Chapter 7
Updated Dec. 2009
XU Zhengchuan
Fudan University

• 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
7-2

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 Network
2.Public Switched Data Networks (PSDNs)
7-3

Site A
56 kbps
Leased
Line
PBX
Figure 7-2: Leased Line Networks for Voice and
Data
Leased Line Voice Network
Site B
OC3 Leased Line
PBX
PBX
T3
Leased
Line
T1
Leased
Site C
56 kbps
Leased
Line
Site D
PBX
Networks Have a
PBX at Each Site
T1
Leased
Line
56 kbps
Leased
Line
PBX
Site E
7-5

Site A
Figure 7-2: Leased Line Networks for Voice and
Data
Leased Line Data Network
Site B
Router
OC3 Leased Line
T3
Leased
Line
Router
T1
56 kbps
Leased
Line
Site C
Router 56 kbps
Leased
Line
Site D
Router
Router at Each Site
T1
Leased
Line
56 kbps
Leased
Line
Router
Site E
7-6

• 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
7-7

Figure 7-4: Leased Line Speeds
North American Digital Hierarchy
Line
56 kbps
Speed Typical Transmission
Medium
56 kbps 2-Pair Data-Grade UTP
T1 1.544 Mbps 2-Pair Data-Grade UTP
56 kbps leased lines are hardly used today because they are so slow.
T1 lines are very widely used because they are in the speed range of greatest corporate demand —
128 kbps to a few megabits per second.
7-8

Figure 7-4: Leased Line Speeds, Continued
North American Digital Hierarchy
Line
T1
Speed Typical Transmission
Medium
1.544 Mbps 2-Pair Data-Grade UTP
Fractional T1
Bonded T1s (multiple
T1s acting as a single line)
128 kbps, 256 kbps,
384 kbps, 512 kbps,
768 kbps
2-Pair Data-Grade UTP
A few multiples of
1.544 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.
7-9

Figure 7-4: Leased Line Speeds, Continued
North American Digital Hierarchy
Line
T1
T3
Speed Typical Transmission
Medium
1.544 Mbps 2-Pair Data-Grade UTP
44.736 Mbps Optical Fiber
The jump from T1 to T3 speeds is extremely large.
Few firms need T3 speeds, and they only need these 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.
7-10

Figure 7-4: Leased Line Speeds, Continued
SONET/SDH Speeds
Line
OC3/STM1
OC12/STM4
OC48/STM16
OC192/STM64
Speed (Mbps) Typical Transmission
Medium
155.52 Optical Fiber
622.08 Optical Fiber
2,488.32 Optical Fiber
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 Europe
SONET speeds measured in OC numbers, SDH in STM numbers
Speeds are multiples of 51.84 Mbps
Used mostly by carriers
7-11

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
7-13

Figure 7-7: Public Switched Data Network (PSDN)
Site A
Site B
POP
Point of Presence
Public Sw itched Data
Netw ork (PSDN)
POP
POP
POP One Private
Line Access
Line per Site
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 cloud
Site C have to know how the network operates.
Site E
7-14

Figure 7-7: Public Switched Data Network (PSDN)
Site A
Site B
POP
Point of Presence
Public Sw itched Data
Netw ork (PSDN)
POP
POP
POP One Private
Line Access
Line per Site
In Public Switched Data Networks, the customer needs a single leased line
Site C Site D points of presence (POPs)
Site E
7-15

Figure 7-8: Virtual Circuit Operation
Virtual
Circuit
Frame w ith
VC Number 47
Sw itch A Sw itch B
Sw itch D
Sw itch A Sw itching Table
Virtual Circuit Port
47
270
982
5
2
3
3
1
Sw itch C
Circuit is a mesh of switches.
Sw itch E
This gives reliability.
7-16

Figure 7-8: Virtual Circuit Operation
Virtual
Circuit
Frame w ith
VC Number 47
Sw itch A Sw itch B
Sw itch D
Sw itch A Sw itching Table
Virtual Circuit Port
47
270
982
5
2
3
3
1
Sw itch C
Virtual
Mesh switching is slow because each switch must evaluate and select the best one.
Server
This creates expensive switching.
7-17

Figure 7-8: Virtual Circuit Operation
Virtual
Circuit
Before communication begins between sites, the PSDN computes a best path called a virtual circuit.
Sw itch A
Frame w ith
VC Number 47
Sw itch C
Virtual
Circuit
Sw itch D
Sw itch A Sw itching Table
Virtual Circuit Port
47
270
982
5
2
3
3
1
Sw itch E
Server
7-18

Figure 7-8: Virtual Circuit Operation
Virtual
Circuit
Frame w ith
VC Number 47
Sw itch A
Sw itch D
Sw itch A Sw itching Table
Virtual Circuit Port
47
270
982
5
2
3
3
1
Each frame has a virtual circuit number instead of a destination address.
Sw itch B
Each switch looks up the VC number in its switching table, sends the frame out the indicated port.
Sw itch C
VCs greatly reduce switching costs.
Virtual
Circuit
Sw itch E
Server
7-19

Public Switched Data
Networks (PSDNs)
Frame Relay
ATM
Metropolitan Area Ethernet
Carrier IP Networks

• 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
7-21

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
7-22

Figure 7-10: Frame Relay Network Elements
1.
Access Device Customer
Premises A
Router or Dedicated
Frame Relay
Access Device
And CSU/DSU
POP
Customer
Premises B
Customer
Premises C
7-23

Site A
PC
Site B
Figure 7-10: Frame Relay Network Elements
Server
Access Device
(Frame Relay
Access Device)
T1 CSU/DSU at
Physical Layer
T1 Line
Frame Relay at
Data Link Layer
Access Device
(Router) T3 CSU/DSU at
Physical Layer
T3 Line
ATM etc. at
Data Link Layer
7-24

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
7-25

Figure 7-10: Frame Relay Network Elements
Customer
Premises A
Switch
2.
Leased Access
Line to POP
POP
Customer
Premises B
Customer
Premises C
7-26

Figure 7-10: Frame Relay Network Elements
Customer
Premises A
3.
Port
Speed
Charge at
POP
Switch
POP has a switch with ports
Switch
The port speed charge is based on the port speed used
The port speed charge usually
Is the biggest part of PSDN costs
Customer
Premises B
POP
Customer
Premises C
7-27

Figure 7-10: Frame Relay Network Elements
2.
PVCs are multiplexed over a single leased line
Customer
Premises A the second-most expensive part of Frame Relay service
Switch
PVC 2
PVCs 1&2
POP
PVC 1
4.
PVC
Charges
PVC prices depend on
PVC speed
PVC 1
PVC 2 PVC 1
Customer
Premises B
Customer
Premises C
7-28

• Frame Relay PVC Numbers are called data link control indicators (DLCIs)
• Pronounced “Dull’ seas”
• Usually 10 bits long
• 2 10 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
PVC 1-2
Site 2
Site 1
Leased
Line
POP PSDN
PVC 1-3
Site 3
7-29

Figure 7-10: Frame Relay Network Elements
Customer
Premises A
For management of equipment on the there is an extra charge.
PVC 2
PVC 2
Customer
Premises B
5.
Management
Charges
PVCs 1&2
POP
PVC 1
PVC 1
Customer
Premises C
PVC 1
7-30

Figure 7.15: Frame Relay Pricing
Details
• To Determine Needs for Each Site
New
Not in Book
• 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
30 kbps needed
56 kbps PVC
Sum
1,056 kbps
Site Being
Analyzed
800 kbps needed, 1 Mbps PVC 7-31

Figure 7.15: Frame Relay Pricing
Details
• Step 2: Determine Port Speed
New
Not in Book
– 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
7-32

Figure 7.15: Frame Relay Pricing
Details
New
Not in Book
• 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
7-33

Figure 7.15: Frame Relay Pricing
Details
• Example
New
Not in Book
– The Situation
• Headquarters and two branch offices.
• Branches communicate with HQ at 200 kbps
• Branches communicate with each other at 40 kbps
B1
HQ
B2
7-34

Figure 7.15: Frame Relay Pricing
Details
• Example
– For HQ
New
Not in Book
• How many connections will HQ need?
• What are their speeds?
HQ
• 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?
7-35

Figure 7.15: Frame Relay Pricing
Details
• Example
– For Each Branch
New
Not in Book
B1
• 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?
7-36

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
7-37

• 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
7-38

Site A
56 kbps
Leased
Line
PBX
Figure 7-2: Leased Line Networks for Voice and
Data
Leased Line Voice Network
Site B
OC3 Leased Line
PBX
PBX
T3
Leased
Line
T1
Leased
Site C
56 kbps
Leased
Line
Site D
PBX
Networks Have a
PBX at Each Site
T1
Leased
Line
56 kbps
Leased
Line
PBX
Site E
7-40

Site A
Figure 7-2: Leased Line Networks for Voice and
Data
Leased Line Data Network
Site B
Router
OC3 Leased Line
T3
Leased
Line
Router
T1
56 kbps
Leased
Line
Site C
Router 56 kbps
Leased
Line
Site D
Router
Router at Each Site
T1
Leased
Line
56 kbps
Leased
Line
Router
Site E
7-41

Site A
Figure 7-3: Full Mesh and Pure Hub-and-Spoke
Topologies for Leased Line Data Networks
Site B
Full Mesh Topology
OC3 Leased Line
In a full mesh topology, there is a leased line between each pair of sites
T3
Leased
Line
Highly reliable
Leased
Line
T3
Leased
Line
T1
Leased
Line
Site C Site D
7-42

Figure 7-3: Full Mesh and Pure Hub-and-Spoke
Topologies for Leased Line Data Networks
Site A Pure Hub-and-Spoke Topology
Site B
In a pure hub-and-spoke one leased line from the hub site to each other site.
Very inexpensive.
T3
Leased
Line
Very unreliable.
T3
Leased
Line
Site D
Few companies use either of these extreme topologies.
They have some backup links.
Site C
7-43

• Test Your understanding
• Page 314
• 2
• Page 317
• 3
• Page 319
• 4
7-44

Public Switched Data
Networks (PSDNs)
Frame Relay
ATM
Metropolitan Area Ethernet
Carrier IP Networks

• 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
7-46

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
7-47

• 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
7-48

• Test Your understanding
• Page 330
• 10
7-49

Public Switched Data
Networks (PSDNs)
Frame Relay
ATM
Metropolitan Area Ethernet
Carrier IP Networks

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
7-51

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
7-52

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
7-53

• Test Your understanding
• Page 332
• 11
• Page 332
• 12
7-54

Virtual Private Network (VPNs)

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)
7-56

Figure 7-14: Virtual Private Networks (VPNs)
Site-to-Site
VPN
Tunnel
Protected
Server
VPN
Gatew ay
Corporate
Site A
Remote access VPNs protect traffic for individual users
VPN
Internet
Remote
Access
VPN
A VPN is communication over the
Internet w ith added security
VPN
Gatew ay
Protected
Client
Corporate
Site B
Remote
Corporate
PC
7-57

Figure 7-14: Virtual Private Networks (VPNs)
Site-to-Site
VPN
Tunnel
Protected
Server
VPN
Gatew ay
Corporate
Site A
Internet
Site-to-site VPNs protect traffic between sites
Will dominate VPN traffic
Host-to-Host
VPN Remote
Access
VPN
A VPN is communication over the
Internet w ith added security
VPN
Gatew ay
Protected
Client
Corporate
Site B
Remote
Corporate
PC
7-58

• 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
7-59

Figure 7-15: IPsec Transport and Tunnel Modes
Site
Network
Transport Mode
Secure Connection
Site
Network
Extra
Software,
Digital
Certificate, and Setup
Required
Secure in Site
Network
Secure on the Internet
Secure in Site
Network
IPsec is the strongest VPN security technology.
Extra
Software,
Digital
Certificate, and Setup
Required
IPsec transport mode gives host-to-host security however, 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.
7-60

Figure 7-15: IPsec Transport and Tunnel Modes
Site
Network
IPsec
Gateway
Tunnel Mode
Tunneled
Connection
IPsec
Gateway
Site
Network
No Extra
Software,
Digital
Certificate, or Setup
Required
No
Security in Site
Network
Secure on the Internet
No
Security in Site
Network
No Extra
Software,
Digital
Certificate, or Setup
Required
In IPsec tunnel mode, there is only security over the Internet between IPsec gateways at each site
No security within sites, but no software, setup or certificates on the individual hosts
Inexpensive compared to transport mode
7-61

Figure 7-16: SSL/TLS for Browser –Webserver
Communication
PC w ith
Brow ser Already
Installed
2.
Protects All Application Layer Traf f ic
That Is SSL/TLS Aw are
(WWW and Sometimes E-Mail)
Webserver w ith Built-in
SSL/TLS Support
1. SSL/TLS Operates at the Transport Layer
No additional sof tw are 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.
7-62

• Test Your understanding
• Page 334
• 13
• Page 335
• 14
• Page 337
• 15
• Page 338
• 16
7-63

Figure 7-18: Market Perspective
• Rapid Growth
– VPNs
– Metro Ethernet
• Stagnant
– Leased line networks
– Frame Relay
– ATM
7-64

• Test Your understanding
• Page 340
• 17
7-65

WAN Essentials

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)
7-67

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
7-68

• Page 313
• Test Your understanding
• 1
7-69

• 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
7-71

• 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
7-72

• 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
7-73

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
7-74

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
7-75