Teknik Akses Jaringan - Non Carrier Sense
Pertemuan 11
Matakuliah : H0484/Jaringan KomputerTahun : 2007
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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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•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
Random Access Protocols
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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
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
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– 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
Pure ALOHA
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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]
time
Collision
Wasted Time Colliding with B
Packet C
Packet B
Packet A
t0t0-1 t0+1
Collision
Overlap with start of
packet B
Overlap with end of packet B
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• Rather than sending a packet at any time, send along time slot boundaries• Collisions are confined to one time slot
Slotted ALOHA
time
Collision
Wasted Time Colliding with
B
Packet CPacket B
Packet A
t0t0-1 t0+1
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Efficiency of ALOHA