Transcript Packet A - Binus Repository

Matakuliah : H0484/Jaringan Komputer
Tahun
: 2007
Teknik Akses Jaringan Carrier Sense
Pertemuan 12
Learning Outcomes
Pada akhir pertemuan ini, diharapkan mahasiswa
akan mampu :
• Menjelaskan teknik akses ke jaringan dengan carrier sense
Bina Nusantara
Outline Materi
• CSMA
• Back-off strategy
Bina Nusantara
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, ppersistent CSMA
Bina Nusantara
1 - Persistent
•
•
•
•
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)
Packet A
Packet B
delay
Station B
listens
Bina Nusantara
Station B
sends
time
Collision in CSMA
• 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.
Packet A
Packet B
delay
Station B
listens
Bina Nusantara
Station B
sends
Packet X
Packet Y
Collision still possible
over long propagation delays
time
Non Persistent
• 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
Packet A
Packet B
Random
delay
Station B
listens
Bina Nusantara
Station B
sends
time
p-Persistent
• 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
Bina Nusantara
CSMA with Collision Detection
• 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.
Bina Nusantara
Ethernet - CSMA/CD
•
•
Ethernet uses CSMA/CD
Listen-while-talk protocol
• A station listens even while it is transmitting, and if a collision
is detected, stops transmitting
Packet B
Packet A
Packet B
delay
Station B
listens
Bina Nusantara
Station B
sends
Packet B size
Not transmitted
Station B detects collision,
Stops sending
time
Minimum size Ethernet packet
•
•
•
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
B
Ethernet
Packet A
t
Packet
B
Packet A
d=propagation delay
t+d
Packet
B
B transmit at t+d, just before packet A arrives. B sees collision and transmits a runt” packet Packet arrives at t+2d
Bina Nusantara
Comparison of multiple access
G (transmission attempts per packet time)
Bina Nusantara