1. Local Area Networks (LAN): The Basics
Chapter Seven
Local Area Networks (LAN): The Basics
Data Communications and Computer Networks: A Business User’s Approach, Fourth Edition

2. Outline Introduction Primary tasks of LAN Pros and Cons of LAN
Basic Network Topologies
Current LAN topologies
Medium Access Control Protocols
Data layers
Industrial LAN products
Wired Ethernet
IBM Token Ring
FDDI
Wireless Ethernet
Business Application
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3. Introduction
A LAN
is a communication network that interconnects a variety of data communicating devices within a small geographic area and broadcasts data at high data transfer rates with very low error rates
Since the LAN first appeared in the 1970s, its use has become widespread in commercial and academic environments
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4. Primary Function of Local Area Networks
To provide access to hardware and software resources that will allow users to perform one or more of the following activities:
File serving
A large storage disk drive acts as a central storage repository
Print serving
Providing the authorization to access a particular printer, accept and queue print jobs, and providing a user access to the print queue to perform administrative duties
Video transfers
High speed LANs are capable of supporting video image and live video transfers
Manufacturing support
LANs can support manufacturing and industrial environments
Academic support
In classrooms, labs, and wireless
support
Interconnection between multiple systems
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5. Advantages of Local Area Networks
Ability to share hardware and software resources
Individual workstation might survive network failure
Component and system evolution are possible
Support for heterogeneous forms of hardware and software
Access to other LANs and WANs (Figure 7-1)
Private ownership
Secure transfers at high speeds with low error rates
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6. Primary Function of Local Area Networks (continued)

7. Advantages and Disadvantages of Local Area Networks (continued)
Equipment and support can be costly
Level of maintenance continues to grow
Private ownership?
Some types of hardware may not interoperate
Just because a LAN can support two different kinds of packages does not mean their data can interchange easily
LAN is only as strong as its weakest link, and there are many links
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8. Basic Network Topologies
1. Star network
2. Hierarchical network
3. Mesh network
4. Bus network
5. Ring Network
6. Hybrid Network
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9. 1. Star network
All circuits radiate from a central node, typically a host computer
can be either or both point-to-point or multi-point circuits
See Figure 11-2
Example:
PBX is an example of star network
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10. FIGURE Star network.
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Applications
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11. Another application (to p12)
FIGURE Dow Corning’s U.S. data communications network has a classic star topology with the center at the company’s headquarters in Midland, Michigan.
Another application
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12. FIGURE Continued
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13. Disadv: if host computer fails, entire network is down
may become overloaded and unable to keep up with all messages that need to transmit
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14. 2. Hierarchical network
has a tree structure, with a top node called root node
See Figure 11-3
most likely implemented where the lower level nodes at the second or third level of computers
transmit is done from bottom node to the root node and vice-versa for replying information
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15. FIGURE 11-3 Hierarchical network.
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16. Advantage:
there is no single point of failure in the network
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17. 3. Mesh network
Similar to hierarchical network except there are more interconnections between nodes at different levels
In this respect, the higher level of node may not be exited at all
See Figure 11-4
Example:
Public telephone network
mobile or cellular phone system
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18. FIGURE Mesh network.
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19. 4. Bus network
is a telecommunications medium to which multiple nodes are attached
usually implemented in situations where the distance between all nodes is limited
Figure 11-5
Problems:
limited to number of devices as each node would contribute a certain amount of signals loss on the cable
when problem occurred, fault is difficult to detect
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20. FIGURE Bus network.
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21. 5. Ring Network
Associated with networks where nodes are relatively close together and each device is connected to the ring
Figure 11-6
signals are passed from one device to another one until destination is reached
this network is less subjected to attenuation (why?)
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22. FIGURE Ring network.
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Footnote

23. FIGURE 11-7 Ring network with two channels.
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24. to activate a device, all nodes must work together
to activate a device, all nodes must work together. Thus, if one node fails, potential exits that the entire ring network will be out of service
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25. 6. Hybrid Network
various network typology discussed are used to combine into hybrid network
if two or more networks are operated under different protocols, then a gateway device is needed to connect them together
Figure 11-8
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26. FIGURE 11-8 Hybrid network.
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27. Basic Local Area Network Topologies
Local area networks are interconnected using one of four basic configurations:
Bus/tree
Star-wired bus
Star-wired ring
Wireless
Comparison
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28. Bus/Tree Topology The original topology
Workstation has a network interface card (NIC) that attaches to the bus (a coaxial cable) via a tap
Data can be transferred using either baseband digital signals or broadband analog signals
Baseband signals are
bidirectional and more outward in both directions from the workstation transmitting
Broadband signals are
usually uni-directional and transmit in only one direction
Because of this, special wiring considerations are necessary
Buses can be split and joined, creating trees
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29. Bus/Tree Topology (continued)

30. Bus/Tree Topology (continued)

31. Bus/Tree Topology (continued)
Need to
Resolve this!
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32. Bus/Tree Topology (continued)
Need a
Turnaround
technique
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33. Star-Wired Bus Topology
Logically operates as a bus, but physically looks like a star
Star design is based on hub
All workstations attach to hub
Modular connectors and twisted pair make installation and maintenance of star-wired bus better than standard bus
Unshielded twisted pair usually used to connect workstation to hub, but can be connected using
twisted pair, coaxial cable, or fiber-optic cable
Hub takes incoming signal and immediately broadcasts it out all connected links, thus one talks everyone can listen (Disadv in security)
It also known as the shared network
All devices are sharing the network medium
Hubs can be interconnected to extend size of network
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34. Star-Wired Bus Topology (continued)

35. Star-Wired Bus Topology (continued)

36. Star-Wired Ring Topology
Logically operates as a ring but physically appears as a star
Based on MAU (Multistation Access Unit) which functions similarly to a hub
Where a hub immediately broadcasts all incoming signals onto all connected links, the MAU passes the signal around in a ring fashion
Like hubs, MAUs can be interconnected to increase network size
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37. Star-Wired Ring Topology (continued)

38. Star-Wired Ring Topology (continued)
MAU takes
Place here!
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39. Wireless LANS Not really a specific topology (Mesh!)
Workstation in a wireless LAN can be anywhere as long as it is within transmitting distance to an access point
Several versions of IEEE standard define various forms of wireless LAN connections
Workstations reside within Basic Service Set, while multiple basic service sets create an Extended Service Set
How it works?
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40. Wireless LANS (continued)
Two basic components necessary:
Client radio
Usually a PC card with an integrated antenna installed in a laptop or workstation
Access point (AP)
An Ethernet port plus a transceiver
AP acts as a bridge between the wired and wireless networks and can perform basic routing functions
Workstations with client radio cards reside within Basic Service Set, while multiple basic service sets create an Extended Service Set
Their standards
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41. Wireless LANS (continued)
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42. Wireless LANS (continued)
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43. Wireless LANS standards (continued)
IEEE
Original wireless standard, capable of transmitting data at 2 Mbps
IEEE b
Second wireless standard, capable of transmitting data at 11 Mbps
In actual tests, 11 Mbps b devices managed 5.5 Mbps (from a July 2000 test by Network Computing)
With directional antennae designed for point-to-point transmission (rare), b can transmit for more than 10 miles
With an omni-directional antenna on typical AP, range may drop to as little as 100 feet
Recent standards
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44. IEEE 802.11a IEEE 802.11g Other standards
One of the more recent standards
Capable of transmitting data at 54 Mbps (theoretical) using the 5-GHz frequency range
IEEE g
The other recent standard
Also capable of transmitting data at 54 Mbps (theoretical) but using the same frequencies as b (2.4-GHz)
Is backwards compatible with b
Other standards
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45. Wireless LANS (continued)
HiperLAN/2 (European standard, 54 Mbps in 5-GHz band)
To provide security, most systems use either:
Wired Equivalent Privacy (WEP) – provides either 40- or 128-bit key protection
WPA or some other more advanced standard
Wireless LANs may also be configured without an access point
These configurations are called “ad-hoc”
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46. Wireless LANS (continued)
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47. Comparison of Bus, Star-Wired Bus, Star-Wired Ring, and Wireless Topologies

48. Medium Access Control Protocols
How does a workstation get its data onto the LAN medium?
A medium access control protocol is the software that allows workstations to “take turns” at transmitting data
Two basic categories:
Contention-based protocols
Round-robin protocols
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49. Contention-Based Protocols
Essentially first-come, first-served
Most common example is carrier sense multiple access with collision detection (CSMA/CD)
If no one is transmitting, workstation can transmit
If someone else is transmitting, workstation “backs off” and waits
If two workstations transmit at same time, collision occurs
When two workstations hear collision, they stop transmitting immediately
Each workstation backs off a random amount of time and tries again
Hopefully, both workstations do not try again at exact same time
CSMA/CD is an example of a nondeterministic protocol
Another one CSMA/CA
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50. Contention-Based Protocols (continued)
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51. Contention-Based Protocols (continued)
CA (Collision avoidance)
Protocol does not listen and detect collisions
Instead, tries to avoid collisions before they happen
How does CSMA/CA do this?
All devices, before they transmit, must wait an amount of time called an interframe space (IFS)
Some applications have a short IFS, while others have a long IFS
If two applications want to transmit at same time, the application with shorter IFS will go first
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52. Round-Robin Protocols
Each workstation takes a turn transmitting and the turn is passed around the network from workstation to workstation
Most common example is token ring LAN in which a software token is passed from workstation to workstation
Token ring is an example of a deterministic protocol
Token ring more complex than CSMA/CD
What happens if token is lost? Duplicated? Hogged?
Token ring LANs are losing the battle with CSMA/CD LANs
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53. Round Robin Protocols (continued)
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54. IEEE 802
To better support local area networks, the data link layer of the OSI model was broken into two sublayers:
Logical link control sublayer
Medium access control sublayer
Medium access control sublayer defines frame layout and is more closely tied to a specific medium at the physical layer
Thus, when people refer to LANs they often refer to its MAC sublayer name, such as 10BaseT
IEEE Frame Format
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55. IEEE 802 (continued)
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56. IEEE Frame Format
IEEE 802 suite of protocols defines frame formats for CSMA/CD (IEEE 802.3) and token ring (IEEE 802.5)
Each frame format describes how data package is formed
The two frames do not have the same layout
If a CSMA/CD network connects to a token ring network, the frames have to be converted from one to another
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57. IEEE 802.3 Frame Format (continued)
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58. Wired Ethernet Most common form of LAN today
Star-wired bus is most common topology but bus topology still not totally dead yet
Comes in many forms depending upon medium used and transmission speed and technology
Originality and its development
Ethernet Standards
Its trends
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59. Wired Ethernet (continued)
Originally, CSMA/CD was 10 Mbps
Then 100 Mbps was introduced
Most NICs sold today are 10/100 Mbps
Then 1000 Mbps (1 Gbps) was introduced
1000 Mbps introduces a few interesting wrinkles:
Transmission is full-duplex (separate transmit and receive), thus no collisions
Prioritization is possible using 802.1p protocol
Topology can be star or mesh (for trunks)
10 Gbps is now being installed in high-end applications
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60. Wired Ethernet (continued)
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61. Wired Ethernet (continued)
One of the latest features is power over Ethernet (PoE) – ie supply electricity current over the medium
What if you have a remote device that has an Ethernet connection?
It will require a power connection
What if you don’t have an electrical outlet nearby?
Use PoE
Power to drive Ethernet NIC is sent over wiring along with usual Ethernet signals
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62. IBM Token Ring
Deterministic LAN offered at speeds of 4, 16 and 100 Mbps
Very good throughput under heavy loads
More expensive components than CSMA/CD
Losing ground quickly to CSMA/CD
May be extincted soon
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63. Fiber Distributed Data Interface (FDDI)
Based on token ring design using 100 Mbps fiber connections
Allows for two concentric rings
Inner ring can support data travel in opposite direction or work as backup
Token is attached to outgoing packet, rather than waiting for outgoing packet to circle entire ring
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64. Wireless Ethernet
As we have already seen, IEEE has created the b, a, and g wireless standards
IEEE n (100 Mbps) will be ratified soon and started appearing in product form in 2007
Latest wireless Ethernet is using MIMO technology (multiple input multiple output)
Sender and receiver have multiple antennas for optimum reception
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65. LANs In Action: A Small Office Solution
What type of system will interconnect 20 workstations in one room and 15 workstations in another room to a central server, which offers:
Internal
A database that contains all customer information
High-quality printer access

66. LANs In Action: A Small Office Solution (continued)

67. LANs In Action: A Small Office Solution (continued)

68. LANs In Action: A Home Office Solution
What if you have two computers at home and want both to share a printer and a connection to the Internet?
Some type of SOHO solution might solve this problem
Essentially a LAN with a 2- or 3-port hub, connecting cables, and software
In some models the hub also acts as a router to the Internet

69. LANs In Action: A Home Office Solution (continued)