designing a large metropolitan area ad hoc network dave johnson rice university departments of cs...

Designing a LargeMetropolitan Area Ad Hoc Network

Dave Johnson

Rice UniversityDepartments of CS and ECE

http://www.monarch.cs.rice.edu/[email protected]

Monarch Project

Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures

Traditional Wireless Networks

Many forms, but all have similar architecture:

• Wireless cellular networks (analog, digital, PCS, 2.5G, 3G, …)• Wireless LANs (proprietary, IEEE 802.11, 802.11b, 802.11a, …)

Relies on a fixed infrastructure:

• Centralized base station or access point• All users in the cell are within wireless transmission range of it• Infrastructure must be planned, installed, managed, maintained, …

Wireless LANWireless Cellular


Wireless Ad Hoc Networking

Sometimes there may be no network infrastructure available:• Remote areas• Unplanned meetings• Emergency relief personnel quickly deployed into an area• Military troops where infrastructure has been destroyed or is untrusted

Sometimes users won’t want to use available infrastructure:• Time to access and register on the service• Cost of using the service• Performance of the existing service and infrastructure• Capacity of the existing service and infrastructure

Can extend coverage range of any existing infrastructure:• Allow users to be further away from infrastructure serving them


Ad Hoc Network Routing

• Some nodes may be out of wireless transmitter range of others

• Need to be able to use other nodes as routers to forward packets

• Need to find new routes as nodes move or network conditions change

A B C


Dynamic Source Routing (DSR) Protocol

DSR divides routing problem for ad hoc networks into two parts:

• Route Discovery: only try to find a route to some destination when you don’t have one and need one

• Route Maintenance: only while you’re actually using a route, try to keep it working or fix it in spite of changes

Unique properties of our protocol:

• All aspects of protocol operation are entirely on-demand

• Nodes ignore all topology changes not affecting them

• Overhead scales automatically as movement increases

• Zero overhead when stationary and found routes already

• Can support unidirectional links and asymmetric routes


Route Discovery Overview

To discover a route to some address:

• Broadcast a ROUTE REQUEST with a unique request id in it

• When receiving a ROUTE REQUEST:

– If target is yourself, return the recorded route to the initiator in aROUTE REPLY packet; initiator caches the route

– Else, if recently seen a request with this id, drop the REQUEST

– Otherwise, append your own address to a route record in the packet and rebroadcast the ROUTE REQUEST

• Optimizations reduce frequency and spread of ROUTE REQUESTS


Route Maintenance Overview

After transmitting a packet to the next hop:• Listen for link-level per-hop acknowledgement (present in many

wireless LANs), or• Listen for that node sending packet to its next hop (passive

acknowledgement), or• Set a bit in the packet to request an explicit next-hop acknowledgement

When a problem with forwarding is detected:• Send a ROUTE ERROR to original sender, describing the broken link• Sender removes the broken link from its cache• May use other routes in cache or perform a new Route Discovery if

needed


DSR Implementation and Testbed

Tested and demonstrated from Dec 1998 through Mar 1999:

• 5 cars, driving 20–25 MPH, looping between A and B

• 2 stationary nodes (E1 and E2) about 3 WaveLAN hops apart

• All routing between ad hocnetwork nodes done with DSR

• Integrated into Internet andMobile IP, allows nodes tojoin the ad hoc network

• Ftp, telnet, UDP CBR audio,real-time kinematic (RTK)GPS corrections, real-timestatistics and positionlogging


DSR Testbed Photos


Preliminary DSR QoS Demonstration

Audio/video using Windows NetMeeting over DSR network:

• DARPA GloMo PI meeting, Eatontown, NJ, July 2000

• QuickCam Pro USB camera, microphones, and speakers

NetMeeting inmoving car

NetMeeting in hotel


DSR QoS Demo Endpoint Configuration


Screen View with Running NetMeeting


Current Primary Funding: NASA


Metropolitan Architecture Overview

Architecture consists of 3 types of nodes:

• Mobile nodes move about freely within the ad hoc network and communicate only through wireless

• Base stations are fixed (stationary):

– Communicate with mobile nodes using same wireless

– Also communicate over a backbone that may be wired orlong-range wireless (e.g., separate frequency)

• Mobile Location Register is either one base station or special node:

– Maintains the registration table to record which base station is serving each mobile node

– Each base station maintains a registration table cache


Maintaining the Registration Table

Each time a base station hears a ROUTE REQUEST initiated by a mobile node, it forwards it to the Mobile Location Register:

• To ensure that the freshest information is kept in the registration table, the registration is tied to both the hop count and the ROUTE REQUEST identifier

• More recent ROUTE REQUEST identifiers always take precedence

• Between two ROUTE REQUESTs with the same identifier value, the one with the shorter hop count takes precedence

• If source mobile node not currently registered or if new ROUTE REQUEST takes precedence over current registration for that mobile node, then Mobile Location Register updates its registration table

• Records this base station as the current base station serving that mobile node


Updating Registration on ROUTE REPLY

Can also update registration when mobile node answers a ROUTE REPLY from a base station:

• The target mobile node includes an additional field in its ROUTE REPLY to carry what would have been that node’s next ROUTE REQUEST identifier

• Allows this ROUTE REPLY to be ordered relative to ROUTE REQUESTs initiated by the mobile node

• Mobile Location Register thus able to choose the freshestinformation for registration

Not used on normal ROUTE REPLYs since these don’t goto a base station to allow registration table update


Implicit Registrations

ROUTE REQUESTs and REPLYs at a base station giveadditional registration hints:

• The source route lists a sequence of nodes that terminates atthe base station

• Mobile Location Register can remember most recent such base station for each of these nodes as an implicit registration

• These implicit registrations cannot be used to update the registration table entries, since their freshness cannot be determined

• But they can be cached and used to optimize paging (described later)


Source Route Selection

A node sending a packet determines the source route:

• Check Route Cache for a route to destination or to a base station

• If none, initiate Route Discovery:

– Transmit ROUTE REQUEST with hop limit ofhb = maximum hop count a node can be from a base station

– Mobile nodes send ROUTE REPLY as in normal DSR

– Base station also returns a ROUTE REPLY along the reverse path

• Mobile node selects route from Route Cache:

– Prefer direct route more than route through a base station

– Only use base station route if hop count does not exceed hb

– Within this, next preference is to shortest route

• Source route on packet terminates at destination or at base station


Route Discovery and Base Station Processing

Source node discovering a route to destination node:

• Dotted line packets are only sent if needed


Destination Base Station Processing

If destination base station has no Route Cache entry:• Query the Mobile Location Register; if reply indicates this base

station,try local Route Discovery for the destination

• Otherwise, return packets to source base station, marked as “stale”


Paging

Used to locate a destination node globally when Mobile Location Register has no registration for destination but needs one

Similar to cellular systems, and can use their paging optimizations

Example (simple) paging policy:

• If Mobile Location Register has no implicit registration for destination,it sends PAGE to all base stations

• Each base station does local Route Discovery, indicating it is a page

• If Mobile Location Register has implicit registration,it sends PAGE only to that base station

• If no response, send PAGE to all recent implicit base stations for it,and perhaps then to nearby base stations

• If no response, continue pages with exponential back, limited times


Initial Evaluation Methodology

Evaluated architecture on 3 main metrics:

• Packet Delivery Ratio (PDR): fraction of data packets originated by a source node that are received by destination node

• Packet Overhead: number of control packets sent by routing protocol

• Path Length: number of times a delivered packet was transmittedover the wireless medium

Communication model was Constant Bit Rate (CBR):

• 20 randomly selected pairs of nodes, independent of location

• 4 packets per second each

Latency over wired backbone set to 50 ms

Nominal wireless transmission range increased to 1.5 km:

• Data rate 2 Mbps (compatible with bandwidth utilization in CDMA 2000)


Evaluation Movement Traces

Collected real traces of real mobility of city busses:

• Seattle, Washington, King County metro bus system

• Total system is 1200 busses over 2000 square mile area

• Busview software allows Internet users to monitor locations(http://busview.its.washington.edu/busview_launch.jsp)

• Recorded traces and filtered to smooth anomalous data

• Used Tuesday, November 20, 2001, 7:00–8:30 am “rush hour” trace

• Average of 950 to 975 busses active in trace at any time

Average over 5 separate portions of movement trace runs:

• Each between 350 and 700 simulated seconds (total 2550 seconds)

• Memory consumption of simulator limited our run lengths (we hit thelimits of 32-bit Pentium 4’s on which we ran ns-2!)


Example Busview Display


Base Station Placements

• Approximately 2000 square miles coveredby the network

• Each red dot is a fixed base station withwired backbone connection

• Red square suggests scale of areaserved by each base station

• Each green dot is location of a bus atthis example moment in time

• Total of 8 fixed base stations

• Base stations placed closer togetherin more densely populated areas

• Each mobile node should be within5 hops of a base station (hb = 5)


Initial Results

• Packet Delivery Ratio (PDR) averaged 97.66%(95% confidence interval was between 96.47% and 98.85%)

• PDR in individual scenarios ranged between 96.21% and 98.37%.

• Approximately 43% of delivered packets were delivered without a base station (direct routeswere shorter)

• Approximately 32% ofpackets through basestation traversedwide area links

• Average latency in eachscenario was between5.6 and 10.8 ms

• Bimodal path lengthdistribution (mean is 3.5):


Routing Overhead

Average of 3.29 overhead packets for each data packet successfully delivered

About an order of magnitude worse than plain DSR using50 nodes on a 1500 m × 300 m area for DSR:

• Partially due to inefficient paging in our current simulations

• Partially due to the higher number of nodes participating in flooding each Route Discovery

• And of course number of nodes is very different(for example: 502 = 2500 vs. 10002 = 1,000,000)

• Due to the hierarchical nature of our architecture,effect of this large increase in node population is reduced


Conclusion

Overhead is still the big challenge in a large ad hoc network

• Our hierarchical architecture keeps overhead manageable but could still be improved such as by paging optimizations

The correct value of hb is hard to set exactly in advance:

• Could pick a conservative (large enough) value

• Or could modify protocol to have node increase limit when needed(similar to an expanding ring search)

New source of movement models for ad hoc networking simulation:

• Real traces of real mobility of real busses

• May be able to provide more realistic results for many kinds of simulation studies

• There are still no real traces of communication patterns

designing a large metropolitan area ad hoc network dave johnson rice university departments of cs...

Documents

route requestoptimizations

route error

recorded route

route record

new route discovery

route reply packet initiator

existing infrastructure

available infrastructure