1. Chapter 7 Local Area Networks : The Basics
Data Communications and
Computer Networks: A
Business User’s Approach
Chapter 7
Local Area Networks : The Basics

2. Last time Types of errors and their prevention Error detection
Parity – 50%
CRC – can detect nearly all errors
Error correction
1. Do nothing
2. Return an error message to the transmitter
3. Fix the error with no further help from the transmitter

3. Three Major Types of Networks
Local Area Network (LAN)
Serves users within a confined geographical area (usually within a mile).
Metropolitan Area Network (MAN)
Covers a geographic area the size of a city or suburb. The purpose of a MAN is often to bypass local telephone companies when accessing long-distance service.
Wide Area Network (WAN)
Covers a wide geographical area, such as a state or a country. Examples: Tyment, Telenet, Uninet, and Accunet.

4. Emphasis Definition of a Local Area Network (LAN)
Physical and Logical LAN Topologies
Different Medium Access Control Protocols
Common LAN Systems
Ethernet (1st commercially available LAN)
Token ring
FDDI (fiber data distributed interface) ring

5. Introduction
A local area network 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
very low error rates.
(WANs now do this too)
Since the local area network first appeared in the 1970s, its use has become widespread in commercial and academic environments.

6. Functions of a Local Area Network
Data Communications and Computer Networks
Chapter 7
Functions of a Local Area Network
File server - A large storage disk drive that acts as a central storage repository.
Print server - Provides the authorization to access a particular printer, accept and queue print jobs, and provides a user access to the print queue to perform administrative duties.
Interconnection - A LAN can provide an interconnection to other LANs and to wide area networks.

7. Functions of a Local Area Network
Data Communications and Computer Networks
Chapter 7
Functions of a Local Area Network
Video transfers - High speed LANs are capable of supporting video image and live video transfers.
Manufacturing support - LANs can support manufacturing and industrial environments.
Distributed processing - LANs can support network operating systems which perform the operations of distributed processing.

8. Advantages of Local Area Networks
Data Communications and Computer Networks
Chapter 7
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.

9. Data Communications and Computer Networks
Chapter 7

10. Disadvantages of Local Area Networks
Data Communications and Computer Networks
Chapter 7
Disadvantages of Local Area Networks
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.
A LAN is only as strong as it weakest link, and there are many links.

11. Basic Network Topologies
Data Communications and Computer Networks
Chapter 7
Basic Network Topologies
Topology: the geometric configuration
Separate from protocols!
Local area networks are interconnected using one of four basic configurations:
1. Bus/tree
2. Star-wired bus
3. Star-wired ring
4. Wireless

12. Basic Network Topologies
Logical vs physical design (topologies)
1. Logical design: how the data moves around the network from workstation to workstation
2. Physical design: how the network physically appears if drawn on a sheet of paper (laid out)
Names of topologies usually refer to the physical design.

13. Bus/Tree Topology Data Communications and Computer Networks
Chapter 7
Bus/Tree Topology
The original topology – 1970’s
Workstation has a network interface card (NIC) provides a physical connection to a network
Data can be transferred using either
baseband digital signals
broadband analog signals.

14. Bus/Tree Topology Data Communications and Computer Networks
Chapter 7
Bus/Tree Topology
Workstation has a network interface card (NIC) provides a physical connection to a network
Attaches to the bus (a coaxial cable) via a tap.
NIC is an electronic device that performs the necessary signal conversions and protocols operations so that the workstation can send and receive data on the network.
Tap is a passive device
Does not alter the signal
Does not require electricity to operate

15. Data Communications and Computer Networks
Chapter 7

16. LAN Topologies: Bus
Consists of a single wire to which individual stations are attached
Each end has a terminator attached to it
Data travels the entire length of the cable
Transmission from any stations travels entire medium (both directions)
Inexpensive and easy to install
Ethernet is the common form of a bus topology system

17. Data Communications and Computer Networks
Chapter 7

18. Bus/Tree Topology Baseband signals Digital signals – 10 Mbps
Bidirectional and more outward in both directions from the workstation transmitting.
Easy to install and maintain
Fewer than 100 workstations
Buses can be split and joined, creating trees.

19. Data Communications and Computer Networks
Chapter 7

20. Bus/Tree Topology Broadband signals
Usually uni-directional and transmit in only one direction.
Analogy and FDM for multiple channels (amplification necessary). Because of this, special wiring considerations are necessary.
100 to 1000 workstations over larger distances due to easy amplification
Buses can be split and joined, creating trees.

21. Data Communications and Computer Networks
Chapter 7
Twice the propagation
delay of baseband

22. Data Communications and Computer Networks
Chapter 7

23. Advantages/disadvantages of a bus
Data Communications and Computer Networks
Chapter 7
Advantages/disadvantages of a bus
Difficult to add new devices if no tap exists.
No tap existing means cutting into the line
As such, this topology is loosing popularity
Plenty still around; Ethernet uses this.

24. Star-wired Topologies
Data Communications and Computer Networks
Chapter 7
Star-wired Topologies
Stars versus a single line
Two types:
Star-wired bus
(often call the star topology)
Star-wired ring

25. 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.
Hub is an unintelligent device that immediately transmits whatever data it receives to all connections
Unshielded twisted pair usually used to connect workstation to hub.
Hub takes incoming signal and immediately broadcasts it out all connected links.
Hubs can be interconnected to extend size of network.
Very popular!

26. Star-wired bus physical topology for a LAN

27. Interconnection of two hubs in a star-wired bus LAN

28. Star-wired Bus Topology
Data Communications and Computer Networks
Chapter 7
Star-wired Bus Topology
Modular connectors and twisted pair make installation and maintenance of star-wired bus better than standard bus.
Hubs can be interconnected with twisted pair, coaxial cable, or fiber optic cable.
Biggest disadvantage: when one station talks, everyone hears it. This is called a shared network. All devices are sharing the network medium.

29. Star-wired Ring Topology
Data Communications and Computer Networks
Chapter 7
Star-wired Ring Topology
Logically operates as a ring but physically appears as a star.
Star-wired ring topology is based on MAU (multi-station 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 in one direction.
Like hubs, MAUs can be interconnected to increase network size.

30. Ring topology viewed logically

31. NIC Repeaters for Star-wired Ring
Data Communications and Computer Networks
Chapter 7
NIC Repeaters for Star-wired Ring
All data must pass through a NIC repeater
Performs basic functions:
Bypass: data does not copy to the workstation, used for inactive devices
Copy: data from ring copied to workstation
Write: data from the workstation copied to the ring
Regenerates: recopies data back onto ring

32. Three possible operations of the workstation repeater
on a ring topology

33. Physical organization of a ring topology
Looks like a star!

34. Multi-station access unit on a ring topology
The MAU replaces the hub for this topology

35. Wireless Topology Data Communications and Computer Networks
Chapter 7
Wireless Topology
Not really a specific topology since a workstation in a wireless LAN can be anywhere as long as it is within transmitting distance to an access point.
Range varies from 50 to 800 ft with speeds of 2 to 11 Mbps
Most wireless LANs include a wired LAN backbone
Usually requires a wireless NIC

36. Stanford’s wireless network

37. Wireless Topology – different forms
Data Communications and Computer Networks
Chapter 7
Wireless Topology – different forms
Newer IEEE and b (Wi-Fi) standard defines various forms of wireless LAN connections.
Speeds up to 11 Mbps with b standard.
Workstations reside within a basic service set, while multiple basic service sets create an extended service set.

38. Wireless Topology Data Communications and Computer Networks
Chapter 7
Wireless Topology
Acceptable transmission ranges broken up into areas:
Basic service set – that surrounding an access point
Extended service set – collection of basic service sets
Workstations reside within a basic service set, while multiple basic service sets create an extended service set.

39. Wireless Topology Data Communications and Computer Networks
Chapter 7
Wireless Topology
Two basic components necessary: the client radio, usually a PC card with an integrated antenna, and the access point (AP), which is an Ethernet port plus a transceiver.
The AP acts as a bridge between the wired and wireless networks and can perform basic routing functions.
Workstations with client radio cards reside within a basic service set, while multiple basic service sets create an extended service set.

40. Single-cell wireless LAN configuration

41. Multiple-cell wireless LAN configuration

42. Ad-hoc configuration for a wireless LAN

43. Wireless Topology Data Communications and Computer Networks
Chapter 7
Wireless Topology
With directional antennae designed for point-to-point transmission (rare), b can work for more than 10 miles.
With an omni-directional antenna on a typical AP, range may drop to as little as 100 feet.
Distance is inversely proportional to transmission speed - as speed goes up, distance goes down.

44. Wireless Topology Data Communications and Computer Networks
Chapter 7
Wireless Topology
In actual tests, 11 Mbps b devices managed 5.5 Mbps
To provide security, most systems use Wired Equivalent Privacy (WEP), which provides either 40- or 128-bit key protection. (how good?)
What will Bluetooth’s impact be on b (Wi-Fi)?

45. Other Wireless Standards
IEEE (older 2 Mbps)
IEEE b (11 Mbps, 2.4 GHz)
Also called Wi-Fi (wireless fidelity)
IEEE a (54 Mbps, 5 GHz, in 2002)
IEEE g (54 Mbps, 2.4 GHz, in 2002)
HiperLAN/2 (European standard, 54 Mbps in 5 GHz band)

46. Data Communications and Computer Networks
Chapter 7

47. Summary of topologies Logical vs physical topologies
Bus and star-rings - old technologies
Still some around
You’ll probably use
Star-wired bus (star)
With bus or routers (now much easier to use)
Wireless network (Wi-fi)

48. Protocols
Last time topologies
Now how the data actually flows

49. Medium Access Control (MAC) Protocols
Data Communications and Computer Networks
Chapter 7
Medium Access Control (MAC) 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.
Since a LAN is a broadcast network, it’s imperative that only one workstation at a time be allowed to transmit data onto the network. (for multiple channels, one channel per workstation)
Switches have changed that, but we’ll take about that in Ch8.

50. Medium Access Control Protocols
Data Communications and Computer Networks
Chapter 7
Medium Access Control Protocols
Three basic categories:
1. Contention-based protocols (1st come; 1st serve)
2. Round robin protocols (take your turn!!)
3. Reservation protocols (requests please!)

51. Contention-Based Protocols
Data Communications and Computer Networks
Chapter 7
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, a workstation can transmit.
If someone else is transmitting, the workstation “backs off” and waits.
Half duplex protocol.

52. Contention-Based Protocols
If two workstations transmit at the same time, a collision occurs.
When the two workstations hear the collision, they stop transmitting immediately.
Each workstation backs off a random amount of time and tries again. Workstations use a persistence algorithm to decide when to resubmit.
Hopefully, both workstations do not try again at the exact same time.
CSMA/CD is an example of a non-deterministic protocol (cannot calculate time at which a workstation will transmit)

53. Collisions Data Communications and Computer Networks
Chapter 7
Collisions
Collision window is the interval during which a workstation’s signal can propagate down the bus and back. During this period a workstation might falsely hear no one transmitting and retransmit a signal.
Changes for collisions go up when every workstation wants to send something – heavy traffic!

54. Data Communications and Computer Networks
Chapter 7

55. Collisions
Busy CSMA/CD networks can waste over half their time dealing with collisions.

56. Round Robin Protocols Data Communications and Computer Networks
Chapter 7
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.

57. Token Passing Protocols
Before workstation can transmit it must possess the one and only token
Two types:
Token ring
Token bus

58. Data Transmission on a token ring LAN
Station A has just released the token
Station M copies the data meant for it from B
B removes the data it sent and releases the token

59. Why CSMA/CD more popular than Token Ring
Data Communications and Computer Networks
Chapter 7
Why CSMA/CD more popular than Token Ring
1st LAN MAC method, more installations and equipment.
Token rings have almost always lagged behind in transmission speed.
CSMA/CD less expensive to implement because of widespread marketing and acceptance.
CSMA/CD is a simpler protocol.
Wireless CSMA/CD now exists

60. Token Bus Data Communications and Computer Networks
Chapter 7
Token Bus
Just like the token ring, but a bus instead
Each workstation maintains a list of neighbors that one passes the token to (logical not physical) neighbor
Designed primarily for manufacturing plants since a non-deterministic protocol is not acceptable there. Why?

61. Reservation Protocols
Data Communications and Computer Networks
Chapter 7
Reservation Protocols
Workstation places a reservation with central server.
Workstation cannot transmit until reservation comes up.
Under light loads, this acts similar to CSMA/CD.
Under heavy loads, this acts similar to token ring.
Powerful access method but again losing out to CSMA/CD.
Most common example of reservation protocol is demand priority protocol.

62. Problems with the OSI model for LANs
Problems with broadcast networks!
Routing (no decisions have to be made)
Data link and physical link tightly coupled
Need to specify hardware! Ugh!
Broadcast networks need to be have a medium access control to decide who talks

63. Medium Access Control Sublayer
Data Communications and Computer Networks
Chapter 7
Medium Access Control Sublayer
To better support local area networks, the data link layer of the OSI model was broken into two sublayers:
1. Logical link control sublayer
2. Medium access control (MAC) sublayer
Medium access control sublayer defines the 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.

64. Modification of OSI model to split data link layer into
two sublayers

65. IEEE 802 Frame Formats Data Communications and Computer Networks
Chapter 7
IEEE 802 Frame Formats
The IEEE 802 suite of protocols defines the frame formats for CSMA/CD (IEEE 802.3) and token ring (IEEE 802.5).
Each frame format describes how the data package is formed.
Note how the two frames are different. If a CSMA/CD network connects to a token ring network, the frames have to be converted from one to another.

66. Frame format for IEEE 802.3 CSMA/CD Protocol

67. Frame Format for IEEE 802.5 Token Ring Protocol

68. Local Area Network Systems
Data Communications and Computer Networks
Chapter 7
Local Area Network Systems
The entire system with protocol and topology

69. Local Area Network Systems
Data Communications and Computer Networks
Chapter 7
Local Area Network Systems
Ethernet or CSMA/CD
Most common form of LAN today.
Star-wired bus is most common topology but bus topology also around.
Ethernet comes in many forms depending upon medium used and transmission speed and technology.

70. Ethernet Data Communications and Computer Networks
Chapter 7
Ethernet
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.
10 Gbps is now available.

71. Ethernet Data Communications and Computer Networks
Chapter 7
Ethernet
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).

72. Ethernet Data Communications and Computer Networks
Chapter 7
Ethernet
Cabling can be either UTP (unshielded twisted pair) or optical (but 10 Gbps Ethernet may not work over UTP due to radio frequency interference).
Where 10 Mbps Ethernet has less than 30% utilization due to collisions, 1000 Mbps is limited only by traffic queueing.
Distance with 10 Mbps is limited by CSMA/CD propagation time, whereas 1000 Mbps limited only by media.

74. Ethernet Standards Key
XXXBase or XXX Broad
XXX 10 means 10Mbps

75. Local Area Network Systems
Data Communications and Computer Networks
Chapter 7
Local Area Network Systems
IBM Token Ring
Deterministic LAN offered at speeds of 4, 16 and 100 Mbps using star-ring topology.
Very good throughput under heavy loads.
More expensive components than CSMA/CD.
Losing ground quickly to CSMA/CD. May be extinct soon.

76. Local Area Network Systems
Data Communications and Computer Networks
Chapter 7
Local Area Network Systems
FDDI (Fiber Distributed Data Interface)
Based on the 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 the outgoing packet, rather than waiting for the outgoing packet to circle the entire ring.

77. FDDI dual ring topology

78. Local Area Network Systems
Data Communications and Computer Networks
Chapter 7
Local Area Network Systems
100VG-AnyLAN
Deterministic LAN based on demand priority access method.
Similar to hub topology (star design).
Two levels of priority - normal and high.
Supports a wide-variety of media types.
Losing ground quickly to CSMA/CD. Will be extinct soon?

79. Data Communications and Computer Networks
Chapter 7

80. LANs In Action : A Small Office Solution
Data Communications and Computer Networks
Chapter 7
LANs In Action : A Small Office Solution
What type of system will interconnect twenty 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

81. Data Communications and Computer Networks
Chapter 7

82. Data Communications and Computer Networks
Chapter 7

83. LANs In Action : A Home Office Local Area Network Solution
Data Communications and Computer Networks
Chapter 7
LANs In Action : A Home Office Local Area Network 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 “network-in-a-box” 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.

84. Data Communications and Computer Networks
Chapter 7

85. What we covered Topologies Protocols Systems
Star-bus
Wireless
Protocols
CSMA/CD
Systems
Ethernet
Next time: wireless networks?