EEE449
Computer Networks
The Data Link Layer
Part 2: Media Access Control
En. Mohd Nazri Mahmud
MPhil (Cambridge, UK)
BEng (Essex, UK)
nazriee@eng.usm.my
Room 2.14
Semester 1 2011-2012
Media Access Control
• In shared media, we need to manage the
sharing process before the data transfer
• When nodes are connected and use a
common link (ie multipoint or broadcast
link) we must
– Ensure that each node gets access to the link
– Prevent any data “collision” between nodes
– Handle the collisions when they occur
Semester 1 2011-2012
Media Access Control
• Random access protocols for multiple
access
– Contention-based
– No node is superior to another node
– Node that wishes to send data uses a
procedure to make sending decision
– No predefined schedule
– Collisions damage data
Semester 1 2011-2012
Media Access Control
• Protocol 1: Pure ALOHA
– The earliest random access method
– Developed in 1970’s for ALOHAnet, a pioneering computer
networking system developed at the University of Hawaii
– ALOHAnet became operational in June, 1971, providing the first
public demonstration of a wireless packet data network
– In the 1970s ALOHA random access was employed in the widely
used Ethernet cable based network and then in both Wi-Fi and in
mobile telephone networks in the 1980’s.
– When collision occur, each node waits for a random amount of
time (called backoff time) before retrying
– After a maximum number of failed retry, nodes must give up and
try later
– Prodecure and parameters (discussion)
Semester 1 2011-2012
Media Access Control
• Pure ALOHA throughput analysis
discussion
Semester 1 2011-2012
Media Access Control
• Protocol 2: Slotted ALOHA
– Time is divided into slots whose duration last
for a frame-time
– Nodes must send only at the beginning of the
time slot
– Nodes must wait until the next time slot if they
miss the current one
– The vulnerable time is halved from the Pure
ALOHA case
Semester 1 2011-2012
Media Access Control
Semester 1 2011-2012
Media Access Control
• Protocol 3: CSMA
– Nodes must sense the medium before
attempting to send
– Due to propagation delay a node may
mistakenly sense an idle medium
– The vulnerable time is equal to the
propagation time
– “Sense before send” can reduce collision but
cannot eliminate it
– The first implementation of CSMA was the
Semester 1 2011-2012
Ethernet
Media Access Control
• Protocol 3: CSMA
– Can employ one of the four access modes:
Nonpersistent,1-persistent, P-persistent or Opersistent modes
– Nonpersistent mode
• When the sender (station) is ready to transmit data, it checks
the physical medium
• If the medium is idle, transmits a frame
• In case of a busy medium, the sender waits for a random
period of time and attempts to transmit again
Semester 1 2011-2012
Media Access Control
• Protocol 3: CSMA
– 1-persistent mode
• When the sender (station) is ready to transmit
data, it checks if the physical medium
CONTINOUSLY until it becomes idle, and then it
transmits a frame
• If the medium is busy, continue to listen until finds
idle
• In case of a collision, the sender back-off for a
random period of time and attempts to transmit
again
• 1-persistent CSMA is used in the Ethernet
Semester 1 2011-2012
Media Access Control
• Protocol 3: CSMA
– P-persistent
• When the sender is ready to send data, it checks continually
if the medium is busy
• If the medium becomes idle, the sender transmits a frame
with a probability p.
• If the station chooses not to transmit (the probability of this
event is 1-p), the sender waits until the next available time
slot and transmits again with the same probability p.
• In case of a collision, the sender waits for a random period of
time and attempts to transmit again
• p-persistent CSMA is used in WiFi and other packet radio
systems.
Semester 1 2011-2012
Media Access Control
• Protocol 3: CSMA
– O-persistent
• Each node is assigned a transmission order by a supervisor
station
• When medium goes idle, nodes wait for their time slot in
accordance with their assigned transmission order
• The station assigned to transmit first transmits immediately
• The station assigned to transmit second waits one time slot
• Stations monitor the medium for transmissions from other
stations and update their assigned order with each detected
transmission
Semester 1 2011-2012
Media Access Control
• Protocol 3: CSMA
– Although more efficient than ALOHA or
Slotted-ALOHA, the medium remains
unusable for the duration of transmission of
both damaged frames.
– For long frames, the unusable time can be
significant and the capacity wasted can be
considerable
Semester 1 2011-2012
Media Access Control
• Protocol 4: CSMA/CD
– Nodes must sense the medium before attempting to send
– If collision occurred and is detected, node stops transmitting that
frame, transmits a jam signal, and then waits for a random time
interval before trying to resend the frame.
– Only part of the transmission time is used for a partial frame
transmission
– improve CSMA performance by terminating transmission as
soon as a collision is detected, thus shortening the time required
before a retry can be attempted.
– Used in the Wired LAN (The Ethernet) – IEEE 802.3 standard.
– Discussion on procedure
Semester 1 2011-2012
Media Access Control
• What about wireless network?
• Can the 4 protocols work? Why or Why not?
Semester 1 2011-2012
Media Access Control
• Hidden nodes in a wireless network refer to nodes that are out of
range of other nodes or a collection of nodes.
• In a wireless network, it is likely that the node at the far edge of the
access point's range (A) can sense the access point, but it is
unlikely that the same node can sense a node on the opposite end
of the access point's range, B.
• The problem is when nodes A and B start to send packets
simultaneously to the access point. Since node A and B can not
sense the carrier, collision cannot be detected.
• The hidden node problem can be observed easily in widespread
(>50m radius) WLAN setups with many nodes that use directional
antennas and have high upload.
Media Access Control
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Protocol 5: Carrier sense multiple access with collision avoidance
(CSMA/CA)
is a wireless network multiple access method
differs from CSMA/CD due to the nature of the medium, the radio frequency
spectrum.
One of the problems of wireless data communications is that it is not
possible to listen while sending, therefore collision detection is not possible.
Collision avoidance is used to improve CSMA performance by not allowing
wireless transmission of a node if another node is transmitting, thus
reducing the probability of collision
supplemented by the exchange of a Request to Send (RTS) packet sent by
the sender S, and a Clear to Send (CTS) packet sent by the intended
receiver R.
RTS/CTS alerts all nodes within range of the sender, receiver or both, to not
transmit for the duration of the main transmission
CSMA/CA is used in 802.11 based wireless LANs
Discussion of procedures
Media Access Control
• Protocol 5: Carrier sense multiple access with collision
avoidance (CSMA/CA)
– IFS – Interframe Spacing – transmission is deferred for an IFS
even if the medium is idle because a distant (out of range) node
may have already transmitted but the frame has not been
detected.
– Contention Window – After waiting for an IFS and the channel is
still idle, node must wait until its time slot arrive. Node choose a
random number of slot as its waiting time.
– RTS/CTS exchange – When its time slot has arrived, sender
send RTS to request for transmission and wait for receiver to
grant permission and also to alert other nodes to wait.