Matakuliah : H0484/Jaringan Komputer
Tahun
: 2007
Teknik Akses Jaringan - Non Carrier Sense
Pertemuan 11
Learning Outcomes
Pada akhir pertemuan ini, diharapkan mahasiswa
akan mampu :
• Menjelaskan teknik akses ke jaringan secara random
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Outline Materi
• ALOHA
• Slotted ALOHA
• Reservation ALOHA
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Medium Sharing Techniques
Allocation Principles
• Channel partitioning (Multiple Access)
– Divide channel into smaller pieces (time slots, frequency)
– Allocate piece to node for exclusive use
• Random access
– Allow collisions and Recover from collisions
• Scheduled /Taking turns
– Tightly coordinate shared access to avoid collisions
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Broadcast Networks
• N senders and receivers connected by a shared medium
(copper wire, atmosphere, water)
• Shared local access to the same media
• Multiple Access Protocol Determine which host is allowed to
transmit next to a shared medium
•Channel partition: TDMA, FDMA, CDMA
•Random access: ALOHA, CSMA/CD, CSMA/CA
• Local Area Network (LAN)
• Ethernet, Fast Ethernet, Gigabit Ethernet, Wireless
Ethernet, or 802.11 b/a, or WiFi
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Multiple-access protocols
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Medium Access Methods
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Random Access Protocols
• General principles:
• No Carrier Sensing
• Carrier Sensing
• Wait for silence
• Then talk (transmit)
• Listen while talking
• Backoff if there is transmission .
• Repeat
• Protocols also add a random increasing timeout
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No Carrier Sensing
ALOHA
– Useful in which uncoordinated users are
competing for the use of a single shared channel.
– Pure Aloha (no global time synchronization) and
Slotted Aloha (global time synch. required)
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ALOHA Network
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Pure ALOHA
–
–
–
–
–
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PURE ALOHA
Stations transmit whenever they have information to
send at all times.
Collision will occur when two hosts try to transmit
packets at the same time
If first bit of a frame overlaps with last bit of an earlier
frame then both will be destroyed
Colliding frames destroyed detected by listening to the
channel
Such a system is called a contention system
Pure ALOHA
– If no ACK by timeout, then wait a randomly selected
delay to avoid repeated collisions, then retransmit
– Too short : large number of collisions
– Too long : underutilization
– Frames are the same size
– In LAN feedback is immediate. For satellite broadcast
270msec delay
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Pure ALOHA - Collision
• No synchronization
• Send without awaiting for beginning of slot
• Collision of packets can occur when a packet overlaps another
packet:
– Packet sent at t0 collide with packets sent at [t0-1] or at [t0+1]
Wasted Time Colliding with B
Overlap
Overlap
with start of with end of
packet B
packet B
Packet A
Packet B
Packet C
t0-1
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t0
Collision
t0+1
Collision
time
Slotted ALOHA
• Rather than sending a packet at any time, send along time
slot boundaries
• Collisions are confined to one time slot
Wasted Time
Colliding with B
Packet A
Packet B Packet C
time
t0-1
t0
Collision
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t0+1
Efficiency of ALOHA
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