams over a
point-to-point link
 Standards-based
 Support
Asynchronous serial
Synchronous serial
Support High-Speed Serial
Interface (HSSI)
Integrated Services Digital
Network (ISDN)
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7.2.2 HDLC and PPP
 PPP has two sub-protocols:
Link Control Protocol - responsible for establishing, maintaining and
terminating the point-to-point link.
Network Control Protocol - provides interaction with different Network
layer protocols.
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7.2.2 HDLC and PPP
 Link Control Protocol negotiates:
Authentication – PAP and CHAP
compression
error detection
multilink
PPP callback
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7.2.2 HDLC and PPP
 Phases of PPP sessions:
Link-establishment - Receipt of the configuration
acknowledgement frame completes this phase
Authentication (optional) - provides password protection to
identify connecting routers
NCP negotiation - The show interfaces command reveals the
LCP and NCP states.
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Activity 7.2.2.5
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Activity 7.2.2.5
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7.2.3 Configuring PPP
 Change encapsulation from HDLC to PPP on both ends
of link
 Configure desired optional features
ppp multilink
Configures load balancing across multiple links.
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7.2.3 Configuring PPP
Verification and troubleshooting commands:
 show interfaces serial - Displays the encapsulation and
the states of the Link Control Protocol (LCP).
 show controllers - Indicates the state of the interface
channels and whether a cable is attached to the
interface
 debug serial interface - Verifies the incrementation of
keepalive packets.
 debug ppp - Provides information about the various
stages of the PPP process, including negotiation and
authentication
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7.2.3.3 Configuring PPP
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7.2.4 PPP Authentication
 Occurs after establishment of the link but before the
Network Layer protocol configuration
 Password Authentication Protocol (PAP)
PAP sends the username/password pair across the link
repeatedly in clear text
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7.2.4 PPP Authentication
 Challenge Authentication Protocol (CHAP)
uses a three-way handshake.
1.PPP establishes the link phase.
2.Local router sends a challenge message to the remote router.
3. Remote router uses the challenge and a shared secret password to generate a
one-way hash.
4. Remote router sends back one-way hash to the local router.
5. Local router checks the response against its own calculation, using the
challenge and the same shared secret.
6. Local router acknowledges authentication if values match.
7. Local router immediately terminates connection if the values do not match.
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7.2.4 PPP Authentication
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7.2.4 PPP Authentication
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7.2.5.3 LAB – Configuring PAP and CHAP
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Describe Frame Relay
 Nonbroadcast multiaccess network
 Packet switching with variable length packets
 STDM
 Virtual circuit between two DTE devices
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Describe Frame Relay
 Data link connection identifier (DLCI)
 Inverse ARP
 LMI
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Describe Frame Relay
Service parameters:
 Committed information rate (CIR)
 Excess information rate (EIR)
 Discard eligible (DE) frames
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Describe Frame Relay
Traffic management:
 Forward explicit congestion notification (FECN)
 Backward explicit congestion notification (BECN)
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Summary
 Circuit switching WAN technologies create a physical
circuit between end devices before sending data
 Packet and cell switching WAN technologies use virtual
circuits to send data across the network
 Layer 2 encapsulation changes as frames move across
a WAN
 PPP permits many advanced features including
authentication, compression, and load balancing
 Frame Relay is a packet switched technology using
switched or permanent virtual circuits
 Frame Relay uses parameters such as CIR to establish
the bandwidth used on each VC
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