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