designing a large metropolitan area ad hoc network dave johnson rice university departments of cs...
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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
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
DSR Testbed Photos
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
DSR QoS Demo Endpoint Configuration
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
Screen View with Running NetMeeting
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
Current Primary Funding: NASA
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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)
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
Route Discovery and Base Station Processing
Source node discovering a route to destination node:
• Dotted line packets are only sent if needed
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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”
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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)
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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!)
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
Example Busview Display
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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)
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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):
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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
Monarch ProjectDavid B. Johnson [email protected] Mobile Networking Architectures
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