1. Business Data Communications and Networking, 6th ed.
FitzGerald and Dennis

2. Copyright © 1999 John Wiley & Sons, Inc.
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3. Metropolitan and Wide Area Networks
Chapter 9
Metropolitan and Wide Area Networks

4. Objectives of Chapter 9 Become familiar with…
the common carriers and the nature of competition,
how to improve MAN and WAN performance,
several factors in selecting MAN and WAN services.

5. Objectives of Chapter 9 Understand ...
the role of common carriers in organizational MANs and WANs,
the four basic categories of MAN and WAN circuits,
dialed circuit services,
dedicated circuit services,
switched circuit services,
packet network services.

6. INTRODUCTION

7. Introduction
Metropolitan area networks (MANs) typically span from 3 to 30 miles and connect backbone networks (BNs), and LANs.
Wide area networks (WANs) connect BNs and MANs across longer distances, often hundreds of miles or more.
Most organizations cannot afford to build their own MANs and WANs, so they rent or lease circuits from common carriers such as AT&T, MCI, BellSouth, PACTEL or NYNEX.

8. The Telephone Network
Most countries have a federal agency that regulates data and voice communications. In the United States, this agency is the Federal Communications Commission (FCC). Each state or province also has its own public utilities commission (PUC) to regulate communications within its borders.
The communications industry in North America operates as a series of private companies that are regulated by the government.

9. The Telephone Network
A common carrier is a private company that sells or leases communications services and facilities to the public. A common carrier that also provides local telephone services is called a local exchange carrier (LEC), while one that provides long distance services is called an interexchange carrier (IXC).
In the United States, 90 percent of the telephone system used to be run by one common carrier, AT&T.

10. DIALED CIRCUIT SERVICES

11. Dialed Circuit Services
Dialed circuit services are the simplest and one of the most common types of MAN and WAN connections. This type of connection uses the normal telephone network. To use dialed circuit services, the user simply lease connection points into the common carrier’s network, then dials the host computer using a modem, and connects to the host system.
Dialed circuit services may use different circuit paths between the two computers each time a number is dialed.

12. Dialed Circuit Services

13. Direct Dialing
Direct dialing (also called dial-up) is the most commonly used direct circuit service. Every time you call your Internet service provider from your home phone, you are using direct dialing.
Charges for direct dialing are based on the distance between the two telephones (in miles) and the number of minutes the connection is used.

14. Wide Area Telephone Service (WATS)
Wide Area Telephone Service (WATS) are special rate service that allows direct circuit calls for both voice and data transmission to be purchased in large quantities.
WATS is limited to one direction only; it is either outward dialing or inward dialing. In general, inward WATS uses the toll free 800 and 888 area code series in North America, and similar numbers in other countries.

15. Wide Area Telephone Service (WATS)
The United States and Canada are divided into about 60 different WATS service areas. The geographical coverage for WATS is determined by the band of service to which the customer subscribes.

16. DEDICATED CIRCUIT SERVICES

17. Motown Café Network

18. Dedicated Circuit Services
There are two main problems with dialed circuits.
Each connection goes through the regular telephone network on a different circuit, which may vary in quality.
The data transmission rates on these circuits are usually low 28.8 to 56 Kbps.
One alternative is to establish a private dedicated circuit, which the user leases from the common carrier for their exclusive use, 24 hrs/day, 7 days/week.

19. Dedicated Circuit Services

20. Dedicated Circuit Services
Dedicate circuits are billed at a flat fee per month and the user has unlimited use of the circuit. Dedicate circuits therefore require more care in network design than dialed circuits.
There are five types of dedicated circuits:
Voice grade circuits
Wideband analog services
T Carrier circuits
SONET circuits
Digital subscriber line circuits

22. Voice Grade Circuits
Voice grade circuits are analog circuits that work in exactly the same manner as traditional telephone lines, except that you do not dial them.
Dedicated voice grade channels often have conditioning (or equalization) done on them to improve data transmission quality by reducing noise and distortion.

23. Wideband Analog Services
Wideband analog services are similar to voice grade circuits but they provide much greater bandwidth.
Typically wideband analog services provide one 48,000 hertz bandwidth channel for use with frequency division multiplexing or as 12 individual voice grade channels (4000 Hz each).

24. T Carrier Circuits
T Carrier circuits are dedicated digital circuits and are the most commonly used form of dedicated circuit services in North America today.
Instead of a modem, a channel service unit (CSU) or data service unit (DSU) are used to connect the circuit into the network.

25. T Carrier Circuits
A T-1 circuit (a.k.a. a DS-1 circuit) provides a data rate of Mbps. T-1’s allow 24 simultaneous 64 Kbps channels (with TDM) which transport data, or voice messages using pulse code modulation.
A T-2 circuit (6.312 Mbps) is basically a multiplexed bundle of four T-1 circuits.
A T-3 circuit ( Mbps) is equal to the capacity of 28 T-1 circuits.
A T-4 circuit ( Mbps) is equal to the capacity of 178 T-1s.
Fractional T-1, (FT-1) offers portions of a Mbps T-1 for a fraction of its full costs.

26. T Carrier System T-Carrier Designation DS Designation Speed T-1 T-2
64 Kbps
1.544 Mbps
6.312 Mbps
Mbps
Mbps

27. Synchronous Optical Network (SONET)
The synchronous optical network (SONET) has recently been accepted by the U.S. standards agency (ANSI) as a standard for optical (fiber) transmission at gigabits per second speed.
The international telecommunications standards agency (ITU-T) also recently standardized a version of SONET under the name of synchronous digital hierarchy (SDH). The two are very similar and can be easily interconnected.

28. Synchronous Optical Network (SONET)
SONET transmission speeds begin at the OC-1 level (optical carrier level 1) of Mbps. Each succeeding rate in the SONET fiber hierarchy is defined as a multiple of OC-1.
Several common carriers (e.g. MCI) now use OC-12 circuits at Mbps to carry digitized voice traffic.

29. SONET SONET Designation SDH Designation Speed OC-1 OC-3 OC-9 OC-12
51.84 Mbps
Mbps
Mbps
Mbps
Mbps
1.244 Gbps
1.866 Gbps
2.488 Gbps
9.952 Gbps
STM-1
STM-3
STM-4
STM-6
STM-8
STM-12
STM-16

30. NY Information Technology Center

31. Digital Subscriber Line (DSL)
Digital Subscriber Line (DSL) is one of the most promising proposals now under consideration by the ITU-T to significantly increase the data rates over traditional telephone lines.
The reason for the limited capacity on voice telephone circuits lies with the telephone and the switching equipment at the end office. The actual cable is capable of providing much higher data rates.

32. Digital Subscriber Line (DSL)
DSL services are quite new and not all common carriers offer them.
Two general categories of DSL services have emerged in the marketplace.
Symmetric DSL (SDSL) provides the same transmission rates (up to 128 Kbps) in both directions on the circuits.
Asymmetric DSL (ADSL) provides different data rates to (up to 640 Kbps) and from (up to Mbps) the carrier’s end office. It includes an analog channel for voice transmissions.

33. Digital Subscriber Line (DSL)
A new version of ADSL called Very high rate Digital Subscriber Line (VDSL) has been designed for use over local loops of 1000 feet or less. It uses FDM to provide three channels:
Normal analog channel
Upstream digital 1.6 Mbps channel
Downstream digital Mbps channel.

34. Digital Subscriber Line (DSL)
One potential competitor to DSL is the “cable modem” a digital service offered by cable television companies which offers an upstream rate of Mbps and a downstream rate of 2-30 Mbps.
A few cable companies offer downstream services only, with upstream communications using regular telephone lines.

35. CIRCUIT SWITCHED SERVICES

36. Circuit Switched Services
The major problem with dedicated circuit services it that the user must carefully plan all circuits needed.
In contrast, switched circuits work much like dialed circuits. The user buys a connection into the common carrier’s network from the end points of the WAN, without specifying all the interconnecting circuits needed.
The primary differences from dialed circuits is that the circuits are entirely digital and that they offer higher data transmission rates.

37. Circuit Switched Services

38. Narrowband Integrated Services Digital Network
The first generation of Integrated services digital network (ISDN), commonly called narrowband ISDN, combines voice, video, and data over the same digital circuit.
ISDN has long been more of a concept than a reliable service in North America.
Acceptance has been slowed because equipment vendors and common carriers conflicting interpretations of ISDN standards.

39. Narrowband Integrated Services Digital Network
Narrowband ISDN offers two types of service:
Basic rate interface (BRI, basic access service or 2B+D) provides two 64 Kbps bearer (B) channels and one 16 Kbps control signaling (D) channel.
One advantage of BRI is it can be installed over existing telephones lines. (if less than 3.5 miles).
Primary rate interface (PRI, primary access service or 23B+D) provides Kbps ‘B’ channels and one 64 Kbps ‘D’ channel. (basically T-1 service)

40. Broadband Integrated Services Digital Network
The second generation of ISDN is called Broadband ISDN (B-ISDN). B-ISDN is a circuit switched service and is backwardly compatible with ISDN.
B-ISDN is currently offered in three services:
Full duplex channel at Mbps.
Full duplex channel at Mbps.
Asymmetrical service with two simplex channels (Upstream at Mbps, downstream at Mbps).

41. PACKET SWITCHED SERVICES

42. Packet Switched Services
Packet switched services enable multiple connections to exist simultaneously between computers.
With packet switching users buy a connection into the common carrier network, and connects via a packet assembly/ disassembly device (PAD).
Packet switching splits messages into small segments called packets (usually 128 bytes).

43. Packet Switched Services

44. Packet Switched Services
Packets from separate messages are interleaved with other packets for transmission.
Although the packets from one data stream may mix (interleave) with several other data streams during their journey, it is unlikely that packets from two different data streams will travel together during the entire length of their transmission.

45. Packet Switched Services
There are two methods used to route packets:
A Datagram is a connectionless service which adds a destination and sequence number to each packet, in addition to information about the data stream to which the packet belongs. Packets may follow a different route, and are reassembled at the destination.
In a Virtual circuit the packet switched network establishes an end-to-end circuit between the sender and receiver. All packets for that transmission take the same route over the virtual circuit that has been set up for that transmission.

46. Packet Switched Services

47. Packet Switched Services
Packet switched services are often provided by different common carriers than the one from which organizations get their usual telephone and data services.
Therefore, organizations often lease a dedicated circuits from their offices to the packet switched network point-of-presence (POP).

48. X.25
The oldest packet switched service is X.25, a standard developed by ITU-T. X.25 offers datagram, switched virtual circuit, and permanent virtual circuit services.
Although widely used in Europe, X.25 is not widespread in North America. The primary reason is transmission speed, now Mbps (up from 64 Kbps).

49. Frame Relay
Frame relay is a newer packet switching technology that transmits data faster than X.25. It differs from X.25 and traditional networks in three important ways:
1. Frame relay only operates at the data link layer.
2. Frame relay networks do not perform error control.
3. Frame relay defines two connection data rate that are negotiated per connection and for each virtual circuit as it is established. (Committed information rate and Maximum allowable rate).

50. Frame Relay

51. Frame Relay
Different common carriers offer frame relay networks with different transmission speeds: 56 Kbps to 45 Mbps.
At present, frame relay suffers from the same problems as ISDN - a lack of standards.

52. Asynchronous Transfer Mode (ATM)
Asynchronous transfer mode (ATM) is one of the fastest growing new technologies, and is similar to frame relay.
All data are packet-switched, and there is no error control at the intermediate computers within the network; error control is the responsibility of the source and destination.

53. Asynchronous Transfer Mode (ATM)
ATM has three important difference from frame relay:
ATM uses fixed packet lengths of 53 bytes (5 bytes of overhead and 48 bytes of user data), which is more suitable for voice transmissions.
ATM provides extensive quality of service information that enables the setting of very precise priorities among different types of transmissions (i.e. voice, video & ).
ATM is scaleable. It is easy to multiplex basic ATM circuits into much faster ATM circuits.

54. Home Depot Network

55. Nortel’s ATM WAN Backbone

56. Switched Multimegabit Data Service (SMDS)
Switched multimegabit data service (SMDS) is an unreliable packet service like ATM and frame relay.
Like ATM and frame relay, SMDS does not perform error checking; the user is responsible for error checking.
SMDS is not yet a widely accepted standard.

57. Northrop Grumman Network

58. Hacienda La Puente Unified School District network

59. IMPROVING MAN/WAN PERFORMANCE

60. Improving MAN/WAN Performance
Improving MAN/WAN performance is handled in the same way as improving LAN performance.
You begin by checking the devices in the network, by upgrading the circuits between computers, and by changing the demand placed on the network.

61. Performance Checklist
MAN/WAN Performance
Performance Checklist
Increase Computer and Device Performance
Upgrade devices
Change to a more appropriate routing protocol (either static or dynamic)
Increase Circuit Capacity
Analyze message traffic and upgrade to faster circuits where needed
Check error rates
Reduce Network Demand
Change user behavior
Analyze network needs of all new systems
Move data closer to users

62. Improving Device Performance
One way to improve network performance is to upgrade the devices and computers that connect backbones to the WAN.
Another strategy is to examine the routing protocol, either static or dynamic. Dynamic routing will increase performance in networks which have many possible routes from one computer to another, or those in which message traffic is “bursty.”

63. Improving Circuit Capacity
The first step is to analyze the message traffic in the network to find which dedicated point-to-point circuits are approaching capacity.
The capacity may be adequate for most traffic, but not for meeting peak demand. One solution may be to add a circuit switched or packet switched service that is only used when demand exceeds circuit capacity.
Sometimes a shortage of capacity may be caused by a faulty circuit. Before installing new circuits, monitor the existing ones to ensure that they are operating properly.

64. Reducing Network Demand
One step to reduce network demand is to require a network impact statement for all new application software developed or purchased by the organization.
Another approach is to shift network usage from peak or high cost times to lower demand or lower cost times.
The network can be redesigned to move data closer to the applications and people who use them.

65. SELECTING MAN/WAN SERVICES

66. Selecting MAN/WAN Services
A 1995 survey of network managers found that:
45 percent of WAN costs were for network management (primarily support staff salaries).
35 percent was spent on services (leasing data circuits from common carriers).
Only 20 percent was spent on equipment.
The most expensive part of the WAN will be the people require to plan, install, and operate it, so pick one that is easy to manage.
It costs more to lease services from common carriers than to buy hardware, so selection decisions should be driven more by the services.

67. Commonly Available Services
Type of Service Approximation Data Rates
Dialed Circuit Services
Voice-grade Kbps to 56 Kbps
WATS Kbps to 56 Kbps
Dedicated Circuit Services
Voice-grade Kbps to 56 Kbps
Wideband analog Kbps to 274 Mbps
T-carrier Kbps to 274 Mbps
SONET Mbps to 622 Mbps

68. Commonly Available Services
Type of Service Approximation Data Rates
Circuit Switched Services
Narrowband ISDN Kbps to 1.5 Mbps
Broadband ISDN Mbps to 622 Mbps
Packet-Switched services
X Kbps to 2 Mbps
Frame relay Kbps to 45 Mbps
ATM Mbps to 622 Mbps
SMDS Kbps to 45 Mbps

69. Key Issues Vendor capabilities Capacity Flexibility Control
Reliability

70. Value Added Networks and Virtual Private Networks
Several companies offer value added networks (VANs) that are alternatives to building networks by leasing circuits from common carriers. VANs provide additional services over and above those provided by common carriers.
A new type of VAN, called a virtual private network (VPN), or software defined networks, provide circuits that run over the Internet but appear to the user to be private networks.

71. Value Added Networks and Virtual Private Networks
The primary advantage of the VPN is low cost.
There are two important disadvantages of VPNs:
Traffic on the Internet is unpredictable.
There are several competing standards for Internet-based CPN, so not all vendor’s equipment and services are compatible.

72. End of Chapter 9