1 pertemuan 13 teknik akses jaringan - random matakuliah: h0174/jaringan komputer tahun: 2006 versi:...
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Pertemuan 13Teknik Akses Jaringan - Random
Matakuliah : H0174/Jaringan Komputer
Tahun : 2006
Versi : 1/0
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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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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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• 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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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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Pertemuan 14Teknik Akses Jaringan – Carrier Sense
Matakuliah : H0174/Jaringan Komputer
Tahun : 2006
Versi : 1/0
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Learning Outcomes
Pada akhir pertemuan ini, diharapkan mahasiswa akan mampu :
• Menjelaskan teknik akses ke jaringan dengan carrier sense
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Carrier Sense Multiple Access
• ALOHA & Slotted ALOHA are inefficient because stations don’t take into account what other stations are doing before they transmit (Talk-before-listen)
• Sense for carriers (see if anyone else is transmitting) before begin transmitting
• Carrier Sense Multiple Access (CSMA) is Listen Before Talk
• In LAN’s it is possible for stations to detect what other stations are doing and reactively change.
• CSMA can improve performance• These protocols are called carrier sense protocols. • They are named 1-persistent CSMA, non-persistent
CSMA, p-persistent CSMA
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• When station has data to send, it listens to channel
• Channel idle: station transmits packet • Channel busy: station waits till channel is idle• When channel becomes free, a host transmits
its packet immediately (with probability 1)
1- Persistent
Packet Atime
Station Blistens
Packet B
delay
Station Bsends
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• Propagation delay II
– If propagation delay is small zero, collision may still occur
– Station A transmits, Stations B, X, Y simultaneously realize that line is busy and wait
– When line is free, stations B transmits then station X, Y simultaneously transmit. Collision.
Collision in CSMA
Packet Atime
Station Blistens
Packet B Packet Y
delay Collision still possibleover long propagation delays
Packet X
Station Bsends
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• Before sending, station senses channel • If no transmission, station starts sending.• If channel is busy it does not continuously sense the
channel in order to start transmitting• Wait/sleep a random interval before sensing again• As soon as channel is idle, then send a packet
• Random interval reduces collisions• Higher throughput than 1-persistent CSMA when
many senders
Non Persistent
Packet Atime
Station Blistens
Packet B
Randomdelay
Station Bsends
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• Generalization of 1-persistent CSMA• Typically applied to slotted channels• Slot length is chosen as maximum propagation delay
• A station senses the channel, and• If slot is idle, transmit with probability p, or defer with
probability q=1-p• If next slot is idle, transmit with probability p, or defer
with probability 1-p, repeat…• If channel is initially busy, it waits until the next slot
then sense channel continuously until it becomes free and applies the above algorithm
p-Persistent
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• IEEE 802.3 is a standard for a 1-persistent CSMA/CD LAN– If cable is busy, station waits until cable is idle.– If 2 or more stations simultaneously transmit
on an idle cable, they will collide.– All colliding stations then terminate their
transmission and, wait random time and then start process again.
• Ethernet is a specific implementation.
CSMA with Collision Detection
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• Ethernet uses CSMA/CD• Listen-while-talk protocol• A station listens even while it is transmitting, and
if a collision is detected, stops transmitting
Ethernet - CSMA/CD
Packet Atime
Station Blistens
Packet B
delay
Packet B size
Not transmitted
Station Bsends
Packet B
Station B detects collision,Stops sending
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• Ethernet CSMA/CD requires a minimum size packet • If packet B arrives at A and A is no longer transmitting, then
Host A will• Fail to detect the collision• Thinks its packet got through• Thinks the incoming packet is a new packet
• Therefore, to detect a collision:• Minimum packet size >= 2*(prop. delay)*BW
A
Ethernet
Packet A t
d=propagation delay
B
Packet A
Packet B
Packet B
t+d
B transmit at t+d, just before packet A arrives. B sees collision and transmits a runt” packet Packet arrives at t+2d
Minimum size Ethernet packet