wide area networks (wans) chapter 7 updated dec. 2009 xu zhengchuan fudan university
TRANSCRIPT
Wide Area Networks(WANs)
Chapter 7
Updated Dec. 2009
XU Zhengchuan
Fudan University
7-2
Orientation
• Single Networks
– Layers 1 and 2 (so OSI standards dominate)
– Chapters 4-7: Local to long-distance for single networks
• Chapter 4: Wired Ethernet LANs
• Chapter 5: Wireless LANs (WLANs)
• Chapter 6: Telecommunications (and Internet Access)
• Chapter 7: Wide Area Networks (WANs)
• Chapter 8: TCP/IP Internetworking– To link multiple single networks
7-3
WAN Concepts
• Wide Area Networks (WANs)
– Single networks that connect different sites
– So Layer 1 and Layer 2 operation
• WAN Purposes
– Internet access (Chapter 6)
– Link sites within the same corporation
– Provide remote access to individuals who are off site
1.Point-to-point Leased Line Network2.Public Switched Data Networks (PSDNs)
Leased Line Networks
7-5
Figure 7-2: Leased Line Networks for Voice and Data
OC3 Leased Line
T1LeasedLine
T1LeasedLine
T3LeasedLine
56 kbpsLeasedLine
56 kbpsLeasedLine
56 kbpsLeasedLine
Site A Site B
Site DSite E
Site C
Leased Line Voice Network
PBX
PBX
PBXPBX
PBX
Leased Line VoiceNetworks Have aPBX at Each Site
7-6
Figure 7-2: Leased Line Networks for Voice and Data
OC3 Leased Line
T1LeasedLine
T1LeasedLine
T3LeasedLine
56 kbpsLeasedLine
56 kbpsLeasedLine
56 kbpsLeasedLine
Site A Site B
Site D Site E
Site C
Leased Line Data Network
RouterRouter
Router
RouterRouter
Leased Line DataNetworks Have a
Router at Each Site
7-7
Leased Lines: Recap
• Leased Line Characteristics
– Point-to-point circuits
– Always on
– High speeds: 64 kbps (rare) to several gigabits per second
– Leased for a minimum period of time
– Usually offered by telephone companies
7-8
Figure 7-4: Leased Line Speeds
Line Speed Typical TransmissionMedium
56 kbps 56 kbps 2-Pair Data-Grade UTP
T1 1.544 Mbps 2-Pair Data-Grade UTP
North American Digital Hierarchy
56 kbps leased lines are hardly used today because they are so slow.
T1 lines are very widely usedbecause they are in the speed range of greatest corporate demand—
128 kbps to a few megabits per second.
7-9
Figure 7-4: Leased Line Speeds, Continued
Line Speed Typical TransmissionMedium
Fractional T1 128 kbps, 256 kbps,384 kbps, 512 kbps,
768 kbps
2-Pair Data-Grade UTP
T1 1.544 Mbps 2-Pair Data-Grade UTP
North American Digital Hierarchy
Bonded T1s (multipleT1s acting as a singleline)
A few multiples of1.544 Mbps
2-Pair Data-Grade UTP
T1 lines are very widely used.Fractional T1 lines offer lower speeds for companies that need them.
Two or three T1 lines can be bonded for higher speeds.T1, Fractional T1, and Bonded T1s are the most widely used leased lines.
7-10
Figure 7-4: Leased Line Speeds, Continued
Line Speed Typical TransmissionMedium
North American Digital Hierarchy
T3 44.736 Mbps Optical Fiber
The jump from T1 to T3 speeds is extremely large.
Few firms need T3 speeds, and they only needthese speeds for some of their leased lines.
Some carriers offer fractional T3 lines to bridge the T1-T3 gap.
T3 lines and all faster leased lines use optical fiber.
T1 1.544 Mbps 2-Pair Data-Grade UTP
7-11
Figure 7-4: Leased Line Speeds, Continued
Line Speed (Mbps) Typical TransmissionMedium
OC3/STM1 155.52 Optical Fiber
OC12/STM4 622.08 Optical Fiber
SONET/SDH Speeds
OC48/STM16 2,488.32 Optical Fiber
OC192/STM64 9,953.28 Optical Fiber
OC768/STM256 39,813.12 Optical Fiber
For speeds above 50 Mbps, the world uses one technology
Called SONET in the United States, SDH in EuropeSONET speeds measured in OC numbers, SDH in STM numbers
Speeds are multiples of 51.84 Mbps
Used mostly by carriers
Public Switched Data Networks (PSDNs)
7-13
Figure 7-6: Public Switched Data Networks (PSDNs)
• Recap: Leased Line Data Networks– Use many leased lines, which must span long distances
between sites
– This is very expensive
– Company must design and operate its leased line network
• Public Switched Data Networks– Carrier does more of the operational and management
work
– Total cost of technology, service, and management usually lower than leased line networks
7-14
Figure 7-7: Public Switched Data Network (PSDN)
Site A Site B
Site DSite C
One PrivateLine AccessLine per Site
Public Switched DataNetwork (PSDN)
Site E
POP POP
POPPOPPoint of Presence
In Public Switched Data Networks,the PSDN carrier handles all switching.Reduces the load on the network staff.
The PSDN central core is shown as a cloudto indicate that the user firm does not
have to know how the network operates.
7-15
Figure 7-7: Public Switched Data Network (PSDN)
Site A Site B
Site DSite C
One PrivateLine AccessLine per Site
Public Switched DataNetwork (PSDN)
Site E
POP POP
POPPOPPoint of Presence
In Public Switched Data Networks,the customer needs a single leased line
from each site to one of the PSDN carrier’spoints of presence (POPs)
7-16
Figure 7-8: Virtual Circuit Operation
VirtualCircuit
VirtualCircuit
Switch A Switching Table
Virtual Circuit Port47 2
270 3982 3
5 1
Switch A Switch B
Switch E
Switch C
Switch D
Server
Frame withVC Number 47
The internal cloud networkis a mesh of switches.
This creates multiple alternative paths.
This gives reliability.
7-17
Figure 7-8: Virtual Circuit Operation
VirtualCircuit
VirtualCircuit
Switch A Switching Table
Virtual Circuit Port47 2
270 3982 3
5 1
Switch A Switch B
Switch E
Switch C
Switch D
Server
Frame withVC Number 47
Mesh switching is slow becauseeach switch must evaluateavailable alternative pathsand select the best one.
This creates expensive switching.
7-18
Figure 7-8: Virtual Circuit Operation
VirtualCircuit
VirtualCircuit
Switch A Switching Table
Virtual Circuit Port47 2
270 3982 3
5 1
Switch A Switch B
Switch E
Switch C
Switch D
Server
Frame withVC Number 47
Before communication begins betweensites, the PSDN computes
a best path called a virtual circuit.
All frames travel along this virtual circuit.
7-19
Figure 7-8: Virtual Circuit Operation
VirtualCircuit
VirtualCircuit
Switch A Switching Table
Virtual Circuit Port47 2
270 3982 3
5 1
Switch A Switch B
Switch E
Switch C
Switch D
Server
Frame withVC Number 47
Each frame has a virtual circuit numberinstead of a destination address.
Each switch looks up the VC numberin its switching table, sends the frame
out the indicated port.
VCs greatly reduce switching costs.
Public Switched Data Networks (PSDNs)
Frame Relay
ATM
Metropolitan Area Ethernet
Carrier IP Networks
7-21
Figure 7-9: Frame Relay
• Frame Relay is the Most Popular PSDN Service Today
– 56 kbps to 40 Mbps
– This fits the range of greatest corporate demand for WAN speed
– Usually less expensive than a network of leased lines
– Grew rapidly in the 1990s, to be come equal to leased line WANs in terms of market share (about 40%)
– Carriers have raised prices, reducing growth
7-22
Figure 7.14: Pricing Elements in Frame Relay Service
• Frame Relay Pricing– Frame relay access device at site
• CSU/DSU at physical layer (converts between internal, PSDN digital formats)
• Frame Relay at the Data Link Layer
– Leased line from site to POP
– Port on the POP• Pay by port speed• Usually the largest price component
– Permanent virtual circuits (PVCs) among communicating sites
– Other charges
7-23
Figure 7-10: Frame Relay Network Elements
SwitchPOP
Customer Premises B
Customer Premises C
1.Access DeviceCustomer
Premises A
Router or DedicatedFrame Relay
Access Device
And CSU/DSU
7-24
Figure 7-10: Frame Relay Network Elements
Site A
Site B
PC
Server
T1 CSU/DSU atPhysical Layer
Frame Relay atData Link Layer
T3 CSU/DSU atPhysical Layer
ATM etc. atData Link Layer
T1 Line
T3 Line
Access Device(Frame Relay
Access Device)
Access Device(Router)
7-25
Figure 7-10: Frame Relay Network Elements
• CSU/DSU
– Channel service unit (CSU) protects the access line from unapproved voltage levels, etc. coming from the firm. It acts like a fuse in an electrical circuit.
– Data service unit (DSU) converts between internal digital format and digital format of access link to Frame Relay network.
• May have different baud rate, number of states, voltage levels, etc.
DSU
7-26
Figure 7-10: Frame Relay Network Elements
SwitchPOP
Customer Premises B
Customer Premises C
Customer Premises A
2.Leased Access
Line to POP
7-27
Figure 7-10: Frame Relay Network Elements
SwitchPOP
Customer Premises B
Customer Premises C
Customer Premises A
3.Port
SpeedCharge at
POPSwitch
POP has a switch with ports
The port speed charge is basedon the port speed used
The port speed charge usuallyIs the biggest part of PSDN costs
7-28
Figure 7-10: Frame Relay Network Elements
Switch
PVC 2
PVCs 1&2
POP
PVC 2 PVC 1
Customer Premises B
Customer Premises C
Customer Premises A
PVC 1
PVC 1
4.PVC
Charges
2.PVCs are multiplexed
over a single leased linePVC charges usually
are collectivelythe second-mostexpensive part
of Frame Relay service
PVC pricesdepend onPVC speed
7-29
Frame Relay Network PVCs
• Frame Relay PVC Numbers are called data link control indicators (DLCIs)
• Pronounced “Dull’ seas”• Usually 10 bits long• 210 or 1,024 possible PVCs from each site
– Multiplexed over the single leased line to the POP• Leased line must be fast enough to handle the
combined PVC speeds
Site 1 PSDNLeased
Line
Site 2
Site 3
POP
PVC 1-2
PVC 1-3
7-30
Figure 7-10: Frame Relay Network Elements
Switch
PVC 2
PVCs 1&2
POP
PVC 2 PVC 1
Customer Premises B
Customer Premises C
Customer Premises A
PVC 1
PVC 1
5.Management
Charges
Frame Relay networksare managed by the carrier.
For managementof equipment on thecustomer premises,
there is an extra charge.
7-31
Figure 7.15: Frame Relay Pricing Details
• To Determine Needs for Each Site
• Step 1: Determine PVC Needs
– Determine needed speed from this site to each other site
– You will need a virtual circuit to serve the needed speed to each other site
– Sum all the virtual circuit speeds
NewNot in Book
Site BeingAnalyzed
30 kbps needed56 kbps PVC
800 kbps needed, 1 Mbps PVC
Sum1,056 kbps
7-32
Figure 7.15: Frame Relay Pricing Details
• Step 2: Determine Port Speed– You need a port speed equal to or greater than the sum
of the PVCs
– Don’t overdo port speed because port speed is most expensive component
• You can get by with 70%
• 70% of 1,056 kbps is 739 kbps
• Next-higher port speed may be 1 Mbps
• Don’t overdo port speed because port speed is most expensive component
NewNot in Book
7-33
Figure 7.15: Frame Relay Pricing Details
• Step 3: Determine Leased Line Speed
– Remember that port speed is more expensive than leased line speeds
• In general, don’t waste port speed by using a leased line much under its capacity
• If port speed is 1 Mbps, leased line should be T1 with 1.544 Mbps capacity
NewNot in Book
7-34
Figure 7.15: Frame Relay Pricing Details
• Example
– The Situation
• Headquarters and two branch offices.
• Branches communicate with HQ at 200 kbps
• Branches communicate with each other at 40 kbps
HQ
B1
B2
NewNot in Book
7-35
Figure 7.15: Frame Relay Pricing Details
• Example
– For HQ
• How many connections will HQ need?
• What are their speeds?
• What will be their PVC speeds (0 kbps, 56 kbps, 256 kbps, 512 kbps, 1 Mbps)
• If port speeds are 56 kbps, 256 kbps, 384 kbps, 512 kbps, what port speed will be needed?
• What leased line will be needed if speeds are 56 kbps, 256 kbps, 512 kbps, or T1?
HQ
NewNot in Book
7-36
Figure 7.15: Frame Relay Pricing Details
• Example
– For Each Branch
• How many links will the branch need?
• What are their speeds?
• What will be their PVC speeds (0 kbps, 56 kbps, 256 kbps, 512 kbps, 1 Mbps)
• If port speeds are 56 kbps, 256 kbps, 384 kbps, 512 kbps, what port speed will be needed?
• What leased line will be needed if speeds are 56 kbps, 256 kbps, 512 kbps, or T1?
B1
NewNot in Book
7-37
Figure 7.15: Frame Relay Pricing Details
• Other Charges
– Installation charges
– Managed service charges
– Service level agreement (SLA) charges
• Geographical Scope
– Frame Relay systems with broader geographical scope cost more
7-38
Figure 7-6: PSDNs
• Service Level Agreements (SLAs)
– Guarantees for services
– Throughput, availability, latency, error rate, etc.
– An SLA might guarantee a latency of no more than 100 ms 99.99 percent of the time
• SLA guarantees no worse than a certain worst-case level of performance
Leased Line Networks Topology
7-40
Figure 7-2: Leased Line Networks for Voice and Data
OC3 Leased Line
T1LeasedLine
T1LeasedLine
T3LeasedLine
56 kbpsLeasedLine
56 kbpsLeasedLine
56 kbpsLeasedLine
Site A Site B
Site DSite E
Site C
Leased Line Voice Network
PBX
PBX
PBXPBX
PBX
Leased Line VoiceNetworks Have aPBX at Each Site
7-41
Figure 7-2: Leased Line Networks for Voice and Data
OC3 Leased Line
T1LeasedLine
T1LeasedLine
T3LeasedLine
56 kbpsLeasedLine
56 kbpsLeasedLine
56 kbpsLeasedLine
Site A Site B
Site D Site E
Site C
Leased Line Data Network
RouterRouter
Router
RouterRouter
Leased Line DataNetworks Have a
Router at Each Site
7-42
Figure 7-3: Full Mesh and Pure Hub-and-Spoke Topologies for Leased Line Data Networks
Full Mesh Topology
OC3 Leased Line
T1LeasedLine
Site A Site B
Site C Site D
T1LeasedLine
T3LeasedLine
T3LeasedLine
In a full mesh topology,there is a leased line
between each pair of sites
Highly reliableHighly expensive
7-43
Figure 7-3: Full Mesh and Pure Hub-and-Spoke Topologies for Leased Line Data Networks
Pure Hub-and-Spoke Topology
OC3 Leased Line
Site ASite B
Site C
Site D
T3LeasedLine
T3LeasedLine
In a pure hub-and-spoke topology, there is only
one leased line from thehub site to each other site.
Very inexpensive.Very unreliable.
Few companies use either of these extreme topologies.They have some backup links.
7-44
• Test Your understanding
• Page 314
• 2
• Page 317
• 3
• Page 319
• 4
Public Switched Data Networks (PSDNs)
Frame Relay
ATM
Metropolitan Area Ethernet
Carrier IP Networks
7-46
Figure 7-11: ATM
• ATM (Asynchronous Transfer Mode) is a another PSDN
• ATM Provides Speeds Greater than Frame Relay Can Provide– One megabit per second to several gigabits per second
• Not a Competitor for Frame Relay– Most carriers offer both FR and ATM
– Sell based on the customer’s speed range needs
– May even interconnect the two services
7-47
Figure 7-11: ATM, Continued
• Designed to Run over SONET/SDH
• Cell Switching (信元交换 )
– Most frames have variable length (Ethernet, etc.)
– All ATM frames, called cells, are 53 octets long
• 5 octets of header
• 48 octets of data
– Using fixed-length frames is called cell switching
– Short length minimizes latency (delay) at each switch
7-48
Figure 7-11: ATM, Continued
• ATM Has Strong Quality of Service (QoS) Guarantees for Voice Traffic– Not surprising because ATM was created for the
PSTN’s transport core
• For pure data transmission, however, ATM usually does NOT provide QoS guarantees!!
• Manageability, Complexity, and Cost– Very strong management tools for large networks
(designed for the PSTN)– Too complex and expensive for most firms– Not thriving in the marketplace
7-49
• Test Your understanding
• Page 330
• 10
Public Switched Data Networks (PSDNs)
Frame Relay
ATM
Metropolitan Area Ethernet
Carrier IP Networks
7-51
Figure 7-12: Metropolitan Area Ethernet
• Metropolitan Area Networks (MANs)
– MANs are carrier networks that are limited to a large urban area and its suburbs
– Metropolitan area Ethernet (metro Ethernet) is available for this niche
– New but growing very rapidly
7-52
Figure 7-12: Metro Ethernet, Cont.
• Attractions of Metropolitan Area Ethernet
– Very Low Prices Compared to Frame Relay and ATM
– High Speeds: Tens of megabits per second
– Familiar Technology for the Networking Staff
• No need to learn a new technology
7-53
Figure 7-12: Metro Ethernet, Cont.
• Carrier Class Service
– Basic Ethernet standards are insufficient for large wide area networks
– Quality of service and management tools must be developed
– The goal: provide carrier class services that are sufficient for customers
7-54
• Test Your understanding
• Page 332
• 11
• Page 332
• 12
Virtual Private Network (VPNs)
7-56
Figure 7-13: Virtual Private Networks (VPNs)
• Virtual Private Networks (VPNs)
– Virtual private networks (VPN) use the Internet with added security for data transmission
• The Attractions of Internet Transmission
– Lowest cost per bit transmitted
– Universal access to communication partners (Everybody uses the Internet)
7-57
Figure 7-14: Virtual Private Networks (VPNs)
CorporateSite A
VPNGateway
VPNGateway
RemoteAccessVPN
Tunnel
Internet
RemoteCorporate
PC
Site-to-SiteVPN
CorporateSite B
ProtectedClient
ProtectedServer
A VPN is communication over theInternet with added security
Host-to-HostVPN
Remote access VPNsprotect traffic for individual users
7-58
Figure 7-14: Virtual Private Networks (VPNs)
CorporateSite A
VPNGateway
VPNGateway
RemoteAccessVPN
Tunnel
Internet
RemoteCorporate
PC
Site-to-SiteVPN
CorporateSite B
ProtectedClient
ProtectedServer
A VPN is communication over theInternet with added security
Host-to-HostVPN
Site-to-site VPNsprotect traffic between sites
Will dominate VPN traffic
7-59
Figure 7-13: VPNs
• VPN Security Technologies
– IPsec for any type of VPN
• Offers very high security
– SSL/TLS for low-cost transmission
• Secure browser-server transmission
• Remote access VPNs
7-60
Figure 7-15: IPsec Transport and Tunnel Modes
Secure onthe Internet
SiteNetwork
SiteNetworkSecure Connection
Securein Site
Network
Securein Site
Network
Transport Mode
ExtraSoftware,
DigitalCertificate,and SetupRequired
ExtraSoftware,
DigitalCertificate,and SetupRequiredIPsec is the strongest VPN security technology.
IPsec transport mode gives host-to-host securityhowever, software must be added to each host,
each host must have a digital certificate,and each host must be setup (configured).
This is very expensive.
7-61
Figure 7-15: IPsec Transport and Tunnel Modes
IPsecGateway
Secure onthe Internet
SiteNetwork
SiteNetwork
IPsecGateway
TunneledConnection
NoSecurityin Site
Network
NoSecurityin Site
Network
Tunnel Mode
No ExtraSoftware,
DigitalCertificate,or SetupRequired
No ExtraSoftware,
DigitalCertificate,or SetupRequired
In IPsec tunnel mode, there is only security overthe Internet between IPsec gateways at each site
No security within sites, butno software, setup or certificates on the individual hosts
Inexpensive compared to transport mode
7-62
Figure 7-16: SSL/TLS for Browser–Webserver
Communication
Webserverwith Built-in
SSL/TLS Support
PC withBrowser Already
Installed
1. SSL/TLS Operates at the Transport Layer
2.Protects All Application Layer Traffic
That Is SSL/TLS Aware(WWW and Sometimes E-Mail)
No additional software is needed on the user PC.
IPsec works at the internet layer.SSL/TLS works at the transport layer.
Only protects SSL/TLS-aware applications.This primarily means HTTP.
SSL/TLS is built into every browser and webserver.
7-63
• Test Your understanding
• Page 334
• 13
• Page 335
• 14
• Page 337
• 15
• Page 338
• 16
7-64
Figure 7-18: Market Perspective
• Rapid Growth
– VPNs
– Metro Ethernet
• Stagnant
– Leased line networks
– Frame Relay
– ATM
7-65
• Test Your understanding
• Page 340
• 17
WAN Essentials
7-67
Figure 7-1: Wide Area Networks (WANs)
• WANs are Characterized by High Cost and Low Speeds
– High cost per bit transmitted compared to LANs
– Consequently, lower speeds (most commonly 128 kbps to a few megabits per second)
• This speed usually is aggregate throughput shared by many users
– Much slower than LAN speeds (100 Mbps to 1 Gbps to the desktop)
7-68
Figure 7-1: Wide Area Networks (WANs)
• Carriers
– Beyond their physical premises, companies must use the services of regulated carriers for transmission
• Companies do not have rights of way to lay wires beyond their premises
– Customers are limited to whatever services the carriers provide
– Prices for carrier services change abruptly and without technological reasons
– Prices and service availability vary widely from country to country
7-69
• Page 313
• Test Your understanding
• 1
Topics Covered
7-71
WANs
• Wide Area Networks– Carry data between different sites, usually within a
corporation
– High-cost and low-speed lines• 128 kbps to a few megabits per second
– Carriers
– Purposes• Internet access, site-to-site connections, and remote
access for Individuals
– Technologies• Leased line networks, public switched data networks,
and virtual private networks
7-72
Leased Line Networks
• Leased Lines are Long-Term Circuits
– Point-to-Point
– Always On
– High-speeds
• Device at Each Site
– PBX for leased line voice networks
– Router for leased line data networks
• Pure Hub-and-Spoke, Full Mesh, and Mixed Topologies
7-73
Leased Line Networks
• Many Leased Line Speeds
– Fractional T1, T1, and bonded T1 dominate
– Slowest leased lines run over 2-pair data-grade UTP
– Above 3 Mbps, run over optical fiber
– Below about 3 Mbps, 2-pair data grade UTP
– Above 3 Mbps, optical fiber
– North American Digital Hierarchy, CEPT, and other standards below 50 Mbps
– SONET/SDH above 50 Mbps
– Symmetrical DSL lines with QoS
7-74
Public Switched Data Networks
• PSDNs
– Services offered by carriers
– Customer does not have to operate or manage
– One leased line per site from the site to the nearest POP
– By reducing corporate labor, typically cheaper than leased line networks
– Service Level Agreements
– Virtual circuits
7-75
Frame Relay PSDNs
• Frame Relay
– Most popular PSDN
– 56 kbps to about 40 Mbps
– Access devices, CSU/DSUs, leased access lines, POP ports, virtual circuits, management
• Usually POP port speed charges are the biggest cost component
• Second usually are PVC charges
– Leased line must be fast enough to handle the speeds of all of the PVCs multiplexed over it