Transcript WAN Basics

Module 6:
WAN Basics
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© 1999, Cisco Systems, Inc.
Agenda
• WAN Basics
• Transmission Options
• WAN Requirements &
Solutions
CSE: Networking Fundamentals—WAN Basics
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WAN Basics
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What Is a WAN?
• A network that serves users across a broad
geographic area
• Often uses transmission devices provided
by public carriers (Pacific Bell, AT&T, etc.)
–This service is commonly referred to as
“plain old telephone service” (POTS)
• WANs function at the lower three layers of
the OSI reference model
–Physical layer, data link layer, and network
layer
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WAN Devices
WAN Switch
Switches traffic such as Frame Relay,
X.25, and SMDS, and operates at the
data link layer
Modem
Interprets digital and analog signals,
enabling data transmission over
telephone lines
Access Server A concentration point for dial-in and
dial-out connections
CSU/DSU
Adapts a terminal physical interface to a
switch interface in a switched-carrier
network
ISDN Terminal Connects ISDN Basic Rate Interface
(BRI) to other interfaces, such as
EIA/TIA-232
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WAN Terminating Equipment
Physical Cable Types
Router
WAN Provider
(Carrier) Network
EIA/TIA-232
V.35
X.21
HSSI
To Corporate
Network
Modem
Usually on the
Customer’s
Premises
DCE
DTE
Data Terminal Equipment
The Customer’s
Equipment
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Data Circuit-Terminating Equipment
The Service Providers
Equipment
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Circuit Switching
Modem
Modem
WAN
• Dedicated physical circuit established, maintained,
and terminated through a carrier network for each
communication session
• Datagram and data stream transmissions
• Operates like a normal telephone call
• Example: ISDN
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Packet Switching
Multiplexing
Demultiplexing
Modem
Modem
WAN
• Network devices share a point-to-point link to
transport packets from a source to a destination
across a carrier network
• Statistical multiplexing is used to enable devices to
share these circuits
• Examples: ATM, Frame Relay, SMDS, X.25
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WAN Virtual Circuits
• A logical circuit ensuring reliable
communication between two devices
• Switched virtual circuits (SVCs)
– Dynamically established on demand
– Torn down when transmission is complete
– Used when data transmission is sporadic
• Permanent virtual circuits (PVCs)
– Permanently established
– Save bandwidth for cases where certain
virtual circuits must exist all the time
• Used in Frame Relay, X.25, and ATM
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Physical
Layer
OSI Reference Model
CSE: Networking Fundamentals—WAN Basics
SDLC
PPP
LAPB
X.21bis
MAC
Sublayer
SMDS
Data
Link
Layer
LLC
Sublayer
HDLC
Network Layer
Frame Relay
X.25 PLP
WAN Protocols
EIA/TIA-232
EIA/TIA-449
V.24 V.35
HSSI G.703
EIA-530
WAN Protocols
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WAN Protocols
SDLC
IBM’s SNA data link layer communications
protocol
HDLC
Bit-oriented synchronous data link layer protocol
LAPB
Data link layer protocol in the X.25 protocol stack
PPP
Provides router-to-router and host-to-network
connections over sync and async circuits
X.25
Defines connections for remote terminal access
and computer communications in PDNs
ISDN
Permits telephone networks to carry data, voice,
and other source traffic
Frame Relay
Switched data link layer protocol that handles
multiple virtual circuits using HDLC; replacing
X.25 due to higher efficiency
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Transmission
Options
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Transmission Options or WAN
Services
Type of Service
Analog or Digital
Permanent
or Temporary
POTS
Analog
Temporary
ISDN
Digital
Temporary
Leased line —T1/E1
Digital
Permanent
Frame Relay
Digital
Permanent
X.25
Digital
Permanent
DSL
Digital
Temporary
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POTS Using Modem Dialup
Modem
Corporate Network
Telecommuters
Mobile
Users
•
•
•
•
•
•
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Basic
Telephone
Service
Server
Modem
Access Router
Widely available
Easy to set up
Dial on demand
Asynchronous transmission
Low cost, usage-based
Lower bandwidth access requirements
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Integrated Services Digital
Network (ISDN)
LAN
Server
ISDN
Telecommuter/AfterHours, Work-at-Home
BRI
2B+D
BRI/PRI
23B+D
30B+D (Europe)
Company Network
• High bandwidth
• Up to 128 Kbps per basic rate interface
• Dial on demand
• Multiple channels
• Fast connection time
• Monthly rate plus cost-effective,
usage-based billing
• Strictly digital
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ISDN
Basic Rate Interface (BRI)
Primary Rate Interface (PRI)
23B
2B
{
D
64 Kbps
64 Kbps
16 Kbps
}
64 Kbps
144
Kbps
30B
64 Kbps
}
1.536
Mbps
D
• One physical connection
to the ISDN network
• Two logical connections
• Used at remote
telecommuter site
CSE: Networking Fundamentals—WAN Basics
• One physical connection
to the ISDN network
• 23 logical connections
(U.S./Canada)
• 30 logical connections
(Europe)
• Used at central site
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Leased Line
• One connection per physical interface
• Bandwidth: 56 kbps–1.544 Mbps
– T1/E1 and fractional T1/E1
• Cost effective at 4–6 hours daily usage
• Dedicated connections with predictable
throughput
• Permanent
• Cost varies by distance
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Frame Relay
• Permanent, not dialup
• Multiple connections per
physical interface
(permanent virtual circuits)
• Efficient handling of
bursty (peak performance
period) data
• Guaranteed bandwidth
(typical speeds are
56/64 Kbps, 256 Kbps,
and 1.544 Mbps)—
committed information
rate (CIR)
• Cost varies greatly by region
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Permanent Virtual Circuit (PVC)
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Connecting Offices with Frame
Relay
Frame
Relay
What You
Pay For
Peak
{
CIR
{
Traffic
Free If
Available
Time
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X.25
DTE
DTE
DCE
X.25
DCE
•
•
•
•
Very robust protocol for low-quality lines
Packet-switched
Bandwidth: 9.6 kbps–64 kbps
Well-established technology;
large installed base
• Worldwide availability
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Digital Subscriber Line (xDSL)
End User
Ethernet
DSL
“Modem”
DSL
ATM
Corporate
Network
Copper Loop
DSL
“Modem”
Server
• DSL is a pair of “modems” on each end of a copper wire pair
• DSL converts ordinary phone lines into high-speed data conduits
• Like dial, cable, wireless, and T1, DSL by itself is a transmission
technology, not a complete end-to-end solution
• End-users don’t “buy” DSL, they “buy” services, such as highspeed Internet access, intranet, leased line, voice, VPN, and video
on demand
• Service is limited to certain geographical areas
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DSL Modem Technology
DSL Technology
Max. Data Rate
Line Coding
Down/Uplink (bps) Technology
VDSL—
Very-high-data-rate DSL
Baseband
Voice?
Max. Reach
Feet (km)
Key Attributes
51–55M / 1.6–2.3M
13M / 1.6–2.3M
TBD
Yes
1,000 (0.3)
4,500 (1.5)
Very fast—Short reach
No standard yet
ADSL—
Asymmetric DSL
8M / 1M
1.5M / 640K
CAP, DMT,
G.lite
Yes
18,000 (5.5)
Coexists with POTS
Technology of choice
for residential
IDSL—
ISDN DSL
144K / 144K
2B1Q
No
18,000 (5.5)+
(w/repeaters)
Uses existing ISDN CPE
Relatively slow
SDSL—
Symmetric DSL
768K / 768K
2B1Q/CAP
No
22,000 (6.9)
Symmetric
No standard
1.5M–2M / 1.5M–2M
(T1–E1 Symmetric)
OPTIS
No
15,000 (4.6)
Standard still under
development
HDSL2—
High-data-rate DSL
• Trade-off is reach versus bandwidth
• Reach numbers are best-case assuming “clean copper”
• Different Layer 1 transmission technologies, need a
common upper protocol layer to tie them together
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Asynchronous Transfer Mode
(ATM)
• Technology capable of transferring voice, video, and
data through private and public networks
• Uses VLSI technology to segment data, at high
speeds, into units called cells
– 5 bytes of header information
– 48 bytes of payload
– 53 bytes total
Header
5
Data
48
• Cells contain identifiers that specify the data stream
to which they belong
• Capable of T3 (44 Mbps), E3 (34 Mbps), and SONET
transmission speeds (OC-1 at 51.84 Mbps to OC-12+)
• Primarily used in enterprise backbones or WAN links
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Which Service?
• Before deciding,
determine the answers
to some questions:
?
?
– Will employees use the Internet
frequently?
– Will the Internet be used for
conducting business?
– Is a large volume of traffic
between branch offices of the
business anticipated?
– Is videoconferencing or video
training needed between
locations?
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How Services Stack Up
• Frame Relay
56,000 bps to 45,000,000 bps
• ISDN BRI
56,000 to 128,000 bps
• ISDN PRI
1,544,000 bps
• T1/E1/DS1
1,544,000 bps/2,090,000 bps
• Analog modems
33,600 bps
• 56K modems
56,000 bps
• Cable modems
30,000,000 bps
• ADSL modems
9,000,000 bps
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Wide-Area Network
Requirements
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Wide-Area Network
Requirements
•
•
•
•
•
•
Minimize bandwidth costs
Maximize efficiency
Maximize performance
Support new/emerging applications
Maximize availability
Minimize management and maintenance
•
•
•
•
•
•
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Multiservice consolidation
Bandwidth efficiency
Performance and QoS guarantees
Emerging IP services
Carrier-class reliability
Ease of operation and management
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Manage Bandwidth to
Control Cost
Hardware
Costs 8.0%
• Dial-on-demand routing
Software
Costs 2.7%
Maintenance
1.5%
• Bandwidth on demand
• Snapshot routing
• IPX protocol spoofing
• Compression
Transmission Costs
87.8%
Source: Data Communications
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Dial-on-Demand Routing
Main Office
PSTN
Remote Site
Interesting
Traffic
• Dials connection only when needed
• Ideal for low-volume, periodic traffic
• Controls usage costs
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Bandwidth-on-Demand
Main Office
PSTN
Remote Site
Start File
Transfer
• Adds bandwidth when needed
• Configurable thresholds
• Controls usage costs
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Snapshot Routing
Update Request
ISDN
Routing
Table
Link Up
Routing Updates
Routing
Table
Routing
Table
Updated
• Controls exchange of routing updates
• Client initiates request
• Server responds
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IPX Protocol Spoofing
• Without spoofing
High overhead traffic
across WAN
• With spoofing
Spoofing
Much reduced overhead
across WAN
Spoofing
• LAN traffic can be very chatty
• WAN links are expensive
• Solution: Limit unnecessary traffic across WAN
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Compression
• Three types
– Header
– Link
– Payload
• Van Jacobson header compression
– RFC 1144
– Reduces header from 40 to ~5 bytes
TCP/IP Header
Data
CRC
Compression
hdr
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Data
CRC
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Dial Backup
Primary
X
DSU/CSU
Secondary
Autodial
• If a primary link goes down or is too busy
• Load balancing
• Completely customizable
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WAN Summary
• Operate beyond the local LAN’s control
• Customers pay telephone service providers
for WAN connections such as ISDN, xDSL,
Frame Relay, leased line, X.25, etc.
• Switching methods include point-to-point,
circuit switching, packet switching,
dialup, and WAN virtual circuits
• Key devices include WAN switches,
access servers, modems, and CSU/DSUs
• Bandwidth optimization features are
essential for controlling WAN costs
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Presentation_ID
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