Matakuliah
Tahun
Versi
: H0174/Jaringan Komputer
: 2006
: 1/0
Pertemuan 13
Teknik Akses Jaringan - Random
1
Learning Outcomes
Pada akhir pertemuan ini, diharapkan mahasiswa
akan mampu :
• Menjelaskan teknik akses ke jaringan secara random
2
Outline Materi
• ALOHA
• Slotted ALOHA
• Reservation ALOHA
3
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
4
/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
5
Multiple-access protocols
6
Medium Access Methods
7
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
8
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)
9
ALOHA Network
10
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
11
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
12
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
t0
Collision
time
t0+1
Collision
13
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
t0+1
Collision
14
Efficiency of ALOHA
15
Matakuliah
Tahun
Versi
: H0174/Jaringan Komputer
: 2006
: 1/0
Pertemuan 14
Teknik Akses Jaringan – Carrier Sense
16
Learning Outcomes
Pada akhir pertemuan ini, diharapkan mahasiswa
akan mampu :
• Menjelaskan teknik akses ke jaringan dengan carrier
sense
17
Outline Materi
• CSMA
• Back-off strategy
18
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
19
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
time
delay
Station B
listens
Station B
sends
20
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
Station B
sends
Packet X
Packet Y
time
Collision still possible
over long propagation delays
21
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
time
Random
delay
Station B
listens
Station B
sends
22
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
23
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.
24
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
Station B
sends
Packet B size
Not transmitted
time
Station B detects collision,
Stops sending
25
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
26
Comparison of multiple access
G (transmission attempts per packet time)
27