1. Wide Area Networks (WANs)
Chapter 7

2. WAN Essentials

3. Figure 7-1: 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

4. 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)

5. 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

6. Leased Line Networks

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

8. Figure 7-2: Leased Line Networks for Voice and Data

9. Figure 7-2: Leased Line Networks for Voice and Data

10. between each pair of sites
Figure 7-3: Full Mesh and Pure Hub-and-Spoke Topologies for Leased Line Data Networks
In a full mesh topology,
there is a leased line
between each pair of sites
Highly reliable
Highly expensive

11. Figure 7-3: Full Mesh and Pure Hub-and-Spoke Topologies for Leased Line Data Networks
In a pure hub-and-spoke
topology, there is only
one leased line from the
hub site to each other site.
Very inexpensive.
Very unreliable.
Few companies use either of these extreme topologies.
They have some backup links.

12. Figure 7-4: Leased Line Speeds
North American Digital Hierarchy
Line
Speed
Typical Transmission
Medium
56 kbps
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.

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

14. Figure 7-4: Leased Line Speeds, Continued
North American Digital Hierarchy
Line
Speed
Typical Transmission
Medium T1
1.544 Mbps
2-Pair Data-Grade UTP T3
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.

15. Figure 7-4: Leased Line Speeds, Continued
CEPT Hierarchy
Line
Speed
Typical Transmission
Medium
64 kbps
64 kbps
2-Pair Data-Grade UTP E1
2.048 Mbps
2-Pair Data-Grade UTP E3
Mbps
Optical Fiber
In Europe, most countries use the CEPT hierarchy
E1 lines are slightly faster than T1 lines
E3 lines are slightly slower than T3 lines

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

17. Figure 7-5: Business-Class Symmetric Digital Subscriber Line (DSL) Services
HDSL
HDSL2
SHDSL
Uses Existing 1-Pair Voice-Grade
UTP Telephone Access Line to
Customer Premises?*
Yes*
Yes*
Yes*
Downstream Throughput
768 kbps
1.544 Mbps
384 kbps –
2.3 Mbps
Upstream Throughput
768 kbps
1.544 Mbps
384 kbps –
2.3 Mbps
*By definition, DSL always uses 1-pair VG UTP
Many firms use HDSL and HDSL2 lines instead of T1 and fractional T1 speeds

18. Businesses need symmetrical throughput and QoS
Figure 7-5: Business-Class Symmetric Digital Subscriber Line (DSL) Services
HDSL
HDSL2
SHDSL
Target Market
Businesses
Businesses
Businesses
Symmetrical Throughput?
Yes
Yes
Yes
QoS Throughput Guarantees?
Yes
Yes
Yes
Businesses need symmetrical throughput and QoS

19. Public Switched Data Networks (PSDNs)

20. 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

21. Figure 7-7: Public Switched Data Network (PSDN)
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
to indicate that the user firm does not
have to know how the network operates.

22. Figure 7-7: Public Switched Data Network (PSDN)
In Public Switched Data Networks,
the customer needs a single leased line
from each site to one of the PSDN carrier’s
points of presence (POPs)

23. Leased Lines in PSDNs A company has ten sites It wants to use a PSDN
Will it need leased lines even if it is using a PDSN?
How many leased lines will it need?
Between what two locations will each leased line go?

24. 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 percent of the time
SLA guarantees no worse than a certain worst-case level of performance

25. Figure 7-8: Virtual Circuit Operation
The internal cloud network
is a mesh of switches.
This creates multiple alternative paths.
This gives reliability.

26. Figure 7-8: Virtual Circuit Operation
Mesh switching is slow because each switch must evaluate
available alternative paths
and select the best one.
This creates expensive switching.

27. Figure 7-8: Virtual Circuit Operation
Before communication begins between
sites, the PSDN computes
a best path called a virtual circuit.
All frames travel along this virtual circuit.

28. Figure 7-8: Virtual Circuit Operation
Each frame has a virtual circuit number
instead of a destination address.
Each switch looks up the VC number
in its switching table, sends the frame
out the indicated port.
VCs greatly reduce switching costs.

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

30. 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

31. Figure 7-10: Frame Relay Network Elements1.
Access Device
Customer
Premises A
Router or Dedicated
Frame Relay
Access Device
And CSU/DSU
Switch
POP
Customer
Premises B
Customer
Premises C

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

33. 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

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

35. Figure 7-10: Frame Relay Network Elements3.
Port
Speed
Charge at
POP
Switch
Customer
Premises A
POP has a switch with ports
The port speed charge is based
on the port speed used
The port speed charge usually
Is the biggest part of PSDN costs
Switch
POP
Customer
Premises B
Customer
Premises C

36. Figure 7-10: Frame Relay Network Elements2.
PVCs are multiplexed
over a single leased line
PVC charges usually
are collectively
the second-most
expensive part
of Frame Relay service
Customer
Premises A 4.
PVC
Charges
PVCs 1&2
Switch
POP
PVC 2
PVC 1
PVC prices
depend on
PVC speed
PVC 1
PVC 2
PVC 1
Customer
Premises B
Customer
Premises C

37. 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
PVC 1-2
Site 2
PSDN
Site 1
POP
Leased
Line
Site 3
PVC 1-3

38. Figure 7-10: Frame Relay Network Elements
Frame Relay networks
are managed by the carrier.
For management
of equipment on the
customer premises,
there is an extra charge. 5.
Management
Charges
Customer
Premises A
PVCs 1&2
Switch
POP
PVC 2
PVC 1
PVC 1
PVC 2
PVC 1
Customer
Premises B
Customer
Premises C

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

40. 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

41. Figure 7-11: ATM, Continued
Designed to Run over SONET/SDH
Cell Switching
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

42. 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

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

44. 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

45. 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
Rapid Provisioning
Setting up service to a customer
Changing the service (adding more capacity)

46. 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

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

48. Carrier IP Networks Some Carriers Now Offer IP Networks
Essentially, private internets
Operate at Layer 3 instead of at Layers 1 and 2, like Frame Relay, ATM, and Ethernet
Use TCP/IP standards
Operated entirely by the carrier, so no overload in the Internet backbone from connected carries
Access is not open to everyone, so security is enhanced
Also called Private IP Networks

49. Carrier IP Networks Other Advantages Growing Rapidly
Allow companies to use familiar IP technology
Mature management and control standards
Carrier can manage everything if the customer desires that (and will pay)
Offer VoIP as well as data—convergence to reduce technology and management costs
Growing Rapidly
Carriers may soon force Frame Relay users to switch to carrier IP service

50. Virtual Private Network (VPNs)

51. Figure 7-13: 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)

52. Figure 7-14: Virtual Private Networks (VPNs)
Remote access VPNs
protect traffic for individual users

53. Figure 7-14: Virtual Private Networks (VPNs)
Site-to-site VPNs
protect traffic between sites
Will dominate VPN traffic

54. 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

55. Figure 7-15: IPsec Transport and Tunnel Modes
IPsec is the strongest VPN security technology.
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.

56. Figure 7-15: IPsec Transport and Tunnel Modes
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

57. Figure 7-16: SSL/TLS for Browser–Webserver Communication
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.

58. Figure 7-17: SSL/TLS with a Gateway
SSL/TLS gateways turn SSL/TLS into a remote access VPN technology,
Gives access to multiple internal webservers.
Can “webify” some other applications for viewing on browsers as webpages.
Can give access to other servers.

59. SSL/TLS Versus IPsec SSL/TLS IPsec Limited to remote access VPNs
Only moderately strong security
Harder to use with many applications
IPsec
Offers stronger security than SSL/TLS
Both remote access and site-to-site VPNs
Costly to set up in the stronger transport mode
Economically attractive for site-to-site VPNs in tunnel mode

60. Figure 7-18: Market Perspective
Rapid Growth
VPNs
Carrier IP networks
Metro Ethernet
Stagnant
Leased line networks
Frame Relay
ATM