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Local Area Networks : The Basics

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Title: Local Area Networks : The Basics


1
Data Communications and Computer Networks A
Business Users 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
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
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
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
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
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
Data Communications and Computer Networks
Chapter 7
  • Bus/Tree Topology
  • The original topology 1970s
  • 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
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
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
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
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
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
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
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
Stanfords wireless network
37
Data Communications and Computer Networks
Chapter 7

Wireless Topology different forms Newer IEEE
802.11 and 802.11b (Wi-Fi) standard defines
various forms of wireless LAN connections. Speeds
up to 11 Mbps with 802.11b standard. Workstations
reside within a basic service set, while multiple
basic service sets create an extended service set.
38
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
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
Data Communications and Computer Networks
Chapter 7

Wireless Topology With directional antennae
designed for point-to-point transmission (rare),
802.11b 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
Data Communications and Computer Networks
Chapter 7

Wireless Topology In actual tests, 11 Mbps
802.11b 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
Bluetooths impact be on 802.11b (Wi-Fi)?
45
  • Other Wireless Standards
  • IEEE 802.11 (older 2 Mbps)
  • IEEE 802.11b (11 Mbps, 2.4 GHz)
  • Also called Wi-Fi (wireless fidelity)
  • IEEE 802.11a (54 Mbps, 5 GHz, in 2002)
  • IEEE 802.11g (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
  • Youll 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
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, its 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 well take about that in Ch8.
50
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
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
Data Communications and Computer Networks
Chapter 7

Collisions Collision window is the interval
during which a workstations 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
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
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
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
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
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
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
Data Communications and Computer Networks
Chapter 7

Local Area Network Systems The entire system with
protocol and topology
69
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
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
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. Priorit
ization is possible using 802.1p
protocol. Topology can be star or mesh (for
trunks).
72
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.
73

74
Ethernet StandardsKey
  • XXXBase or XXX Broad
  • XXX 10 means 10Mbps

75
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
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
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
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 e-mail
  • 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
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
  • Star-bus
  • Wireless
  • Protocols
  • CSMA/CD
  • Systems
  • Ethernet
  • Next time wireless networks?
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