Internet dan Jaringan Komputer
Komunikasi Data dan Jaringan
Komputer
(Bagian 3)
Dr. Tb. Maulana Kusuma
mkusuma@staff.gunadarma.ac.id
http://staffsite.gunadarma.ac.id/mkusuma
Magister Manajemen Sistem Informasi
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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
applications
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Third Generation LAN
Support for multiple guaranteed classes of
service


Live video may need 2Mbps
File transfer can use background class
Scalable throughput

Both aggregate and per host
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 devices
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LAN Applications (2)
High speed office networks


Desktop image processing
High capacity local storage
Backbone LANs




Interconnect low speed local LANs
Reliability
Capacity
Cost
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LAN Architecture
Protocol architecture
Topologies
Media Access Control (MAC)
Logical Link Control (LLC)
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Protocol Architecture
Lower 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 Physical
Encoding/decoding
Preamble generation/removal
Bit transmission/reception
Transmission medium and topology
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802 Layers Logical Link Control
Interface to higher levels
Flow and error control
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802 Layers Media Access Control
Assembly 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
available
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LAN Protocols in Context
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Topologies
Tree
Bus

Special case of tree
One trunk, no branches
Ring
Star
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LAN Topologies
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Bus and Tree
Multipoint 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 medium
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Frame Transmission - Bus LAN
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Ring Topology
Repeaters 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
frame
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Frame Transmission
Ring LAN
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Star Topology
Each station connected directly to central
node

Usually via two point to point links
Central node can broadcast


Physical star, logical bus
Only one station can transmit at a time
Central node can act as frame switch
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Media Access Control
Where

Central
Greater control
Simple access logic at station
Avoids problems of co-ordination
Single point of failure
Potential bottleneck

Distributed
How

Synchronous
Specific capacity dedicated to connection

Asynchronous
In response to demand
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Asynchronous Systems
Round 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 load
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Logical Link Control
Transmission 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 protocol
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Bus LAN
Signal balancing
 Signal must be strong enough to meet receiver’s
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 Media
Twisted 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 fiber
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Baseband Coaxial Cable
Uses digital signaling
Manchester or Differential Manchester encoding
Entire frequency spectrum of cable used
Single channel on cable
Bi-directional
Few kilometer range
Ethernet (basis for 802.3) at 10Mbps
50 ohm cable
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10Base5
Ethernet 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
10Base5
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10Base2
Cheaper
0.25 inch cable







More flexible
Easier to bring to workstation
Cheaper electronics
Greater attenuation
Lower noise resistance
Fewer taps (30)
Shorter distance (185m)
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Repeaters
Transmits 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 stations
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Baseband Configuration
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Ring LAN
Each 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 ring
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Ring Repeater States
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Listen State Functions
Scan 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
Modify bit as it passes

e.g. to indicate a packet has been copied
(ACK)
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Transmit State Functions
Station has data
Repeater has permission
May receive incoming bits

If ring bit length shorter than packet
Pass back to station for checking (ACK)

May be more than one packet on ring
Buffer for retransmission later
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Bypass State
Signals propagate past repeater with no
delay (other than propagation delay)
Partial solution to reliability problem (see
later)
Improved performance
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Star LAN
Use 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 collisions
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Hubs and Switches
Shared 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
20Mbps
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Switched Hubs
No 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 frames
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Hubs and Switches
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Wireless LAN
Wireless 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 popularity
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Wireless Communications
In 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 LANs
IEEE 802.11a
IEEE 802.11b
IEEE 802.11g
Infrared
Bluetooth
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IEEE 802.11b
Two 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.
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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.
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IEEE 802.11a vs 802.11b vs
802.11g
802.11a 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
Pros
Cons
802.11a
fastest maximum speed;
highest cost;
supports more
shorter range signal that is
simultaneous users;
more easily obstructed
less signal interference
from other devices
802.11b
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 band
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IEEE Lan
Standard
802.11g
Pros
fastest maximum
speed;
supports more
simultaneous users;
signal range is best
and is not easily
obstructed
Cons
costs more than
802.11b;
appliances may
interfere on the
unregulated signal
frequency
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Wireless LAN Applications
LAN Extension
Cross building interconnection
Nomadic access
Ad hoc networks
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LAN Extension
Buildings 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 Interconnection
Point 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 Access
Mobile data terminal

e.g. laptop
Transfer of data from laptop to server
Campus or cluster of buildings
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Ad Hoc Networking
Peer to peer
Temporary
e.g. conference
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Wireless LAN Configurations
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Wireless LAN Requirements
Throughput
Number of nodes
Connection to backbone
Service area
Battery power consumption
Transmission robustness and security
Collocated network operation
License free operation
Hand-off / roaming
Dynamic configuration
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Wireless LAN Technology
Infrared (IR) LANs
Spread spectrum LANs
Narrow band microwave
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Wireless LAN standard –
IEEE 802.11
The IEEE 802.11 standard for wireless
LANs was finalized in 1997.
The standard defines three different
physical layer specifications - 2 are radio
frequency-based and one is infraredbased:



Direct Sequence Spread Spectrum
Frequency-hopping spectrum
Infrared
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Wireless LAN Components
The 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.
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Wireless LAN Components (cont’d)
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Wireless LAN Components (cont’d)
Signals from wireless computers are
transmitted via built-in antennas on the NIC to
the 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.
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Medium Access Control
The 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.
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Medium Access Control (cont’d)
A centralized access mechanism involve
regulation of transmission by a centralized
manager. It is particularly useful for timesensitive or high priority data.
The MAC layer is divided into 2 sub-layers:
The lower layer is called the Distributed
Coordination Function (DCF), operates
similar to CSMA/CD. Used for ordinary traffic.
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Medium Access Control (cont’d)
The upper layer is called the Point
Coordination Function (PCF). PCF is a
centralized MAC algorithm used for
contention-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.
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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.
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Physical Carrier Sense Method
In the physical carrier sense method, a node
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.
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Virtual Carrier Sense Method
When 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
AP’s range, both computers may be able to
transmit to the AP, but can not detect each
other’s signals.
This is known as the hidden node problem.
When it occurs, the physical carrier sense
method will not work.
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Virtual Carrier Sense Method (cont’d)
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 LAN
Infrared 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
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.
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Infrared Wireless LAN (cont’d)
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Bluetooth
Bluetooth is a 1 Mbps wireless standard developed
for piconets, small personal or home networks.
It may soon be standardized as IEEE 802.15.
Bluetooth is designed to facilitate networking of
different hand-held and mobile devices. For example:
 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
 automatic synchronizer : automatically
synchronizes a user’s desktop PC, mobile PC and
mobile phone.
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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
 Contention

Stations content for time on medium
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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
utilization
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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 off
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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
than one port is collision
 Special collision presence signal
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Gigabit Ethernet Configuration
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Gigabit Ethernet - Differences
 Carrier extension
 At least 4096 bit-times long (512 for
10/100)
 Frame bursting
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Gigabit Ethernet - Physical
 1000Base-SX

Short wavelength, multimode fiber
 1000Base-LX

Long wavelength, Multi or single mode fiber
 1000Base-CX

Copper jumpers <25m, shielded twisted pair
 1000Base-T

4 pairs, cat 5 UTP
 Signaling - 8B/10B
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Token Ring (802.5)
 MAC protocol

Small frame (token) circulates when idle

Station waits for token

Changes one bit in token to make it SOF for data frame

Append rest of data frame

Frame makes round trip and is absorbed by transmitting
station

Station then inserts new token when transmission has
finished and leading edge of returning frame arrives

Under light loads, some inefficiency

Under heavy loads, round robin
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Token Ring Operation
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Priority Scheme
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Dedicated Token Ring





Central hub
Acts as switch
Full duplex point to point link
Concentrator acts as frame level repeater
No token passing
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FDDI
 100Mbps
 LAN and MAN applications
 Token Ring
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FDDI Operation
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ATM LANs
Asynchronous Transfer Mode
Virtual paths and virtual channels
Preconfigured or switched
Gateway to ATM WAN
Backbone ATM switch
 Single ATM switch or local network of ATM
switches
Workgroup ATM
 End systems connected directly to ATM switch
Mixed system
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Example ATM LAN
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ATM LAN HUB
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Compatibility
Interaction between end system on ATM
and end system on legacy LAN
Interaction between stations on legacy
LANs of same type
Interaction between stations on legacy
LANs of different types
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