Komunikasi Data dan Jaringan Komputer(Bagian 3)
Dr. Tb. Maulana [email protected]
http://staffsite.gunadarma.ac.id/mkusuma
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LAN Generation First Carrier Sense Multiple Access (CSMA) /
Collision Detection (CD) and Token Ring Terminal to host and client
server Moderate data rates Second Fiber Distributed Data Interface
(FDDI) Backbone High performance workstations Third Asynchronous
Transfer Mode (ATM) Aggregate throughput and real time support for
multimedia applicationsMagister Teknik Elektro 1
Third Generation LANSupport for multiple guaranteed classes of
service
Live video may need 2Mbps File transfer can use background class
Both aggregate and per host
Scalable throughput
Facilitate LAN / WAN internetworking
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LAN Applications (1)Personal computer LANs Low cost Limited data
rate Back end networks and storage area networks Interconnecting
large systems (mainframes and large storage devices) High data rate
High speed interface Distributed access Limited distance Limited
number of devicesMagister Teknik Elektro 3
LAN Applications (2)High speed office networks
Desktop image processing High capacity local storage
Interconnect low speed local LANs Reliability Capacity CostMagister
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Backbone LANs
LAN ArchitectureProtocol architecture Topologies Media Access
Control (MAC) Logical Link Control (LLC)
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Protocol ArchitectureLower layers of OSI model IEEE 802
reference model Physical LLC MAC
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IEEE 802 v OSI
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802 Layers PhysicalEncoding/decoding Preamble generation/removal
Bit transmission/reception Transmission medium and topology
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802 Layers Logical Link ControlInterface to higher levels Flow
and error control
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802 Layers Media Access ControlAssembly of data into frame with
address and error detection fields Disassembly of frame Address
recognition Error detection Govern access to transmission medium
Not found in traditional layer 2 data link control For the same
LLC, several MAC options may be availableMagister Teknik Elektro
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LAN Protocols in Context
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TopologiesTree Bus
Special case of treeOne trunk, no branches
Ring Star
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LAN Topologies
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Bus and TreeMultipoint medium Transmission propagates throughout
medium Heard by all stations Need to identify target station Each
station has unique address Full duplex connection between station
and tap Allows for transmission and reception Need to regulate
transmission To avoid collisions To avoid hogging Data in small
blocks - frames Terminator absorbs frames at end of mediumMagister
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Frame Transmission - Bus LAN
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Ring TopologyRepeaters joined by point to point links in closed
loop Receive data on one link and retransmit on another Links
unidirectional Stations attach to repeaters Data in frames
Circulate past all stations Destination recognizes address and
copies frame Frame circulates back to source where it is removed
Media access control determines when station can insert
frameMagister Teknik Elektro 16
Frame Transmission Ring LAN
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Star TopologyEach station connected directly to central node
Usually via two point to point links Physical star, logical bus
Only one station can transmit at a time
Central node can broadcast
Central node can act as frame switch
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Media Access ControlWhere Central Greater control Simple access
logic at station Avoids problems of co-ordination coSingle point of
failure Potential bottleneck Distributed How Synchronous Specific
capacity dedicated to connection Asynchronous In response to
demandMagister Teknik Elektro 19
Asynchronous SystemsRound robin Good if many stations have data
to transmit over extended period Reservation Good for stream
traffic Contention Good for bursty traffic All stations contend for
time Distributed Simple to implement Efficient under moderate load
Tend to collapse under heavy loadMagister Teknik Elektro 20
Logical Link ControlTransmission of link level PDUs between two
stations Must support multiaccess, shared medium Relieved of some
link access details by MAC layer Addressing involves specifying
source and destination LLC users Referred to as service access
points (SAP) Typically higher level protocolMagister Teknik Elektro
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Bus LANSignal balancing Signal must be strong enough to meet
receivers minimum signal strength requirements Give adequate signal
to noise ration Not so strong that it overloads transmitter Must
satisfy these for all combinations of sending and receiving station
on bus Usual to divide network into small segments Link segments
with amplifies or repeaters
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Transmission MediaTwisted pair Not practical in shared bus at
higher data rates Baseband coaxial cable Used by Ethernet Broadband
coaxial cable Included in 802.3 specification but no longer made
Optical fiber Expensive Difficulty with availability Not used Few
new installations Replaced by star based twisted pair and optical
fiberMagister Teknik Elektro 23
Baseband Coaxial CableUses digital signaling Manchester or
Differential Manchester encoding Entire frequency spectrum of cable
used Single channel on cable Bi-directional BiFew kilometer range
Ethernet (basis for 802.3) at 10Mbps 50 ohm cableMagister Teknik
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10Base5Ethernet and 802.3 originally used 0.4 inch diameter
cable at 10Mbps Max cable length 500m Distance between taps a
multiple of 2.5m
Ensures that reflections from taps do not add in phase
Max 100 taps 10Base5Magister Teknik Elektro 25
10Base2Cheaper 0.25 inch cable
More flexible Easier to bring to workstation Cheaper electronics
Greater attenuation Lower noise resistance Fewer taps (30) Shorter
distance (185m)Magister Teknik Elektro 26
RepeatersTransmits in both directions Joins two segments of
cable No buffering No logical isolation of segments If two stations
on different segments send at the same time, packets will collide
Only one path of segments and repeaters between any two
stationsMagister Teknik Elektro 27
Baseband Configuration
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Ring LANEach repeater connects to two others via unidirectional
transmission links Single closed path Data transferred bit by bit
from one repeater to the next Repeater regenerates and retransmits
each bit Repeater performs data insertion, data reception, data
removal Repeater acts as attachment point Packet removed by
transmitter after one trip round ringMagister Teknik Elektro 29
Ring Repeater States
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Listen State FunctionsScan passing bit stream for patterns
Address of attached station Token permission to transmit
Copy incoming bit and send to attached station
Whilst forwarding each bit e.g. to indicate a packet has been
copied (ACK)Magister Teknik Elektro 31
Modify bit as it passes
Transmit State FunctionsStation has data Repeater has permission
May receive incoming bits
If ring bit length shorter than packetPass back to station for
checking (ACK)
May be more than one packet on ringBuffer for retransmission
later
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Bypass StateSignals propagate past repeater with no delay (other
than propagation delay) Partial solution to reliability problem
(see later) Improved performance
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Star LANUse unshielded twisted pair wire (telephone) Minimal
installation cost May already be an installed base All locations in
building covered by existing installation Attach to a central
active hub Two links Transmit and receive Hub repeats incoming
signal on all outgoing lines Link lengths limited to about 100m
Fiber optic - up to 500m Logical bus - with collisionsMagister
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Hubs and SwitchesShared medium hub Central hub Hub retransmits
incoming signal to all outgoing lines Only one station can transmit
at a time With a 10Mbps LAN, total capacity is 10Mbps Switched LAN
hub Hub acts as switch Incoming frame switches to appropriate
outgoing line Unused lines can also be used to switch other traffic
With two pairs of lines in use, overall capacity is now
20MbpsMagister Teknik Elektro 35
Switched HubsNo change to software or hardware of devices Each
device has dedicated capacity Scales well Store and forward switch
Accept input, buffer it briefly, then output Cut through switch
Take advantage of the destination address being at the start of the
frame Begin repeating incoming frame onto output line as soon as
address recognized May propagate some bad framesMagister Teknik
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Hubs and Switches
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Wireless LANWireless LANs are growing in popularity because they
eliminate cabling and facilitate network access from a variety of
locations. The most common wireless networking standard is IEEE
802.11, often called Wireless Ethernet or Wireless LAN. Broadband
wireless (IEEE 802.16) is now growing in popularityMagister Teknik
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Wireless CommunicationsIn wireless communications signals travel
through space instead of through a physical cable. Two general
types of wireless communications are:
Radio transmission Infrared transmission
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Why Wireless LAN?Mobility Flexibility Hard to wire areas Reduced
cost of wireless systems Improved performance of wireless
systems
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Types of Wireless LANsIEEE 802.11a IEEE 802.11b IEEE 802.11g
Infrared Bluetooth
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IEEE 802.11bTwo forms of the IEEE 802.11b standard currently
exist: Direct Sequence Spread Spectrum (DSSS) systems transmit
signals through a wide range of frequencies simultaneously. The
signal is divided into many different parts and sent on different
frequencies simultaneously. Data rate: ~ 11Mbps. Frequency Hopping
Spread Spectrum (FHSS) divides the frequency band into a series of
channels and then use each frequency in turn. FHSS changes its
frequency channel about every half a second, using a pseudorandom
sequence.Magister Teknik Elektro 42
FHSS is more secure, but is only capable of data rates of 1 or 2
Mbps. IEEE 802.11a is another Wireless LAN standard developed
around the same time as 802.11b. It operates in the 5 GHz band and
is capable of data rates of up to 54 Mbps. IEEE 802.11g combines
the best of both 802.11a and 802.11b. 802.11g supports bandwidth up
to 54 Mbps, and it uses the 2.4 Ghz frequency for greater range.
802.11g is backwards compatible with 802.11b, meaning that 802.11g
access points will work with 802.11b wireless network adapters and
vice versa.Magister Teknik Elektro 43
IEEE 802.11a vs 802.11b vs 802.11g802.11a is the most expensive.
It fits predominately in the business market, whereas 802.11b
better serves the home market. 802.11a supports bandwidth up to 54
Mbps and signals in a regulated 5 GHz range. Compared to 802.11b,
this higher frequency limits the range of 802.11a. The higher
frequency also means 802.11a signals have more difficulty
penetrating walls and other obstructions.
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Although slower than 802.11a, the range of 802.11b is about 7
times greater than that of 802.11a. Because 802.11a and 802.11b
utilize different frequencies, the two technologies are
incompatible with each other. Some vendors offer hybrid 802.11a/b
network gear, but these products simply implement the two standards
side by side. 802.11g offers the best of both worlds and allow for
greater flexibility.
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IEEE LAN Standard 802.11a
Pros
Cons
fastest maximum speed; highest cost; supports more shorter range
signal that is simultaneous users; more easily obstructed less
signal interference from other devices lowest cost; signal range is
best and is not easily obstructed slowest maximum speed; supports
fewer simultaneous users; appliances may interfere on the
unregulated frequency band46
802.11b
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IEEE Lan Standard 802.11g
Prosfastest maximum speed; supports more simultaneous users;
signal range is best and is not easily obstructedMagister Teknik
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Conscosts more than 802.11b; appliances may interfere on the
unregulated signal frequency
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Wireless LAN ApplicationsLAN Extension Cross building
interconnection Nomadic access Ad hoc networks
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LAN ExtensionBuildings with large open areas
Manufacturing plants Warehouses
Historical buildings Small offices May be mixed with fixed
wiring system
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Single Cell Wireless LAN
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Multi Cell Wireless LAN
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Cross Building InterconnectionPoint to point wireless link
between buildings Typically connecting bridges or routers Used
where cable connection not possible
e.g. across a street
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Nomadic AccessMobile data terminal
e.g. laptop
Transfer of data from laptop to server Campus or cluster of
buildings
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Ad Hoc NetworkingPeer to peer Temporary e.g. conference
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Wireless LAN Configurations
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Wireless LAN RequirementsThroughput Number of nodes Connection
to backbone Service area Battery power consumption Transmission
robustness and security Collocated network operation License free
operation HandHand-off / roaming Dynamic configurationMagister
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Wireless LAN TechnologyInfrared (IR) LANs Spread spectrum LANs
Narrow band microwave
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Wireless LAN standard IEEE 802.11The IEEE 802.11 standard for
wireless LANs was finalized in 1997. The standard defines three
different physical layer specifications - 2 are radio
frequencyfrequency-based and one is infraredinfraredbased:
Direct Sequence Spread Spectrum FrequencyFrequency-hopping
spectrum InfraredMagister Teknik Elektro 58
Wireless LAN ComponentsThe smallest building block of a wireless
LAN is called the Basic Service Set (BSS). BSS is a number of
stations executing the same MAC protocol and using the same shared
medium. A BSS may be isolated or connected to a backbone
distribution system via an access point. The distribution system is
usually a wired backbone LAN.Magister Teknik Elektro 59
Wireless LAN Components (contd)
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Wireless LAN Components (contd)Signals from wireless computers
are transmitted via built-in antennas on the NIC to builtthe
nearest access point, which serves as a wireless repeater. Because
of the ease of access, security is a potential problem. The IEEE
802.11 has specified a data link security protocol called Wired
Equivalent Privacy (WEP), which is designed to make the security of
WLAN as good as that of wired LANs.Magister Teknik Elektro 61
Medium Access ControlThe MAC protocol used in 802.11 LANs is
called Distributed Foundation Wireless MAC (DFWMAC). This protocol
provides a distributed access control mechanism with an optional
centralized control built in. A distributed access mechanism
distributes the decision to transmit over all the nodes, using a
carrier sense mechanism, like CSMA/CD.Magister Teknik Elektro
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Medium Access Control (contd)A centralized access mechanism
involve regulation of transmission by a centralized manager. It is
particularly useful for timetimesensitive or high priority data.
The MAC layer is divided into 2 sub-layers: subThe lower layer is
called the Distributed Coordination Function (DCF), operates (DCF),
similar to CSMA/CD. Used for ordinary traffic.
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Medium Access Control (contd)The upper layer is called the Point
Coordination Function (PCF). PCF is a centralized MAC algorithm
used for contentioncontention-free service. All implementations
must support DCF, but PCF is optional. When DCF is employed, 802.11
uses a protocol called CSMA/CD to regulate access to the
medium.Magister Teknik Elektro 64
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA
)Wireless LANs use CSMA/CA . Like CSMA/CD, stations listen before
they transmit and if the line is free, they transmit. Detecting
collisions is more difficult in wireless networks, so wireless LANs
try to avoid collisions to a greater extent than traditional
Ethernet. Two different WLAN MAC techniques are now in use: the
Physical Carrier Sense Method and the Virtual Carrier Sense Method.
Method.Magister Teknik Elektro 65
Physical Carrier Sense MethodIn the physical carrier sense
method, a node method, that wants to send first listens to make
sure that the transmitting node has finished, then waits a period
of time longer. Each frame is sent using the Stop and Wait ARQ, so
by waiting, the listening node can detect that the sending node has
finished and can then begin sending its transmission. With Wireless
LAN, ACK / NAK signals are sent a short time after a frame is
received, while stations wishing to send a frame wait a somewhat
longer time, ensuring that no collision will occur.Magister Teknik
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Virtual Carrier Sense MethodWhen a computer on a Wireless LAN is
near the transmission limits of the AP at one end and another
computer is near the transmission limits at the other end of the
APs range, both computers may be able to transmit to the AP, but
can not detect each others signals. This is known as the hidden
node problem. problem. When it occurs, the physical carrier sense
method will not work.Magister Teknik Elektro 67
Virtual Carrier Sense Method (contd)The virtual carrier sense
method solves this problem by having a transmitting station first
send a request to send (RTS) signal to the AP. If the AP responds
with a clear to send (CTS) signal, the computer wishing to send a
frame can then begin transmitting.
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Infrared Wireless LANInfrared WLAN is less flexible than IEEE
802.11 WLANs because, as with TV remote controls that are also
infrared based, they require line of sight to work. Infrared Hubs
and NICs are usually mounted in fixed positions to ensure they will
hit their targets. The main advantage of infrared WLAN is reduced
wiring. A new version, called diffuse infrared, operates infrared,
without a direct line of sight by bouncing the infrared signal off
of walls, but is only able to operate within a single room and at
distances of only about 50-75 feet. 50-
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Infrared Wireless LAN (contd)
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BluetoothBluetooth is a 1 Mbps wireless standard developed for
piconets, small personal or home networks. piconets, It may soon be
standardized as IEEE 802.15. Bluetooth is designed to facilitate
networking of different hand-held and mobile devices. For example:
hand linking a wireless mouse to a computer, a telephone headset to
a base unit, or a Palm handheld computer to your car to lock or
unlock the door. 3-in-1 phone concept in automatic synchronizer :
automatically synchronizes a users desktop PC, mobile PC and mobile
phone.Magister Teknik Elektro 71
Bluetooth was designed to operate within a very small area (up
to 30 feet). May be extended. Devices are small and cheap. A
Bluetooth network consists of no more than eight devices, but can
be linked to other piconets to from larger networks. Although
Bluetooth uses the same 2.4 GHz band as Wireless LANs, it is not
compatible with the IEEE 802.11 standard and so cannot be used in
locations that use the Wireless LANs.
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Ethernet (CSMA/CD) Carriers Sense Multiple Access with Collision
Detection Xerox - Ethernet IEEE 802.3
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IEEE802.3 Medium Access Control Random Access
Stations access medium randomly Stations content for time on
medium
Contention
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CSMA Propagation time is much less than transmission time All
stations know that a transmission has started almost immediately
First listen for clear medium (carrier sense) If medium idle,
transmit If two stations start at the same instant, collision Wait
reasonable time (round trip plus ACK contention) No ACK then
retransmit Max utilization depends on propagation time (medium
length) and frame length Longer frame and shorter propagation gives
better utilizationMagister Teknik Elektro 75
If Busy? If medium is idle, transmit If busy, listen for idle
then transmit immediately If two stations are waiting,
collision
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CSMA/CD With CSMA, collision occupies medium for duration of
transmission Stations listen whilst transmitting If medium idle,
transmit If busy, listen for idle, then transmit If collision
detected, jam then cease transmission After jam, wait random time
then start again Binary exponential back offMagister Teknik Elektro
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CSMA/CD Operation
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Collision Detection On baseband bus, collision produces much
higher signal voltage than signal Collision detected if cable
signal greater than single station signal Signal attenuated over
distance Limit distance to 500m (10Base5) or 200m (10Base2) For
twisted pair (star-topology) activity on more (starthan one port is
collision Special collision presence signalMagister Teknik Elektro
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Gigabit Ethernet Configuration
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Gigabit Ethernet - Differences Carrier extension At least 4096
bit-times long (512 for bit10/100) Frame bursting
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Gigabit Ethernet - Physical 1000Base-SX 1000Base
Short wavelength, multimode fiber Long wavelength, Multi or
single mode fiber Copper jumpers