1. Chapter 7 Introduction to Local Area Networks
Part II: Understanding Internet Access Technologies

2. Topics Addressed in Chapter 7
What is a local area network?
Business rationale for LANs
LAN alternatives
Emerging alternatives to full-fledged LANs
LAN components
Communication media
Wiring center options and LAN adapters
Print servers and backup devices
Storage area networks
LAN-to-host interfaces

3. What is a Local Area Network?
A local area network (LAN) is a data communication network that serves users in a confined geographic area and uses high transmission speeds (typically 10 mbps to 1 gbps)
Major LAN components include servers, workstations, network operating systems, communication links, and wiring center technologies
LANs are fundamental building blocks for enterprise-wide networks
LANs are the a primary means for business users to access and leverage the Internet

4. Business Rationale for LANs
Some of the major drivers of business interest in LANs are:
cost-effective via resource sharing
support of work groups
potential for management control over computing resources
modular expansion
ability to adapt to changing hardware and software requirements
wide variety of LAN alternatives

5. LAN Alternatives LAN alternatives include
full-fledged LANs with dedicated servers
peer-to-peer LANs with non-dedicated servers
centralized host-based networks with terminals (see Figure 7-1)
interconnections via service bureaus or application service providers
zero-slot LANs
sub-LANs with data switches (see Figure 7-2 and Figure 7-3)
Table 7-1 compares several of these alternatives on a variety of dimensions

6. Figure 7-3

7. Table 7-1

8. Emerging Alternatives to Full-Fledged LANs
Additional alternatives to full-fledged LANs have become increasingly popular. These include:
cable-based home (and SOHO) networks
CEBus (Consumer Electric Bus) and HomePnP networks
HomePNA (Home PhoneLine Networking Alliance) networks
wireless home networks
HomeRF
WiFi & other IEEE WLANs
personal area networks (PANs)
wireless PANs (WPANs)
Bluetooth
IrDA

9. LAN Components
Full-fledged LANs have several major components (see Figure 7-4):
servers
workstations (clients)
communication media
wiring center technologies
network interface cards (LAN adapters)
LAN software

10. Figure 7-4

11. Servers
Servers enable network users to access shared computing and communication services. A variety of servers are found in LANs including:
File servers
Print servers
Database servers
Application servers
Fax servers
Remote access servers
Communication servers
Terminal servers
Thin client servers
Citrix™ servers

12. File, Database, and Application Servers
File servers are used to provide user access to data, programs, and other files stored on the server’s disk drives (see Figure 7-5)
When a user requests a file on a file server, the entire file is sent to the user over the LAN’s communication medium
Database servers enable users to access records and also performs some database processing. For example, in response to a user’s request for a count of the total number of records in the database that meet certain conditions, a database server would send the total, not copies of all the records
SQL Server is an example of a database server (see Figure 7-6)
Application servers provide server-side application processing for network users

13. Server Configuration
A server is a combination of hardware and software. Key aspects of server hardware include:
Disk drive capacity and performance capabilities (access time, disk controllers [EIDE vs. SCSI], etc.)
Memory (including sufficient RAM and cache memory; virtual memory management; ECC [error-correcting code] memory, etc.)
Processor(s) and processor speed (including SMP [symmetrical multiprocessing])
Expansion capabilities (ability to add new peripherals or to create server clusters)
System bus speed and architecture
Expansion bus architecture (EISA vs. PCI vs. Infiniband, etc.)

14. Workstations
Most LAN workstations are microcomputers. Like servers, workstation configuration is an important determinant of performance. Key configuration considerations include:
Memory
Processor version and speed
System bus architecture
Expansion bus architecture
Thin clients, diskless workstations, and terminals may also be found in LANs
Docking stations and wireless connections can be used to enable notebooks, PDAs, and handheld computers to function as LAN workstations

15. Communication Media the physical path between transmitter and receiver
design factors
bandwidth
attenuation: weakening of signal over distances
interference:
number of receivers

16. Communication Media, cont.
Data communication media, including that used in LANs can be broken down into two major classes:
Conducted media that use a conductor such as a wire or fiber optic cable to move a signal from sender to receiver
Radiated media are wireless media such as radio frequencies and infrared light

17. Comparisons

18. Wires Wires are the most widely used data transmission medium in LANs
Advantages include availability and low cost
Some support transmission speeds up to 1 gbps
Disadvantages include susceptibility to signal distortion and transmission errors
Per foot costs depend on shielding, gauge, and number of conducting wires in the cable
In the U.S., (AWG) gauges of 19, 22, 24, 26, and 28 are most common; the lower the gauge, the thicker the wire and the higher the bandwidth
The number of conducting strands in data communication wires varies with 4, 7,8, 10, 12, 15, and 25 being most common
Examples are illustrated in Figure 7-8

19. Figure 7-8

20. Twisted Pair Wires
consists of two insulated copper wires arranged in a regular spiral pattern to minimize the electromagnetic interference between adjacent pairs
often used at customer facilities and also over distances to carry voice as well as data communications
low frequency transmission medium

21. Twisted Pair Wires two varieties STP (shielded twisted pair)
the pair is wrapped with metallic foil or braid to insulate the pair from electromagnetic interference
UTP (unshielded twisted pair)
each wire is insulated with plastic wrap, but the pair is encased in an outer covering

22. Twisted Pair Wires Category 3 UTP (CAT 3) Category 5 UTP (CAT 5) STP
data rates of up to 16mbps are achievable
Category 5 UTP (CAT 5)
data rates of up to 100mbps are achievable
more tightly twisted than Category 3 cables
more expensive, but better performance
“standard”
STP
More expensive, harder to work with
Necessary in some installations

23. Twisted Pair Advantages
inexpensive and readily available
flexible and light weight
easy to work with and install

24. Twisted Pair Disadvantages
susceptibility to interference and noise
attenuation problem
For analog, repeaters needed every 5-6km
For digital, repeaters needed every 2-3km
relatively low bandwidth (3000Hz)

25. Table 7-3

26. Figure 7-18

27. Coaxial Cable
Coaxial cable was the most widely used communication medium in early LANs and is still used in some networks, sometimes alongside other wires (see Figure 7-9)
Figure 7-10 illustrates the typical make-up of coaxial cable. The insulation can be PVC or plenum
RG58 and RG59 are the two most common kinds of coaxial cable found data communication networks
T-connectors and AUI connectors are most common
Data transmission over coaxial cable involves two basic techniques broadband or baseband (see Figure 7-11)
Baseband is more common in LANs

28. Figure 7-10

29. Coax Layers outer jacket (polyethylene) shield (braided wire)
insulating material
copper or aluminum
conductor

30. Figure 7-11

31. Coaxial Cable (or Coax)
bandwidth of up to 400 Mbps
has an inner conductor surrounded by a braided mesh
both conductors share a common center axial, hence the term “co-axial”

32. Coax Advantages higher bandwidth can be tapped easily (pros and cons)
400 to 600Mhz
up to 10,800 voice conversations
can be tapped easily (pros and cons)
much less susceptible to interference than twisted pair

33. Coax Disadvantages
high attenuation rate makes it expensive over long distance
Bulky
Error-prone
Turn radius
Connectors / terminators

34. Fiber Optic Cable
Fiber optic cable comes in three varieties, each with a different way of guiding light pulses from source to destination:
Multimode step-index (see Figure 7-13)
Multimode graded-index (see Figure 7-14)
Single-mode (see Figure 7-15)
All three have the same basic form and characteristics (see Figure 7-12), however, single mode has the smallest diameter core and greatest bandwidth (see Figure 7-16); single mode is also the most expensive
Several types of connectors exist (see Figure 7-17), but ST and SC are most common in LANs

35. Figure 7-12

36. Figure 7-13, 7-14, 7-15

37. Figure 7-16

38. Figure 7-17

39. Fiber Optic Advantages
greater capacity (bandwidth of up to 2 Gbps)
smaller size and lighter weight
lower attenuation
immunity to environmental interference
highly secure due to tap difficulty and lack of signal radiation

40. Fiber Optic Disadvantages
expensive over short distance
requires highly skilled installers
adding additional nodes is difficult

41. Radiated Media
The two most common kinds of radiated media used in wireless LANs are spread-spectrum radio (SSR) and infrared light; SSR is the most widely used
There are two major types of SSR:
Frequency hopping: this SSR technique changes the transmission frequency at regular intervals over a fixed spectrum; data are transmitted at one frequency, then the data are transmitted at a different frequency. Each piece of data is transmitted over several frequencies to increase the probability that it will be successfully received. Both Bluetooth and HomeRF rely on frequency hopping SSR
Direct sequencing: this SSR transmission technique simultaneously transmits the same data on different frequencies in order to increase the probability of success. Most WLANs use direct sequencing

42. Wireless Transmission
transmission and reception are achieved by means of an antenna
directional
transmitting antenna puts out focused beam
transmitter and receiver must be aligned
omnidirectional
signal spreads out in all directions
can be received by many antennas

43. Wireless Examples terrestrial microwave transmission
satellite transmission
broadcast radio
infrared

44. Terrestrial Microwave Transmission
uses the radio frequency spectrum, commonly from 2 to 40 Ghz
transmitter is a parabolic dish, mounted as high as possible
used by common carriers as well as by private networks
requires unobstructed line of sight between source and receiver
curvature of the earth requires stations (called repeaters) to be ~30 miles apart

45. Microwave Transmission Applications
long-haul telecommunications service for both voice and television transmission
short point-to-point links between buildings for closed-circuit TV or a data link between LANs
bypass application

46. Microwave Transmission Advantages
no cabling needed between sites
wide bandwidth
multichannel transmissions

47. Microwave Transmission Disadvantages
line of sight requirement
expensive towers and repeaters
subject to interference such as passing airplanes and rain

48. Satellite Microwave Transmission
a microwave relay station in space
can relay signals over long distances
geostationary satellites
remain above the equator at a height of 22,300 miles (geosynchronous orbit)
travel around the earth in exactly the time the earth takes to rotate

49. Satellite Transmission Links
earth stations communicate by sending signals to the satellite on an uplink
the satellite then repeats those signals on a downlink
the broadcast nature of the downlink makes it attractive for services such as the distribution of television programming

50. Satellite Transmission Process
transponder
dish
dish
22,300 miles
uplink station
downlink station

51. Satellite Transmission Applications
television distribution
a network provides programming from a central location
direct broadcast satellite (DBS)
long-distance telephone transmission
high-usage international trunks
private business networks

52. Principal Satellite Transmission Bands
C band: 4(downlink) - 6(uplink) GHz
the first to be designated
Ku band: 12(downlink) -14(uplink) GHz
rain interference is the major problem
Ka band: 19(downlink) - 29(uplink) GHz
equipment needed to use the band is still very expensive

53. Satellite Advantages can reach a large geographical area
high bandwidth
cheaper over long distances

54. Satellite Disadvantages
high initial cost
susceptible to noise and interference
propagation delay

55. Wiring Center Options
Most LAN architectures use wiring hubs to provide device interconnection
A wiring hub is a central interconnection point for workstations, servers, and other network equipment that is used in numerous LAN implementations to provide node-to-node connections (see Figure 7-19)
Hubs vary in terms of:
LAN architecture (e.g. Ethernet vs. token ring)
number of ports
stand-alone vs. stackable
unmanaged vs. managed
Because hubs do not process the frames transmitted by network-attached computers, they are often considered to be physical layer devices

56. Figure 7-19

57. LAN Switches
LAN switches physically resemble hubs and like hubs, they vary in number of ports, stand-alone vs. stackable, and managed vs. unmanaged
However, because they must read and process transmitted frames in order to provide a switched connection between network-attached devices, they are found at the data link layer (or higher)
Some switches support virtual LANs (VLANs). A VLAN is a logical group of workstations interconnected by one or more LAN switches that function as a self-contained LAN or workgroup

58. Wireless LAN Hubs
Wiring center options for wireless LANs depend on the kind of communication medium that is used (see Figure 7-20)
When SSR is used (e.g. in IEEE ) in WLANs, the wireless hubs are called access points
Access points are often used to create wireless segments of cable-based networks (see Figure 7-20b)

59. Figure 7-20a

60. Figure 7-20b

61. LAN Adapters
LAN adapters, also called network interface cards (NICs), provide the connection between the communication medium and a workstation or server bus (see Figure 7-21 and 7-22)
Adapters are designed to support a specific LAN architecture (e.g. Ethernet or token ring) and support one or more physical connection interfaces. They must be compatible with the computer’s bus architecture
PC cards (PCMCIA) adapters are available for notebook computers and wireless LAN adapters are needed for WLANs

62. Figure 7-21

63. Figure 7-22

64. NIC Functions
LAN adapters have their own onboard architectures (see Figure 7-23) and they carry out several important functions including:
monitoring activity on the communication medium
providing each workstation/server with a unique identification address
recognizing and receiving data transmitted to the computer
creating (building) the frames needed to transmit data on the communication medium
controlling LAN transmission speed
transmission error detection and recovery

65. Figure 7-23

66. Other Important LAN Hardware
Other key LAN hardware components include:
printers and print servers
backup devices and media including
duplexed servers
high-capacity removable disks
mirrored disks
RAID (redundant arrays of independent disks)
optical disk systems
virtual disk systems
magnetic tape backup systems
uninterruptible power supplies (UPSs)

67. Print Servers Print servers manage shared printers and print queues
Some file servers also function as print servers, however, dedicated print servers are very common
Most print servers are capable of managing multiple printers
Primary print server functions include job distribution (routing print jobs to the printers controlled by the print server) and job sequencing (managing the print queues for the individual printers controlled by the print server)

68. Storage Area Networks
Storage area networks (SANs) enable storage to be physically separated from an organization’s servers and to be managed as a central resource while still logically controlled by each server’s application software
There are two main ways of implementing SANs (see Figure 7-24):
centralized
decentralized
Network attached storage systems (NASs) are similar to centralized SANs, but connect to LANs much like a server

69. Figure 7-24

70. LAN-to-Host Interfaces
Today’s LAN-to-host interfaces include:
Installing a LAN adapter in the host and establishing node-to-host connections via LAN wiring center technologies
Dedicated point-to-point connections between LAN nodes and the host
Switched or dedicated connections via modems
Shared/multiplexed connections between LAN nodes and the host
Shared connections via a communication server
Connections via distributed or collapsed backbone networks
LAN-to-host gateways

71. Chapter 7 Introduction to Local Area Networks
Part II: Understanding Internet Access Technologies