TCP for Mobile and Wireless Hosts
Download
Report
Transcript TCP for Mobile and Wireless Hosts
Directional Antennas
in
Ad Hoc Networks
Nitin Vaidya
University of Illinois at Urbana-Champaign
Joint work with
Romit Roy Choudhury, UIUC
Xue Yang, UIUC
Ram Ramanathan, BBN
1
Mobile Ad Hoc Networks
Formed by wireless hosts which may be mobile
Without necessarily using a pre-existing infrastructure
Routes between nodes may potentially contain
multiple hops
2
Mobile Ad Hoc Networks
May need to traverse multiple links to reach a
destination
3
Mobile Ad Hoc Networks (MANET)
Mobility causes route changes
4
Why Ad Hoc Networks ?
Potential ease of deployment
Decreased dependence on infrastructure
5
Many Applications
Personal area networking
cell phone, laptop, ear phone, wrist watch
Military environments
soldiers, tanks, planes
Civilian environments
taxi cab network
meeting rooms
sports stadiums
boats, small aircraft
Emergency operations
search-and-rescue
policing and fire fighting
6
Many Variations
Fully Symmetric Environment
all nodes have identical capabilities and responsibilities
Asymmetric Capabilities
transmission ranges and radios may differ
battery life at different nodes may differ
processing capacity may be different at different nodes
Asymmetric Responsibilities
only some nodes may route packets
some nodes may act as leaders of nearby nodes (e.g.,
cluster head)
7
Many Variations
Traffic characteristics may differ in different ad hoc
networks
bit rate
timeliness constraints
reliability requirements
unicast / multicast / geocast
host-based addressing / content-based addressing /
capability-based addressing
May co-exist (and co-operate) with an infrastructurebased network
8
Many Variations
Mobility patterns may be different
Mobility characteristics
people sitting at an airport lounge
New York taxi cabs
kids playing
military movements
personal area network
speed
predictability
• direction of movement
• pattern of movement
uniformity (or lack thereof) of mobility characteristics among
different nodes
9
Challenges
Limited wireless transmission range
Broadcast nature of the wireless medium
– Hidden terminal problem
Packet losses due to transmission errors
Mobility-induced route changes
Mobility-induced packet losses
Battery constraints
Potentially frequent network partitions
Ease of snooping on wireless transmissions (security
hazard)
10
Question
Can ad hoc networks benefit from the progress made
at physical layer ?
Efficient coding schemes
Power control
Adaptive modulation
Directional antennas
…
Need improvements to upper layer protocols
11
Directional Antennas
12
Using Omni-directional Antennas
A Frozen
node
D
B
S
A
13
Directional Antennas
Not possible
using Omni
D
B
S
C
A
14
Comparison
Omni
Directional
Spatial Reuse
Low
High
(varies inversely with
beamwidth)
Connectivity
Low
High
Interference
Omni
Directional
Cost & Complexity
Low
High
15
Questions
Are Directional antennas beneficial in ad hoc
networks ?
To what extent ?
Under what conditions ?
16
Research Direction
Identify issues affecting directional communication
Evaluate trade-offs across multiple layers
Design protocols that effectively use directional
capabilities
Caveat: Work-in-Progress
17
Preliminaries
18
Hidden Terminal Problem
Node B can communicate with A and C both
A and C cannot hear each other
When A transmits to B, C cannot detect the
transmission using the carrier sense mechanism
If C transmits, collision may occur at node B
A
B
C
19
RTS/CTS Handshake
Sender sends Ready-to-Send (RTS)
Receiver responds with Clear-to-Send (CTS)
RTS and CTS announce the duration of the transfer
Nodes overhearing RTS/CTS keep quiet for that
duration
C
10
RTS (10)
A
B
CTS (10)
D
10
20
IEEE 802.11
Physical carrier sense
Virtual carrier sense using Network Allocation Vector
(NAV)
NAV is updated based on overheard
RTS/CTS/DATA/ACK packets, each of which
specified duration of a pending transmission
Nodes stay silent when carrier sensed busy
(physical/virtual)
21
Antenna Model
22
Antenna Model
2 Operation Modes: Omni & Directional
23
Antenna Model
Omni Mode:
Omni Gain = Go
Idle node stays in Omni mode.
Directional Mode:
Capable of beamforming in specified direction
Directional Gain = Gd (Gd > Go)
24
Directional Neighborhood
A
B
C
A and B are Directional-Omni (DO) neighbors
B and C are Directional-Directional (DD) neighbors
25
A Simple Directional MAC Protocol
(DMAC)
26
DMAC Protocol
A node listens omni-directionally when idle
Only DO links can be used
Sender node sends a directional-RTS using specified
transceiver profile
Receiver of RTS sends directional-CTS
27
DMAC Protocol
DATA and ACK transmitted and received directionally
Nodes overhearing RTS or CTS sets up NAV for that
DOA (direction of arrival)
Nodes defer transmitting only in directions for which
NAV is set
28
Directional NAV (DNAV)
Node E remembers directions in which it has
received RTS/CTS, and blocks these directions.
Transmission initiated only if direction of transmission
does not overlap with blocked directions.
29
Directional NAV (DNAV)
E has DNAV set due to RTS from H. Can talk to B
since E’s transmission beam does not overlap.
30
Example
C
E
D
B
B and C communicate
D & E cannot: D blocked with DNAV
A
D and A communicate
31
Issues with DMAC
Hidden terminals due to asymmetry in gain
A does not get RTS/CTS from C/B
Data
RTS
A
B
C
A’s RTS may interfere with C’s reception of DATA
32
Problems with DMAC
Hidden terminals due to directionality
Due to unheard RTS/CTS
D
B
A
C
A beamformed in direction of D
A does not hear RTS/CTS from B/C
A may now interfere at C
33
Issues with DMAC: Deafness
• Deafness
Z
RTS
A
B
DATA
RTS
Y
RTS
X
X does not know node A is busy.
X keeps transmitting RTSs to node A
With 802.11 (omni antennas), X would be aware that A is busy, and
defer its own transmission
34
Problems with DMAC
Shape of Silenced Regions
Region of interference for
omnidirectional transmission
Region of interference for
directional transmission
35
Problems with DMAC
Since nodes are in omni mode when idle, RTS
received with omni gain
DMAC can use DO links, but not DD links
A
B
C
36
DMAC Trade-off
Benefits
Better Network Connectivity
Spatial Reuse
Disadvantages
– Increased hidden
terminals
– Deafness
– Directional interference
– Uses only DO links
37
Solving DMAC Problems
Are improvements possible to make directional MAC
protocols more effective ?
One possible improvement: Use DD links
38
Using DD Links
Possible to exploit larger range of directional
antennas.
A
C
A & C are DD neighbors, but cannot communicate with DMAC
If A & C could be made to point towards each other, single hop
communication may be possible
39
Multi-Hop RTS: Basic Idea
D
C
A
B
DO neighbors
E
DD neighbors
F
G
A source-routes RTS to D through adjacent DO neighbors
(i.e., A-B-C-D)
When D receives RTS, it beamforms towards A, forming a
DD link.
40
MMAC protocol
D
E
F
H
C
A
B
A transmits RTS in the direction of its DD neighbor,
node D
Blocks H from communicating in the direction H-D
A then transmits multi-hop RTS using source route
A beamforms towards D and now waits for CTS
G
41
D
MMAC protocol
E
F
H
C
A
B
G
D receives MRTS from C and transmits CTS in the direction of A
(its DD neighbor).
A initiates DATA communication with D
H, on hearing RTS from A, sets up DNAVs towards both H-A
and H-D. Nodes B and C do not set DNAVs.
D replies with ACK when data transmission finishes.
42
Performance
Simulation
Qualnet simulator 2.6.1
CBR traffic
Packet Size – 512 Bytes
802.11 transmission range = 250 meters.
Channel bandwidth 2 Mbps
Mobility - none
43
Impact of Topology
• Nodes arranged in
linear configurations
reduce spatial reuse for
directional antennas
44
Impact of Topology
IEEE 802.11 = 1.19 Mbps
DMAC = 2.7 Mbps
IEEE 802.11 = 1.19 Mbps
DMAC = 1.42 Mbps
45
“Aligned” Flows
MMAC
802.11
DMAC
46
“Unaligned” Flows
MMAC
802.11
DMAC
47
“Unaligned” Flows & Topology
MMAC
802.11
DMAC
48
Delay: “Unaligned” Flows & Topology
49
Directional MAC: Summary
Directional MAC protocols can improve throughput
and decrease delay
But not always
Performance dependent on topology
50
Routing using Directional Antennas
51
Motivation
Directional antennas affect network layer, in addition
to MAC protocols
52
Dynamic Source Routing [Johnson]
Sender floods RREQ through the network
Nodes forward RREQs after appending their names
Destination node receives RREQ and unicasts a
RREP back to sender node, using the route in which
RREQ traveled
53
Route Discovery in DSR
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
I
D
N
Represents a node that has received RREQ for D from S
54
Route Discovery in DSR
Y
Broadcast transmission
[S]
S
Z
E
F
B
C
M
J
A
L
G
H
K
I
D
N
Represents transmission of RREQ
[X,Y]
Represents list of identifiers appended to RREQ
55
Route Discovery in DSR
Y
Z
S
E
[S,E]
F
B
C
A
M
J
[S,C]
H
G
K
I
L
D
N
56
Route Discovery in DSR
Y
Z
S
E
F
B
[S,E,F]
C
M
J
A
L
G
H
I
[S,C,G] K
D
N
• Node C receives RREQ from G and H, but does not forward
it again, because node C has already forwarded RREQ once
57
Route Discovery in DSR
Y
Z
S
E
[S,E,F,J]
F
B
C
M
J
A
L
G
H
D
K
I
[S,C,G,K]
N
• Nodes J and K both broadcast RREQ to node D
58
Route Reply in DSR
Y
Z
S
E
RREP [S,E,F,J,D]
F
B
C
M
J
A
L
G
H
K
I
Represents RREP control message
D
N
59
DSR over Directional Antennas
RREQ broadcast by sweeping
To use DD links
60
Route Discovery in DSR
Y
Z
S
E
[S,E,F,J]
F
B
C
M
J
A
L
G
H
D
K
I
[S,C,G,K]
N
• Nodes J and K both broadcast RREQ to node D
61
Trade-off
Larger Tx Range
Few Hop Routes
Fewer Hop Routes
Low Data Latency
Smaller Angle
More Sweeping
High Sweep Delay
High Overhead
62
Route discovery latency … Single flow, grid
topology (200 m distance)
DDSR4
DDSR6
DSR
63
Observations
Advantage of higher transmit range significant only at
higher distance of separation.
Grid distance = 200 m --- thus no gain with higher tx
range of DDSR4 (350 m) over 802.11 (250 m).
However, DDSR4 has sweeping delay. Thus route discovery
delay higher
64
Throughput
DDSR18
DDSR9
DSR
Sub-optimal routes chosen by DSR because destination
node misses the shortest RREQ, while beamformed.
65
Route Discovery in DSR
F
J
RREP
J
D
K
RREQ
N
D receives RREQ from J, and replies with RREP
D misses RREQ from K
66
Delayed RREP Optimization
Due to sweeping – earliest RREQ need not have
traversed shortest hop path.
RREQ packets sent to different neighbors at different points
of time
If destination replies to first arriving RREP, it might
miss shorter-path RREQ
Optimize by having DSR destination wait before
replying with RREP
67
Routing Overhead
Using omni broadcast, nodes receive multiple copies
of same packet - Redundant !!!
• Broadcast Storm Problem
Using directional Antennas – can do better ?
68
Routing Overhead
Use K antenna elements to forward broadcast packet.
K = N/2 in simulations
Footprint
of Tx
Ctrl Overhead
(No. Ctrl Tx) (Footprint of Tx)
=
No. Data Packets
69
Routing Overhead
Control overhead reduces
Beamwidth of antenna element (degrees)
70
Directional Antennas over mobile scenarios
Frequent Link failures
Communicating nodes move out of transmission range
Possibility of handoff
Communicating nodes move from one antenna to another
while communicating
71
Directional Antennas over mobile scenarios
Link lifetime increases using directional antennas.
Higher transmission range - link failures are less frequent
Handoff handled at MAC layer
If no response to RTS, MAC layer uses N adjacent antenna
elements to transmit same packet
Route error avoided if communication re-established.
72
Aggregate throughput over random mobile
scenarios
DDSR9
DSR
73
Observations
Randomness in topology aids DDSR.
Voids in network topology bridged by higher
transmission range (prevents partition)
Higher transmission range increases link lifetime –
reduces frequency of link failure under mobility
Antenna handoff due to nodes crossing antenna
elements – not too serious
74
Conclusion
Directional antennas can improve performance
But suitable protocol adaptations necessary
Also need to use suitable antenna models
… plenty of problems remain
75
Thanks!
www.crhc.uiuc.edu/~nhv
76
77
Adaptive Modulation
Joint work with Gavin Holland and Victor Bahl
78
Adaptive Modulation
Channel conditions are time-varying
Received signal-to-noise ratio changes with time
A
B
79
Adaptive Modulation
Multi-rate radios are capable of transmitting at
several rates, using different modulation schemes
Choose modulation scheme as a function of channel
conditions
Modulation schemes provide
a trade-off between
throughput and range
Throughput
Distance
80
Adaptive Modulation
If physical layer chooses the modulation scheme
transparent to MAC
MAC cannot know the time duration required for the transfer
Must involve MAC protocol in deciding the
modulation scheme
Some implementations use a sender-based scheme for this
purpose [Kamerman97]
Receiver-based schemes can perform better
81
Sender-Based “Autorate Fallback”
[Kamerman97]
Probing mechanisms
Sender decreases bit rate after X consecutive
transmission attempts fail
Sender increases bit rate after Y consecutive
transmission attempt succeed
82
Autorate Fallback
Advantage
Can be implemented at the sender, without making any
changes to the 802.11 standard specification
Disadvantage
Probing mechanism does not accurately detect channel
state
Channel state detected more accurately at the receiver
Performance can suffer
Since the sender will periodically try to send at a rate
higher than optimal
Also, when channel conditions improve, the rate is not
increased immediately
•
•
83
Receiver-Based Autorate MAC
[Holland01mobicom]
Sender sends RTS containing its best rate estimate
Receiver chooses best rate for the conditions and
sends it in the CTS
Sender transmits DATA packet at new rate
Information in data packet header implicitly updates
nodes that heard old rate
84
Receiver-Based Autorate MAC Protocol
C
A
RTS (2 Mbps)
B
CTS (1 Mbps)
Data (1 Mbps)
D
NAV updated
using rate
specified in the
data packet
85
Extra slides
86
Directional Antennas in Random Topologies
Higher transmission range improves connectivity in
addition to achieving fewer hop routes.
E.g. Link a-b not possible using Omni transmission.
87
Effect of Beamwidth in Random Static
Topologies
88