Jaringan Komputer - Dr. Tb. Maulana Kusuma

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Internet dan Jaringan Komputer
Komunikasi Data dan Jaringan
Komputer
(Bagian 2)
Dr. Tb. Maulana Kusuma
[email protected]
http://staffsite.gunadarma.ac.id/mkusuma
Magister Manajemen Sistem Informasi
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Review of OSI Networking Model
Program X
Data
AH Data
Application
Presentation
Presentation
SH Data unit
Transport
TH
Network
Physical
Application
PH Data unit
Session
Data link
Program Y
NH
LH
Session
Data unit
Transport
Data unit
Network
Data unit
Bits
LT
Data link
Physical
Physical transmission medium
Magister Manajemen Sistem Informasi
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Data Link Layer
Means of activating, maintaining and
deactivating a reliable link
Error detection and control
Higher layers may assume error free
transmission
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Introduction
The PDU at the Data Link Layer (DL-PDU)
is typically called a Frame. A Frame has a
header, a data field, and a trailer
Example:
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Framing
Problem: Identify the beginning and the end of a
frame in a bit stream
Solution (bit-oriented Framing): A special bit
pattern (flag) signals the beginning and the end
of a frame (e.g., "01111110")
Problem:

The sequence '01111110' must not appear in the data
of the frame
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Bit-oriented framing and bit stuffing
'Bit stuffing': If the sender detects five
consecutive '1‘ it adds a '0' bit into the bit
stream. The receiver removes the '0' from
each occurrence of the sequence '111110'
Note: The flags itself are not bit-stuffed.
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Flow control
Flow Control is a technique for speed-matching
of transmitter and receiver. Flow control ensures
that a transmitting station does not overflow a
receiving station with data
We will discuss two protocols for flow control:


Stop-and-Wait Protocol
Sliding Window Protocol
For the time being, we assume that we have a
perfect channel between sender and receiver
(no errors)
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Stop-and-wait flow control
Simplest form of flow control
In Stop-and-Wait flow control, the receiver
indicates its readiness to receive data for each
frame
Operations:
1. Sender: Transmit a single frame
2. Receiver: Transmit acknowledgment (ACK)
3. Go to 1.
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Analysis of stop-and-wait
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Analysis of stop-and-wait
Transmission delay is the time that the sender
needs to transmit a frame
Transmission delay is dependent on the size of
a frame and the maximum data rate
Example:
Frame Size = 1000 bit
Data rate of network = 1 Mbps
Transmission delay = 1000 bit / 1 Mbps = 1 ms
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Analysis of stop-and-wait
Propagation delay is the time that a transmitted bit
needs to travel from sender to the receiver
Propagation delay is only dependent on the speed of the
transmission medium and the distance between sender
and receiver.
Speed of light: 300000 km/sec,
Speed in guided media (approx.): 200000 km/sec
Example:
Distance = 1000 km
Propagation delay = 1000 km / (200000 km/sec)
= 5 ms
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Sliding window flow control
Major Drawback of Stop-and-Wait Flow
Control:

Only one frame can be in transmission at a time
Sliding Window Flow Control



Allows transmission of multiple frames
Assigns each frame a k-bit sequence number
Range of sequence number is [0..2k-1], i.e., frames
are counted modulo 2k
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Operation of sliding window
Sending Window:


At any instant, the sender is permitted to send
frames with sequence numbers in a certain
range
The range of sequence numbers is called the
sending window
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Operation of sliding window
Receiving Window:

The receiver maintains a receiving window
corresponding to the sequence numbers of
frames that are accepted
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Operation of sliding window
Operations at the sender:
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Operation of sliding window
Operations at the sender:
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Operation of sliding window
Operations at the receiver
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Operation of sliding window
Operations at the receiver
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Operation of sliding window
How is “flow control” achieved?


Receiver can control the size of the sending
window
By limiting the size of the sending window
data flow from sender to receiver can be
limited
Interpretation of ACK N message:

Receiver acknowledges all packets until (but
not including) sequence number N
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Analysis of sliding window
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Error control
Two basic approaches to handle bit errors:
Error-detecting codes plus retransmission (Automatic
Repeat reQuest / ARQ)


Used if retransmission of corrupted data is feasible
Receiver detects error and requests retransmission of a frame.
Error-correcting codes



Used if retransmission of the data is not possible
Data are encoded with sufficient redundancy to correct bit errors
Examples: Hamming Codes, Reed Solomon Codes, etc.
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Error detection techniques
Error Detection Techniques:


Parity Checks
Cyclic Redundancy Check (CRC)
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Parity checks
General Method:

Append a parity bit to the end of each character in a
frame such that the total number of '1' in a character
is:
even (even parity) or
odd (odd parity)
Example:



With ASCII code, a parity bit can
be attached to an 7-bit character
ASCII "G" = 1 1 1 0 0 0 1
with even parity =
with odd parity =
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Cyclic-Redundancy Codes
General Method:

The transmitter generates an n-bit check sequence
number from a given k-bit frame such that the
resulting (k+n)-bit frame is divisible by some number
The receiver divides the incoming frame by the
same number
If the result of the division does not leave a
remainder, the receiver assumes that there was
no error
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Cyclic-Redundancy Codes
CRC is used by all advanced data link
protocols, for the following reasons:


Powerful error detection capability
CRC can be efficiently implemented in
hardware
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Additional facts on CRC
CRC can be efficiently implemented in hardware
by a set of XOR gates and a shift register
The following generator polynomials are widely
used:
CRC-12:
CRC-16:
CRC-CCITT:
CRC-32:
P(x) = x12 + x11 + x3 + x2 + x + 1
P(x) = x16 + x15 + x2 + 1
P(x) = x16 + x12 + x5 + 1
P(x) = x32 + x26 + x23 + x22 + x16
+ x12 + x11 + x10 + x8 + x7
+ x5 + x4 + x2 + x + 1
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ARQ error control
Two types of errors:


Lost frames
Damaged Frames
Most Error Control techniques are based on (1)
Error Detection Scheme (e.g., Parity checks,
CRC), and (2) Retransmission Scheme
Error control schemes that involve error
detection and retransmission of lost or corrupted
frames are referred to as Automatic Repeat
ReQuest (ARQ) error control
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ARQ error control
All retransmission schemes use all or a
subset of the following procedures:




Receiver sends an acknowledgment (ACK)
if a frame is correctly received
Receiver sends a negative acknowledgment
(NAK) if a frame is not correctly received
The sender retransmits a packet if an ACK is
not received within a timeout interval
All retransmission schemes (using ACK, NAK
or both) rely on the use of timers
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ARQ error control
Note: Once retransmission is used, a sequence
number is required for every data packet to
prevent duplication of packets
Both ACKs and NAKs can be sent as special
frames, or be attached to data frames going in
the opposite direction (Piggybacking)
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ARQ schemes
The most common ARQ retransmission
schemes:



Stop-and-Wait ARQ
Go-Back-N ARQ
Selective Repeat ARQ
The protocol for sending ACKs in all ARQ
protocols are based on the sliding window
flow control scheme
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Stop-and-wait ARQ
Stop-and-Wait ARQ is an addition to the Stopand-Wait flow control protocol:




Frames have 1-bit sequence numbers (SN = 0 or 1)
Receiver sends an ACK (1-SN) if frame SN is
correctly received
Sender waits for an ACK (1-SN) before transmitting
the next frame with sequence number 1-SN
If sender does not receive anything before a timeout
value expires, it retransmits frame SN
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Stop-and-wait ARQ
Lost frame
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Stop-and-wait ARQ
Lost ACK
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Go-back-N ARQ
Go-Back-N uses the sliding window flow control
protocol. If no errors occur the operations are
identical to Sliding Window
Operations:



A station may send multiple frames as allowed by the
window size
Receiver sends a NAKi if frame i is in error. After that,
the receiver discards all incoming frames until the
frame in error was correctly retransmitted
If sender receives a NAKi it will retransmit frame i and
all packets i+1, i+2,... which have been sent, but not
been acknowledged
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Go-back-N ARQ
Lost frame
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Go-back-N ARQ
Lost ACK
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Details Go-back-N ARQ
Scenario 1:
A transmits frame i, and B detects error in frame i, but
has received frames i-1, i-2,... correctly
➨ B sends NAKi
Scenario 2:
Frame i is lost or B does not recognize frame i
Assume that A sends frame i+1 and B receives it
➨ B sends NAKi, or A will timeout and retransmit frame i
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Manajemen Sistem Informasi
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and all subsequent
frames
Details Go-back-N ARQ
Scenario 3: B receives frame i and sends
ACK(i+1) which is lost
➨ B may send an ACK(i+k) later which also
acknowledges all frames < i+k (ACKs are
“cumulative”)
or
A retransmits frame i and all subsequent frames
Scenario 4: NAKi is lost
➨ A will eventually time out
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Example of Go-back-N ARQ
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Selective-repeat ARQ
Similar to Go-Back-N ARQ. However, the sender
only retransmits frames for which a NAK is
received
Advantage over Go-Back-N:

Fewer Retransmissions.
Disadvantages:



More complexity at sender and receiver
Each frame must be acknowledged individually (no
cumulative acknowledgements)
Receiver may receive frames out of sequence
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Selective-repeat ARQ
Lost frame
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Example of Selective-repeat ARQ
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Analysis of ARQ protocols
What is the efficiency of the discussed
ARQ protocols?
A number of assumptions:


ACKs and NAKs are never lost, and frames
are not dropped.
Sizes of ACKs, NAKs, and frame headers are
negligible.
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Error correction techniques
Forward error correction (FEC)
Hybrid-ARQ (H-ARQ)



Type-I H-ARQ
Type-II H-ARQ
Type-III H-ARQ
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Networking
Point to point communication not usually
practical


Devices are too far apart
Large set of devices would need impractical
number of connections
Solution is a communications network
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Simplified Network Model
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Two types of networks at the
data link layer


Broadcast Networks: All stations share a single
communication channel
Point-to-Point Networks: Pairs of hosts (or routers)
are directly connected
Broadcast Netw ork

Point-to-Point Netw ork
Typically, local area networks (LANs) are broadcast and
wide area networks (WANs) are point-to-point
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Networking
Computer network A collection of
computing devices that are connected in
various ways in order to communicate and
share resources
Usually, the connections between
computers in a network are made using
physical wires or cables
However, some connections are wireless,
using radio waves or infrared signals
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Networking
The generic term node or host refers to
any device on a network
Data transfer rate The speed with which
data is moved from one place on a
network to another
Data transfer rate is a key issue in
computer networks
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Switching Networks
Long distance transmission is typically
done over a network of switched nodes
Nodes not concerned with content of data
End devices are stations

Computer, terminal, phone, etc.
A collection of nodes and connections is a
communications network
Data routed by being switched from node
to node
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Nodes
Nodes may connect to other nodes only,
or to stations and other nodes
Node to node links usually multiplexed
Network is usually partially connected
Some redundant connections are desirable
for reliability
Two different switching technologies
 Circuit switching
 Packet switching

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Simple Switched Network
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Circuit Switching
Dedicated communication path between
two stations
Three phases



Establish
Transfer
Disconnect
Must have switching capacity and channel
capacity to establish connection
Must have intelligence to work out routing
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Circuit Switching - Applications
Inefficient


Channel capacity dedicated for duration of
connection
If no data, capacity wasted
Set up (connection) takes time
Once connected, transfer is transparent
Developed for voice traffic (phone)
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Public Circuit Switched Network
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Telecomm Components
Subscriber
 Devices attached to network
Local Loop
 Subscriber loop
 Connection to network
Exchange
 Switching centers
 End office - supports subscribers
Trunks
 Branches between exchanges
 Multiplexed
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Circuit Switch Elements
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Circuit Switching Concepts
Digital Switch
 Provide transparent signal path between devices
Network Interface
Control Unit
 Establish connections
Generally on demand
Handle and acknowledge requests
Determine if destination is free
construct path
 Maintain connection
 Disconnect
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Blocking or Non-blocking
Blocking



A network is unable to connect stations
because all paths are in use
A blocking network allows this
Used on voice systems
Short duration calls
Non-blocking


Permits all stations to connect (in pairs) at
once
Used for some
data connections
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Space Division Switching
Developed for analog environment
Separate physical paths
Crossbar switch
 Number of crosspoints grows as square of number of
stations
 Loss of crosspoint prevents connection
 Inefficient use of crosspoints
All stations connected, only a few crosspoints in
use
 Non-blocking
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Crossbar Matrix
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Time Division Switching
Partition low speed bit stream into pieces that share
higher speed stream
e.g. TDM bus switching
 based on synchronous time division multiplexing
 Each station connects through controlled gates to
high speed bus
 Time slot allows small amount of data onto bus
 Another line’s gate is enabled for output at the same
time
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Control Signaling Functions
Audible communication with subscriber
Transmission of dialed number
Call can not be completed indication
Call ended indication
Signal to ring phone
Billing info
Equipment and trunk status info
Diagnostic info
Control of specialist equipment
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Control Signal Sequence
Both phones on hook
Subscriber lifts receiver (off hook)
End office switch signaled
Switch responds with dial tone
Caller dials number
If target not busy, send ringer signal to target subscriber
Feedback to caller

Ringing tone, engaged tone, unobtainable
Target accepts call by lifting receiver
Switch terminates ringing signal and ringing tone
Switch establishes connection
Connection release when Source subscriber hangs up
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Packet Switching
Data transmitted in small packets
 Typically 1000 octets
 Longer messages split into series of packets
 Each packet contains a portion of user data plus
some control info
Control info
 Routing (addressing) info
Packets are received, stored briefly (buffered) and past
on to the next node
 Store and forward
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Use of Packets
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Advantages
Line efficiency
 Single node to node link can be shared by many
packets over time
 Packets queued and transmitted as fast as possible
Data rate conversion
 Each station connects to the local node at its own
speed
 Nodes buffer data if required to equalize rates
Packets are accepted even when network is busy
 Delivery may slow down
Priorities can be used
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Switching Technique
Station breaks long message into packets
Packets sent one at a time to the network
Packets handled in two ways


Datagram
Virtual circuit
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Datagram
Each packet treated independently
Packets can take any practical route
Packets may arrive out of order
Packets may go missing
Up to receiver to re-order packets and
recover from missing packets
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Virtual Circuit
Preplanned route established before any
packets sent
Call request and call accept packets establish
connection (handshake)
Each packet contains a virtual circuit identifier
instead of destination address
No routing decisions required for each packet
Clear request to drop circuit
Not a dedicated path
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Virtual Circuits v Datagram
Virtual circuits

Network can provide sequencing and error control

Packets are forwarded more quickly
No routing decisions to make

Less reliable
Loss of a node looses all circuits through that node
Datagram

No call setup phase
Better if few packets

More flexible
Routing can be used to avoid congested parts of the network
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Circuit v Packet Switching
Performance



Propagation delay
Transmission time
Node delay
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External Virtual Circuit and Datagram
Operation
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Internal Virtual Circuit and Datagram
Operation
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