Wireless Mobile Communications: Part 2 – Mobile Ad-hoc Networks (MANET)

Jae H. Kim, Ph.D.

Manager/Associate Technical Fellow

Boeing Phantom Works

jae.h.kim@boeing.com

(253) 657-7685

Outline

PART 2:

• Wireless LAN MAC Protocols

• Mobile Ad-hoc Network (MANET)

• Proactive• Reactive• Hybrid

Wireless LANMAC Protocol

Wireless Protocol Layers

Application Processing

Propagation Model Mobility

Frame Processing Radio Status/Setup

CS/RadioRTS/CTSFrame Wrapper

Ack/Flow Control

Clustering

Packet Store/Forward VC Handle

FlowControl Routing

IP Wrapper IP/Mobile IP

RSVPTransport Wrapper TCP/UDP Control

Channel

Radio

MAC Layer

Network

IP

Transport

Application

RTP Wrapper RCTP

Packet Store/Forward

Clustering

Routing

Link Layer

Application Setup

Data Plane Control Plane

Media Access Control (MAC) Layer

• MAC protocol: - Coordination and scheduling of transmissions among competing neighbors

• Goals: - Low latency, good channel utilization; best effort and real time support

• MAC layer clustering: - Aggregation of nodes in a cluster (= cell) for MAC enhancement- Different from network layer clustering, partitioning such as used for routing

• CDMA (Code Division Multiple Access)• FDMA (Frequency Division Multiple Access)• TDMA (Time Division Multiple Access)

- ALOHA- Slotted ALOHA- CSMA (Carrier Sense Multiple Access)- DAMA (Demand Assigned Multiple Access)- PRMA (Packet Reservation Multiple Access)- Reservation TDMA- MACA (Multiple Access with Collision Avoidance)- Polling

• SDMA (Space Division Multiple Access)

MAC Protocols

• MAC protocol coordinates transmissions from different stations in order to minimize/avoid collisions

(1) Random Access: CSMA, MACA

(2) Channel Partitioning: TDMA, FDMA, CDMA

(3) “Taking turns”: Polling

• Goal is efficient, fair,

simple, decentralized

Multiple Access

Random Access

• A node transmits at random (i.e., no a priory coordination among nodes) at full channel data rate

• If two or more nodes “collide,” they retransmit at random times

• The random access MAC protocol specifies how to detect collisions and how to recover from them (via delayed retransmissions, for example)

• Examples of random access MAC protocols:

(a) Slotted ALOHA – 36% throughput

(b) ALOHA – 18% throughput

(c) CSMA and CSMA/CD

Slotted ALOHA

• Time is divided into equal size slots (= full packet size)• a newly arriving station transmits a the beginning of the next

slot• if collision occurs (assume channel feedback, eg the receiver

informs the source of a collision), the source retransmits the packet at each slot with probability P, until successful.

• Success (S), Collision (C), Empty (E) slots• S-ALOHA is channel utilization efficient; it is fully decentralized

Pure (unslotted) ALOHA

• Slotted ALOHA requires slot synchronization

• A simpler version, pure ALOHA, does not require slots

• A node transmits without awaiting for the beginning of a slot

• Collision probability increases (packet can collide with other packets which are transmitted within a window twice as large as in S-Aloha)

• Throughput is reduced by one half, i.e., S= 1/2e

Carrier Sense Multiple Access (CSMA)

• CSMA: listen before transmit. If channel is sensed busy, defer transmission

• Persistent CSMA: retry immediately when channel becomes idle (this may cause instability)

• Non persistent CSMA: retry after random interval• Note: collisions may still exist, since two stations

may sense the channel idle at the same time ( or better, within a “vulnerable” window = round trip delay)

• In case of collision, the entire packet transmission time is wasted

Collision Detection

• CSMA/CD: carrier sensing and deferral like in CSMA. But, collisions are detected within a few bit times.

• Transmission is then aborted, reducing the channel wastage considerably.

• Typically, persistent transmission is implemented

• CSMA/CD can approach channel utilization =1 in LANs (low ratio of propagation over packet transmission time)

• Collision detection is easy in wired LANs (eg, E-net): can measure signal strength on the line, or code violations, or compare tx and receive signals

• Collision detection cannot be done in wireless LANs (the receiver is shut off while transmitting, to avoid damaging it with excess power)

• Sense channel idle for DISF (Distributed Inter Frame Space)- transmit frame (no Collision Detection)- receiver returns ACK after SIFS (Short Inter Frame Space)

• If channel sensed busy, then binary backoff

• NAV: Network Allocation Vector (min time of deferral)

IEEE 802. 11 MAC - CSMA Protocol

• CSMA inefficient in presence of hidden terminals

• Hidden terminals: A and B cannot hear each other because of obstacles or signal attenuation; so, their packets collide at B

• Solution? CSMA/CA (Collision Avoidance)

Hidden Terminal Effect

• CTS “freezes” stations within range of receiver (but possibly hidden from transmitter); this prevents collisions by hidden station during data

• RTS and CTS are very short: collisions during data phase are thus very unlikely (similar effect as Collision Detection)

• Note: IEEE 802.11 allows CSMA, CSMA/CA and “polling” from AP

Collision Avoidance

Access methods

• MAC-DCF CSMA/CA (mandatory)- Collision avoidance via randomized “back-off” mechanism- Minimum distance between consecutive packets- ACK packet for acknowledgements (not for broadcasts)

• MAC-DCF w/ RTS/CTS (optional)- Distributed Foundation Wireless MAC- Avoids hidden terminal problem

• MAC- PCF (optional)- Access point polls terminals according to a list

IEEE 802.11 - MAC Layer

• Priorities– defined through different inter frame spaces– no guaranteed, hard priorities– SIFS (Short Inter Frame Spacing)

• highest priority, for ACK, CTS, polling response– PIFS (PCF IFS)

• medium priority, for time-bounded service using PCF– DIFS (DCF, Distributed Coordination Function IFS)

• lowest priority, for asynchronous data service

t

medium busySIFS

PIFS

DIFSDIFS

next framecontention

direct access if medium is free DIFS

802.11 - MAC layer (cont.)

– station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment)

– if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type)

– if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time)

– if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness)

t

medium busy

DIFSDIFS

next frame

contention window(randomized back-offmechanism)

slot timedirect access if medium is free DIFS

IEEE 802.11 MAC - CSMA/CA

• Sending unicast packets– station has to wait for DIFS before sending data

– receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC)

– automatic retransmission of data packets in case of transmission errors

t

SIFS

DIFS

data

ACK

waiting time

otherstations

receiver

senderdata

DIFS

contention

IEEE 802.11 MAC - CSMA/CA (cont.)

• Sending unicast packets– station can send RTS with reservation parameter after waiting for DIFS

(reservation determines amount of time the data packet needs the medium)

– acknowledgement via CTS after SIFS by receiver (if ready to receive)

– sender can now send data at once, acknowledgement via ACK

– other stations store medium reservations distributed via RTS and CTS

t

SIFS

DIFS

data

ACK

defer access

otherstations

receiver

senderdata

DIFS

contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

IEEE 802.11 MAC - RTS/CTS

• IEEE 802.11 DCF: Congestion control achieved by dynamically choosing the contention window, CW

• When transmitting a packet, choose a backoff interval in the range [0,CW]– cw is contention window

• Count down the backoff interval when medium is idle– Count-down is suspended if medium becomes busy

• When backoff interval reaches 0, transmit RTS

IEEE 802.1 MAC - DCF

IEEE 802.11 MAC – DCF (Cont.)

B1 and B2 are backoff intervalsat nodes 1 and 2CW = 31

data

waitB1 = 5

B2 = 15

B1 = 25

B2 = 20

data

waitB2 = 10

Example

• The time spent counting down backoff intervals is a part of MAC overhead

• Choosing a large CW leads to large backoff intervals and can result in larger overhead

• Choosing a small CW leads to a larger number of collisions (when two nodes count down to 0 simultaneously)

Congestion Avoidance

• Since the number of nodes attempting to transmit simultaneously may change with time, some mechanism to manage congestion is needed

• IEEE 802.11 DCF: Congestion control achieved by dynamically choosing the contention window CW

Congestion Control

• When a node fails to receive CTS in response to its RTS, it increases the contention window– cw is doubled (up to an upper bound)

• When a node successfully completes a data transfer, it restores CW to CWmin

Binary Exponential Backoff in DCF

Channel Partitioning (e.g., CDMA)

• CDMA (Code Division Multiple Access): exploits spread spectrum (DS or FH) encoding scheme

• unique “code” assigned to each user; I.e., code set partitioning• Used mostly in wireless broadcast channels (cellular, satellite,etc)• All users share the same frequency, but each user has own

“chipping” sequence (i.e., code)• Chipping sequence like a mask: used to encode the signal• encoded signal = (original signal) x (chipping sequence)• decoding: inner product of encoded signal and chipping sequence

(note: the inner product is the sum of the component-by-component products)

• To make CDMA work, chipping sequences must be chosen orthogonal to each other (i.e., inner product = 0)

CDMA Encode/Decode

CDMA (cont.)

CDMA Properties:• protects users from interference and jamming (in WW II)• protects users from radio multipath fading • allows multiple users to “coexist” and transmit

simultaneously with minimal interference (if codes are “orthogonal”)

• requires “chip synch” acquisition before demodulation• requires careful transmit power control to avoid “capture”

by near stations in near-far situations• FAA requires use of SS (with limits on tx power) in the

Unlicensed Spectrum region (ISM), e.g., 900 MHz and 2.4 GHz (WaveLANs)

• CDMA used in Qualcomm cellphones (channel efficiency improved by factor of 4 with respect to TDMA)

• Frequency spectrum sliced into frequency subbands (e.g., 125 subbands in a 25 MHz range)

• Time is subdivided into slots; each slot can carry several bits (slow FH)

• A typical packet covers several time slots• A transmitter changes frequency slot by slot

(frequency hopping) according to unique, predefined sequence; all users are clock and slot synchronized

• Ideally, hopping sequences are “orthogonal” (i.e., non overlapped); in practice, some conflicts may occur

Frequency Hopping (FH)

Mobile Ad-Hoc Networks (MANET)

Routing Protocols

• Distance Vector:– Destination-sequenced distance vector (DSDV), Bellman-Ford– Routing control overhead linearly increasing with network size– Convergence problems (count to infinity); potential loops

• Link State:– Open Shortest Path First (OSPF)– Link update flooding overhead caused by frequent topology changes

Not scalable to network size and mobility …

Proactive, Table Driven Routing

0

5

1

2

4

3

Routing table at node 5 :

Tables grow linearly with # nodes

Control overhead grows with network size and mobility

Destination Next Hop Distance

0 2 3

1 2 2

… … …

Distance Vector

• At node 5, based on the link state pkts, topology table is constructed:

• Dijkstra’s Algorithm can then be used for the shortest path

0

5

1

2

4

3

{1}

{0,2,3}

{1,4}

{2,4}

{2,3,5}

{1,4,5}

0 1 2 3 4 5

0 1 1 0 0 0 0

1 1 1 1 1 0 0

2 0 1 1 0 1 1

3 0 1 0 1 1 0

4 0 0 1 1 1 1

5 0 0 1 0 1 1

Link State Routing

• Routes are established “on demand” as requested by the source

• Only the active routes are maintained by each node

• Channel/Memory overhead is minimized• Two leading methods for route discovery:

source routing and backward learning (similar to LAN interconnection routing)

Reactive, On-Demand Routing

Routing Protocol Choices?

OSPFv2 is one of the most heavily used IGPs in the Internet today

Commercially, we have alternatives:• Cisco EIGRP (proprietary, in Distance Vector

class of protocols)• RIP (legacy Distance Vector, considered

inferior for large networks)• IS-IS (OSI link-state protocol, many of same

issues as with OSPFv2)

What is OSPFv2?

• A “link state” routing protocol for unicast traffic

• Simple concept:– Assign costs to links– Give every router a complete map of the

network– Execute a shortest path calculation for

every destination– Build a routing table with next-hop

information for all destinations

OSPFv2 Area Hierarchy

• OSPFv2 uses an “area hierarchy” to summarize groups of nodes– The backbone is called “Area 0”– Every additional area must attach to the

backbone– Routes to different areas are summarized

(aggregated) before re-distribution– Cost of area hierarchy is loss of precision in

the routes, complexity, and topology restrictions

• OSPFv2 also uses route summarization between “autonomous systems”– This is method of scaling in ADNS

Basic OSPFv2 Operation

• Routers transmit “Hello” messages every 10 seconds to each neighbor– Hello messages also contain a list of

neighbors from whom Hellos have been received

– If you see yourself in your neighbor’s Hello message, you know you have a 2-way link

– Peer routers then synchronize their databases

• Routers use a reliable flooding algorithm to disseminate link and network information

OSPFv2 Problems

• Flooding of “link state advertisements” causes overhead to grow with a) number of nodes, b) mobility

• Hello message traffic over slow links• Convergence time (operationally it is a lot

larger than one would expect) and route “flapping”

• Difficult for different areas to peer with one another

OSPFv2 over MANET?

• No interface type defined for wireless, broadcast-based networks– “Ethernet”-like interface (broadcast) does not

function correctly in wireless network– Point-to-multipoint interface creates too much

overhead (does not capitalize on broadcast capability)

• No support for Quality-of-Service-based link metrics (for load balancing)– Used to be in the protocol specification, but

was removed

Other Alternatives

• The Internet Engineering Task Force (IETF) is considering the standardization of new “Mobile Ad-hoc Network (MANET)” routing protocols

– Optimized for wireless operation– Various strategies for scaling to large

networks– Designed for most severe of NBN conditions

(when regular infrastructure breaks down or is non-existent)

MANETInstrumentation

TraditionalInstrumentation Architecture

• MANET provides connectivity across

battlefield w/o large infrastructure enables geometric pairing allows arbitration of engagements

w/in target’s subnet provides channel to report position/

location of players, casualtyassessment results

3. reportresults

2. arbitrate

1. shoot

What is MANET ?

• Mobility

• Need to scale to large numbers (100’s to 1000's up to 10, 000’s)

• Unreliable radio channel (e.g., fading, external interference)

• Limited bandwidth

• Limited power

• Need multimedia applications (QoS)

Wireless Multihop Routing

Placeholder for

mobile ad-hoc networking Applications

(animation)

• Proactive: Conventional table-driven routing– Optimized Link State Routing (OLSR)– Destination Sequenced Distance Vector (DSDV)– Fisheye State Routing (FSR)– Source Tree Adaptive Routing (STAR)– Hierarchical State Routing (HSR)

• Reactive: On-Demand routing:– Ad-hoc On-Demand Distance Vector (AODV)– Dynamic Source Routing (DSR)– Temporarily Ordered Routing Algorithm (TORA)– Location Assisted Routing (LAR)

• Hybrid Routing:– Zone routing

Mobile Ad-hoc Routing Protocols

OLSR Protocol

• Optimized Link State Routing (OLSR)–developed by INRIA, France–categorized as “proactive” protocol

• Most like OSPF–Shortest Path First (SPF)-based algorithm–Unreliable flooding algorithm

• Sets up distribution tree to disseminate routing information (nodes are called Multipoint Relays)

OLSR Example

•20 nodes

•100 bi-directional links (not shown)

•19 tree links (shown)

•16 leaves (not filled)

•4 non-leaf nodes

•Only 4 non-leaf nodes forward updates generated by the source

•In flooding, all 20 nodes would forward updates.

source

AODV Routing Protocol

• Ad-hoc on-demand distance vector– Developed by Perkins, Royer, Das– categorized as “reactive” MANET protocol

• Unknown routes are queried for by a flooding algorithm

• Recently used routes are cached for future use

• Several implementations exist, and extensions for QoS and IPv6 defined

AODV Example

destination

source

destination

RREP reverses successful flooding path

RREQ flooded with increasing scope

• Use hierarchical routing to reduce table size and table update overhead

• Proposed hierarchical schemes include:– Fisheye (implicit hierarchy induced by "scope")– Zone routing (hybrid scheme)– Landmark Routing

Hierarchical Routing

• Topology data base at each node– Similar to link state (e.g., OSPF)

• Routing information is periodically exchanged with neighbors only ( “Global” State Routing)

– Similar to distance vector, but exchange entire topology matrix

• Routing update frequency decreases with distance to destination

– Higher frequency updates within a close zone and lower frequency updates to a remote zone

– Highly accurate routing information about the immediate neighborhood of a node; progressively less detail for areas further away from the node

Fisheye State Routing

1

2

3

4

5

67

8

9

9

10

11

12

14 1516 17

18 19

20

21

2223

2425

26

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3234

35

36

Hop=1

Hop=2

Hop>2

13

Scope of Fisheye

0

5

1

2

4

3

0:{1}1:{0,2,3}2:{5,1,4}3:{1,4}4:{5,2,3}5:{2,4}

101122

LST HOP

0:{1}1:{0,2,3}2:{5,1,4}3:{1,4}4:{5,2,3}5:{2,4}

212012

LST HOP

0:{1}1:{0,2,3}2:{5,1,4}3:{1,4}4:{5,2,3}5:{2,4}

221101

LST HOP

Message Reduction in FSR

• Logical subnet: group of nodes with functional affinity with each other (e.g., they move together)

• Node logical address = <subnet, host>

• A Landmark is elected in each subnet

• Every node keeps Fisheye Link State table/routes to neighbors up to hop distance N

• Every node maintains routes to all Landmarks

Landmark Routing Protocol

Logical Subnet

Landmark

Landmark Routing (cont.)

• A packet to local destination is routed directly using Fisheye tables

• A packet to remote destination is routed to corresponding Landmark based on logical addr

• Once the packet gets within Landmark scope, the direct route is found in Fisheye tables

• Benefits: dramatic reduction of both routing overhead and table size; scalable to large networks

Landmark

Logical Subnet

References

• References- UCLA Course note, “Ad-hoc Wireless Routing,” CS 218- Fall 2002.

- UCLA Course note, “Ad-hoc Nets – MAC Layer: Part 1,” CS 218- Fall 2002.

- UCLA Course note, “Ad-hoc Nets – MAC Layer: Part 2,” CS 218- Fall 2002.

• Books- James D. Solomon, Mobile IP The Internet Unplugged, Prentice Hall, 1998.

- Charles E. Perkins, Mobile IP Design Principles and Practices, Addison-Wesley, 1998.

- Christian Huitema, Routing in the Internet, Prentice Hall, 2000.

- Jochen Schiller, Mobile Communications, Addison-Wesley, 2000.

- Charles E. Perkins, Ad-Hoc Networking, Addison-Wesley, 2001.

• IETF Working Group URLhttp://www.ietf.org/html.charters/mobileip-charter.html