Transcript Introduction to Wireless Communications & Networks

Medium Access Control
Data link layer=logical link control + MAC
Logical link control hides the physical
connection from higher layers, while MAC is the
part interfacing with physical characteristics
If link is dedicated, MAC may not be necessary
What is a link is broadcast in nature?
Need coordination
Multiple access communications
3
2
4
1
Shared Multiple
Access Medium
5
M

1. Any transmission from any station can be heard by any other stations
2. If two or more stations transmit at the same time, collision occurs
Figure 6.1
Wireless LAN: share wireless medium and require MAC
Figure 6.6
Medium Access Control
MAC: Medium access control or Multiaccess control or
Multiple access control
MAC: protocols coordinating the use of shared resource
(“channel”)
Classification





Centralized vs distributed
Deterministic vs random
Static vs dynamic
Reservation-based vs demand based
QoS based or fair sharing
Approaches to sharing transmission medium
Medium Sharing Techniques
Static
Channelization
Partitioned channels
are dedicated to
individual users, so
no collision at all.
Good for steady traffic
and achieve efficient
usage of channels
Dynamic Medium
Access Control
Scheduling
Schedule a
orderly access
of medium.
Good for heavier
traffic.
Minimize the
incidence of
collision to
achieve reasonable
usage of medium.
Good for bursty traffic.
Random Access
Try and error. if no collision,
that is good, otherwise wait a
random time, try again.
Good for light traffic.
Figure 6.2
Duplexing
Control transmissions in both directions
Forward (downlink)
Reverse (uplink)
Time division duplexing (TDD)
 Forward and reverse use the same channel but at different time
assignment
 Time slots in a frame divided into uplink and downlink
Frequency division duplexing (FDD)
 Forward and reverse transmissions use different frequency
channels
Channelization (Deterministic MAC)
Resource available in the network will be shared




Time
Space
Frequency
Code
Classification
 FDMA
 TDMA
 CDMA
 SDMA
FDMA

A channel is assigned to a user for the entire duration of a call.
No other user can access the channel during that time. When
call terminates, the same channel is re-assigned to another user.

FDMA used in nearly all first generation mobile systems: AMPS
(30 KHz channels), NMT, Japanese TACS/NTT
Bandwidth
Channel 1
Channel 2
Channel 3
Channel 4
Time
TDMA
The whole channel is assigned to each communicating user, but
users are multiplexed in time domain. Each user is assigned a
particular time slot, during which it communicates using the entire
frequency spectrum

The data rate of the channel is the sum of data rates of all the
multiplexed transmissions

Channel interference between transmissions in two adjacent
slots, limits the number of users sharing the channel
Channel 3
Time
Channel 2
Channel 1
Channel 4
Channel 3
Channel 2
Channel 1
Bandwidth

CDMA
It’s a spread-spectrum technique, allows multiple users to share
the same channel by multiplexing transmissions in code space.
Different signals from different users are encoded by different
codes (keys) and coexist both in time and frequency domains

A code is represented by a wideband pseudo noise (PN) signal

When decoding a transmitted signal at the receiver, due to low
cross-correlation of (orthogonal) codes, other transmissions
appear as noise. This property enables multiplexing of multiple
transmissions on the same channel with minimal interference

Maximum allowable interference (from other transmissions) limits
the number of simultaneous transmissions on the same channel
Bandwidth

All channels share bandwidth
Time
CDMA

Spreading the signal can be performed using

Direct Sequencing (DS): the narrow band data signal is
multiplied by a wideband pseudo noise (PN) signal (code).
Multiplication in the time domain translates to convolution in
the spectral domain, resulting in a wideband signal

Frequency Hopping (FH): the carrier frequency rapidly hops
among a set of possible frequencies according to some
pseudo random sequence (code). The set of frequencies
spans a large bandwidth. Thus the bandwidth of the
transmitted signal appears as largely spread
MAC efficiency
•Suppose bit rate of medium is R, then number
of bits “wasted” in access coordination is 2tpropR.
And suppose average length of packets is L.
Then efficiency in use of the medium is:
Efficiency =
L
L + 2tpropR
a=tpropR / L
=
1
1+2
tpropR
=
1
1+2a
L
i.e., the ratio of (one-way) delay-bandwidth product
to the average packet length.
Suppose a = 0.01, then efficiency = 1/1.02 = 0.98
a = 0.1, then efficiency = 1 / 2 = 0.50
Examples of efficiency
Ethernet (CSMA-CD):
 Efficiency = 1/(1+6.44a) where a = tpropR/L.
Token-ring networks:
 Efficiency = 1/(1+a’ ) where a = ring-latency
in bits/L where ring-latency contains:
 The sum of bit delays introduced at each ring
adapter.
 Delay-bandwidth product where delay is the time
required for a bit to circulate around the ring.
(a)
Typical LAN structure and network interface card
A LAN connects servers, workstations,
Printers, etc., together to achieve sharing
1. NIC is parallel with memory
but serial with network
2. ROM stores the implementation of MAC
3. Unique physical address burn into ROM
4. A hardware in NIC recognizes physical,
broadcast & multicast addresses.
5. NIC can be Set to “promiscuous” mode
to catch all transmissions.
(b)
Ethernet
Processor
RAM
ROM
RAM
14
Figure 6.10
IEEE 802 LAN standards
Network Layer
Network Layer
LLC
802.2 Logical Link Control
MAC
Physical
Layer
802.3
CSMA-CD
802.5
Token Ring
802.11
Wireless
LAN
Various Physical Layers
IEEE 802
Data Link
Layer
Other
LANs
Physical
Layer
OSI
One LLC and several MACs, each MAC has an associated set of physical layers.
MAC provides connectionless transfer. Generally no error control because of relatively error free.
MAC protocol is to direct when they should transmit frames into shared medium.
Figure 6.11
The MAC sublayer provides unreliable datagram service
Unreliable Datagram Service
MAC
MAC
MAC
PHY
PHY
PHY
Important: all three MAC entities must cooperate to provide datagram service, I.e., the interaction
between MAC entities is not between pairs of peers, but rather all entities must monitor all
16 frames.
Figure 6.12
LLC provides three HDLC services: 1. Unacknowledged connectionless service, recall HDLC has
unnumbered frames; 2. Reliable connection-oriented service in the form of HDLC ABM mode;
3. Acknowledged connectionless service, need to add two unnumbered frames to HDLC frame set.
C
A
A
Reliable Packet Service
C
LLC
LLC
LLC
MAC
MAC
MAC
PHY
PHY
PHY
LLC can provide reliable packet transfer service
17
Figure 6.13
•LLC provides additional addressing, i.e., SAP (Service Access Point). Like PPP, LLC can
support several different network connections with different protocols at the same time.
•Typical SAPs: IP: 06, IPX: E0, OSI packets: FE etc.
•In practice, LLC SAP specifies in which buffer the NIC places the frame, thus allowing the
appropriate network protocol to retrieve the data.
1 byte
Destination
SAP Address
1
Source
SAP Address
1 or
2
Control
Source SAP Address
Destination SAP Address
C/R
I/G
1
Information
7 bits
I/G = Individual or group address
1
7 bits
C/R = Command or response frame
LLC PDU structure and its support for several SAPs
Figure 6.14
Header overhead: TCP & IP: >=20
LLC:
3 or 4
MAC:
26
IP Packet
LLC
PDU
MAC frame
LLC
Header
IP
Data
MAC
Header
FCS
LLC PDU and MAC frame
Figure 6.15
IEEE 802.3 MAC frame
802.3 MAC Frame
7
1
Preamble
SD
Synch
6
Destination
Address
Start
frame
0
Single address
1
Group address
0
Local address
1
Global address
6
Source
Address
2
Length Information Pad
4
FCS
64 to 1518 bytes
• Destination address is either single address
or group address (broadcast = 111...111)
• Addresses are defined on local or universal
basis
• 246 possible global addresses
Unicast address: a single host address, multicast address: a group of hosts,
Broadcast address: all hosts.
Figure 6.52
Random Access
Why random access?
 Reaction time (i.e., 2 times of propagation delay)
is very important for performance, e.g., in Stopand-Wait, when reaction time is small (i.e., the ACK
will arrive soon) the performance is very good,
however, if reaction time is large, then performance
is very bad.
 Therefore, proceed the transmission without waiting
for ACK and deal with collision/error after the fact,
i.e., random access.
Three types of random accesses:
 ALOHA, slotted ALOHA, and CSMA-CD
ALOHA
Basic idea:
 let users transmit whenever they have data to be sent.
 When collision occurs, wait a random time ( why? ) and
retransmit again.
Differences between regular errors &collision
 Regular errors only affect a single station
 Collision affects more than one
 The retransmission may collide again
 Even the first bit of a frame overlaps with the last bit of a
frame almost finished, then two frames are totally
destroyed.
ALOHA random access scheme
Suppose L: the average frame length, R: rate, X=L/R: frame time
1. Transmit a frame at t=t0 (and finish transmission of the frame at t0+X )
2. If ACK does not come after t0+X+2tprop or hear collision, wait for random time: B
3. Retransmit the frame at t0+X+2tprop+B
Two modes: collide only from time to time and snowball effect collision
First transmission
t0-X
t0
t0+X
Vulnerable
period
Retransmission
t0+X+2tprop
t0+X+2tprop
Time-out Backoff period: B
When collision occurs?
t
Retransmission
if necessary
Vulnerable period: t0-X to t0+X, (2X seconds) if any other frames are transmitted during
the period, the collision will occur.
Therefore the probability of a successful transmission is the probability that there is no
additional transmissions in the vulnerable period.
Figure 6.16
The performance of ALOHA
•Let S be the arrival rate of new frames in units of frames/X seconds,
S is also the throughput of the system.
•Let G be the total arrival rate in units of frames/X seconds, G
contains the new and retransmissions and is the total load.
•Assume that aggregate arrival process resulting from new and
retransmitted frames has a Poisson distribution with an average
number of arrivals of 2G frames/2X seconds, i.e.,
(2G)k -2G
P[k transmissions in 2X seconds] =
e
k!
Therefore, the throughput of the system is:
, k=0,1,2,…
S=GP[no collision] =GP[0 transmission in 2X seconds]
(2G)0 e-2G =G e-2G
=G
0!
What results can be obtained from the graph?
1.peak value at G=0.5 with S=0.184
2.for any given S, there are two values of G, corresponding to
the two modes: occasional collision mode with S  G and
frequent collision mode with G >> S
0.368
0.4
0.35
0.3
S
Ge-G
0.25
0.184
0.2
0.15
0.1
Ge-2G
0.05
8
4
2
1
0.5
0.25
0.125
0.0625
0.03125
0.01563
0
G
Throughput S versus load G for ALOHA and slotted ALOHA
Figure 6.17
Slotted ALOHA
Synchronize the transmissions of stations
–All stations keep track of transmission time slots and are allowed
to initiate transmissions only at the beginning of a time slot.
Suppose a packet occupies one time slot
–Vulnerable period is from t0-X to t0, i.e., X seconds long.
Therefore, the throughput of the system is:
S=GP[no collision] =GP[0 transmission in X seconds]
(G)0 -G
=G
e =G e-G
0!
Slotted ALOHA random access scheme
t0=(k+1)X
First transmission
kX
(k+1)
X
Retransmission
t0+X+2tprop
t0+X+2tprop =nX
Time-out Backoff period: B
t
Retransmission
if necessary
Vulnerable period: t0-X to t0 , i.e., X seconds long
Figure 6.16
Peak value at G=1 with S=0.368 for slotted ALOHA, double compared with ALOHA.
In LAN, propagation delay may be negligible and uncoordinated access of shared medium
is possible but at the expense of significant wastage due to collisions and at very low throughput.
Throughput of ALOHAs is not sensitive to the reaction time because stations act independently.
0.368
0.4
0.35
0.3
S
Ge-G
0.25
0.184
0.2
0.15
0.1
Ge-2G
0.05
8
4
2
1
0.5
0.25
0.125
0.0625
0.03125
0.01563
0
G
Throughput S versus load G for ALOHA and slotted ALOHA
Figure 6.17
CSMA (Carrier sensing multiple access)
Problem with ALOHAs: low throughput
because the collision wastes
transmission bandwidth.
Solution: avoid transmission that are
certain to cause collision, that is CSMA.
Any station listens to the medium, if
there is some transmission going on the
medium, it will postpone its
transmission.
Suppose tprop is propagation delay from one extreme end to the other extreme end of the medium.
When transmission is going on, a station can listen to the medium and detect it.
After tprop, A’s transmission will arrive the other end; every station will hear it and refrain
from the transmission, so A captures the medium and can finish its transmission.
But in ALOHAs, it is X or 2X
Vulnerable period = tprop
Station A
begins
transmission at
t=0
A
Station A
captures
channel
at t=tprop
A
In LAN,generally, tprop < X
sense
sense
sense
sense
CSMA random access scheme
Figure 6.19
Three different CSMA schemes
Based on how to do when medium is
busy:
 1-persistent CSMA
 Non-persistent CSMA
 p-persistent CSMA
1-persistent CSMA
sense channel when want to transmit a packet, if channel is busy, then
sense continuously, until the channel is idle, at this time, transmit the
frame immediately.
If more than one station are sensing, then they will begin transmission
the same time when channel becomes idle, so collision. At this time,
each station executes a backoff algorithm to wait for a random time, and
then re-sense the channel again.
Problem with 1-persistent CSMA is “high collision rate”.
Non-persistent CSMA
sense channel when want to transmit a packet, if channel is
idle, then transmit the packet immediately. If busy, run backoff
algorithm immediately to wait a random time and then
re-sense the channel again.
Problem with non-persistent CSMA is that when the channel
becomes idle from busy, there may be no one of waiting
stations beginning the transmission, thus waste channel
bandwidth,
p-persistent CSMA
sense channel when want to transmit a packet, if channel is
busy, then persist sensing the channel until the channel
becomes idle. If the channel is idle, transmit the packet
with probability of p, and wait, with probability of 1-p,
additional propagation delay tprop and then re-sense again
Throughput versus load G for 1-persistent
(three different a=tprop/X )
S
0.6
0.53
0.5
0.4
0.01
0.45
0.3
0.2
0.16
0.1
0.1
64
32
16
8
4
2
1
0.5
0.25
0.13
0.06
0.03
0
0.02
1-Persistent
CSMA
G
1
Figure 6.21 - Part 2
Throughput versus load G for non-persistent
(three different a=tprop/X )
S
0.81
0.9
0.8
Non-Persistent
CSMA
0.7
0.01
0.6
0.51
0.5
0.4
0.3
0.14
0.2
0.1
0.1
64
32
16
8
4
2
1
0.5
0.25
0.13
0.06
0.03
0.02
0
G
1
1-persistent is sharper than non-persistent.
a=tprop/X has import impact on the throughput.
When a approaches 1, both 1-persistent and non-persistent is
worse than ALOHAs.
Figure 6.21 - Part 1
CSMA-CD
When the transmitting station detects a collision, it stops its
transmission immediately, Not transmit the entire frame
which is already in collision.
The time for transmitting station to detect a collision is 2tprop.
In detail: when a station wants to transmit a packet, it
senses
channel, if it is busy, use one of above three algorithms (i.e.,
1-persistent, non-persistent, and p-persistent schemes).
The transmitter senses the channel during transmission. If a
collision occurred and was sensed, transmitter stops its left
transmission of the current frame; moreover, a short
jamming signal is transmitted to ensure other stations that a
collision has occurred and backoff algorithm is used to
schedule a future re-sensing time.
The implication: frame time X >= 2tprop, , since X=L/R, which
means that there is a minimum limitation for frame length.
The reaction time in CSMA-CD is 2tprop
A begins to
transmit at
t=0
A
B
A
B
A
B
A detects
collision at
t= 2 tprop-
B begins to
transmit at
t= tprop-
B detects
collision at
t= tprop
It takes 2 tprop to find out if channel has been captured
Figure 6.22
1.
2.
3.
When a is small, i.e, tprop << X, the CSMA-CD is best and all CSMAs are
better than ALOHAs.
When a is approaching 1, CSMAs become worse than ALOHA.
ALOHAs are not sensitive to a because they do not depend on reaction time.
1
CSMA/CD
1-P CSMA
Non-P CSMA
0.8
max
Slotted Aloha
0.6
0.4
Aloh
a
0.2
0
0.01
0.1
1
a = t /X
prop
Maximum achievable throughput of random access schemes
Figure 6.24