Wireless%20Data%20Tutorial

1
Wireless Data Tutorial
Phil Karn Senior Staff Engineer Qualcomm karn_at_qual
comm.com http//people.qualcomm.com/karn
2
Introduction

• "Data" really means packet data
• Or more specifically, Internet access
• could be a private net that uses TCP/IP
• Everything else is an Internet application
• e.g., CDMA asynch data fax

3
Tutorial Topics

• The Internet and its architecture
• Generic considerations for IP over wireless
• Adapting existing digital voice systems to packet
  data
• IS-95 CDMA, Globalstar, GSM
• Systems designed specifically for packet data
• CDPD, HDR
• Ad-hoc packet radio networks
• IEEE 802.11

4
Introduction to the Internet

• Evolved from DARPA-sponsored packet networking
  research begun in the 1960s
• ARPANET begun in 1969 as first packet switched
  network
• What became TCP/IP conceived in 1974 as means to
  interconnect ARPANET with ARPA packet radio
  networks

5
The Internet Problem

• Given a variety of applications, transmission and
  networking technologies, including those not yet
  invented, how can we unify them into a single,
  seamless network?
• Cerf Kahn, A Protocol for Packet Network
  Interconnection, IEEE Transactions on
  Communications, May 1974
• describes the basic design of what became TCP/IP
• TCP/IP was originally one protocol, later split
• established Cerf Kahn as the Internets
  grandfathers

6
Key Internet Concepts

• End-to-end principle
• push complexity and features to upper layers
• I.e., out of network to user computers
• Simplified, 4-layer reference model
• Connectionless network layer
• every packet contains full source dest
  addresses
• easy to implement on variety of physical networks
• Flexible transport protocols
• TCP and UDP meet virtually all needs

7
The End-to-End Principle

• Saltzer, Reed and Clark, 1981
• Many traditional low-level network functions are
  better done at the endpoints, I.e., at higher
  protocol levels
• Some functions can sometimes be justified within
  the network as a performance enhancement
• IMHO, one of the most important CS papers of all
  time
• http//people.qualcomm.com/karn/library.html has
  links

8
End-to-End in the Internet

• The end-to-end principle is widely accepted, is
  fundamental to the Internet architecture, and
  largely explains its success
• Nevertheless, some old-guard Bell-heads still
  refuse to accept it on ideological grounds
• Sort of like the theory of biological evolution
• Telcos dont like being thought of as dumb bit
  pipe providers, even if that is their only real
  competence
• The end-to-end Internet architecture is a
  powerful tool in the hands of end users
• significant political and economic implications

9
The Internet Reference Model
Application
Host-to-Host (end-to-end)
Internet
Subnet
10
The Internet Reference Model

• Application Layer
• covers OSI application presentation layers
• HTTP, Telnet, FTP, SMTP, POP, DNS, etc
• End-to-End Layer
• OSI transport session layers
• TCP UDP
• Internet Layer
• OSI network (upper part)
• IP
• Subnet Layer
• OSI network (lower part), link, physical

11
How the Internet Model Differs from OSI

• Fewer layers
• Presentation merged into application
• Session transport layers merged into end-to-end
• Single connectionless Internet layer
• simple, least-common-denominator service
• Subnetwork layer deliberately unspecified
• may be a simple point-to-point link, a complete
  network with internal routing, or tin cans
  string
• Strong end-to-end emphasis
• Put functions at endpoints whenever possible
• Keep the network itself as simple as possible

12
The Major Internet Protocols
SMTP
Telnet
DNS
FTP
DHCP
ICMP
IP
Other subnetworks
Dial
IS95
ISDN
13
Connectionless Networks

• Similar to postal system
• perhaps an unfortunate metaphor
• Full addresses in every packet
• network handles each packet independently
• Any notion of a connection is strictly
  end-to-end the network doesnt know about them
• facilitates scaling to very large networks
• Service is usually best-effort
• Far easier to implement
• Standard examples Ethernet, IP

14
The Internet Protocol (IP) - RFC791

• The protocol that defines The Internet
• Datagram based (connectionless)
• 32-bit address space (IPv4)
• written as 4 bytes in dotted decimal format,
  e.g., 129.46.101.170
• Maximum datagram size 64KB
• Best-effort delivery service, optional QOS
• Fragmentation/reassembly for subnets with smaller
  packet size limits

15
Internet Services

• IP is best effort. Packets may be
• Lost (frequently, alas)
• Corrupted (very rarely, thanks to link CRCs)
• Delivered out of order (when routes change)
• Duplicated (rarely)
• Upper layer entities must anticipate and recover
  on an end-to-end basis

16
The IP Header
0
TOS
Ver
IHL
Total Length
M F
D F
Frag offset
0
Identification
4
TTL
Protocol
8
Header Checksum
Source Address
12
16
Destination Address
17
End-to-End Protocols

• User Datagram Protocol (UDP)
• defined in RFC 768
• Transmission Control Protocol (TCP)
• defined in RFC793
• Internet Control Message Protocol (ICMP)
• defined in RFC792
• error reporting, diagnostic testing
• Others exist, but are rare
• because TCP and UDP cover nearly all needs

18
The UDP Header
0
Source Port
Destination Port
Checksum
Length
4
19
UDP Applications

• Short transactions
• Domain Name System (DNS)
• Network File System (NFS)
• Real-time applications
• Voice over IP
• Multicasting
• Conferencing, broadcasting

20
TCP

• Connection-oriented
• Reliable
• sequence numbering, retransmission
• Bi-directional
• though many applications are unidirectional
• Featureless byte stream
• records, messages, etc, imposed by application

21
TCP vs UDP

• Many applications could use TCP or UDP
• TCP tends to be easier to use
• UDP tends to be more efficient and robust
• especially if application protocol is idempotent

22
Connections

• A socket is an IP address, port pair
• A connection is defined by a pair of sockets,
  I.e, the 4-tupleIP source address, source
  port,IP destination address, destination port
• Note that many different connections can share
  the same socket on one end
• I.e., the analogy to a hardware outlet isnt
  exact
• This permits well known ports for servers

23
TCP Connection Management

• 3-way handshake opens bi-directional
  point-to-point connection
• Either side can issue a close and continue to
  receive data indefinitely
• Designed to handle simultaneous opens
• though rarely used in practice
• Great care taken to detect and recover from lost,
  duplicated or reordered packets
• When both sides close, the connection terminates

24
The TCP Header
0
Source Port
Destination Port
4
Sequence Number
8
Acknowledgement Number
Window
offs
flags
12
16
Checksum
Urgent Pointer
25
Wireless IP Considerations

• Performance
• Reliability/availability
• usually much lower than wired links
• Cost
• Routing/mobility
• Addressing
• Security

26
Wireless Performance Issues

• Lower speeds and higher packet loss rates than
  wired networks
• Connectivity usually not continuous
• incomplete wireless coverage
• cost
• limited battery energy
• Transport protocols (e.g., TCP), applications and
  users must all adapt to these properties

27
Transport Performance

• TCP adapts to variable throughput and delay
• already deals with many wireless performance
  issues
• High loss rates, intermittent connectivity more
  problematic
• Research ongoing
• IETF Performance Implications of Link
  Characteristics (PILC) working group

28
Transmission Control

• TCP - not the application - packetizes user byte
  stream, deciding how much to send and when
• TCPs name (Transmission Control Protocol)
  emphasizes the importance of this function
• TCPs rules
• A few big packets are better than many tinygrams
• Assume most timeouts are congestion-related

29
Nagle Algorithm

• Early TCPs sent every application write in a
  separate packet
• This was death for character-at-a-time logins
  over slow links
• link header 40 bytes TCP/IP header 1 byte
  data
• Nagle algorithm (RFC896, Jan 1984) applies simple
  heuristic
• If data avail for a max packet, send it
• Else, send only if no unacked data in flight
• I.e., stop-and-wait until requested throughput gt
  1 packet/round trip time

30
TCP Retransmissions

• The Internet can drop packets
• As a reliable protocol, TCP detects lost
  packets with timers and retransmits them
• Congestion is the main cause of packet loss
• Ergo, overly aggressive TCP retransmission
  strategies can cause congestion collapse!
• links are busy, but little useful work being done
  because few packets reach their destinations

31
Round Trip Time Estimation

• TCP must adapt to changing Internet propagation
  delays due to queuing delays, changing routes,
  speed-of-light delays, etc
• Packets are also lost occasionally
• It is hard to tell whether an overdue packet has
  been lost or is simply delayed longer than usual
• TCP doesnt have enough info in the header to
  reliably distinguish ACKs for successive
  retransmissions of the same data

32
TCP Network Delay Modeling

• TCP models Internet delay as a gaussian RV with a
  slowly varying mean and standard deviation
• Retransmission Timeout (RTO) set to mean delay
  4 standard deviations
• This is a tradeoff between
• maximizing throughput with packet loss
• minimizing unnecessary retransmissions
• Round trip time (RTT) measurements made by timer
  started when certain sequence number sent,
  stopped when it is acked

33
Estimating Round Trip Times

• Mean and standard deviation estimates made with
  exponential smoother
• mean (7/8)mean (1/8)(rtt)
• sdev (3/4)sdev (1/4)abs(rtt-mean)
• RTO mean 4sdev
• If rtt has low variance, then RTO will be only a
  little greater than the mean round trip time
• If rtt has high variance, then RTO will be much
  greater than the mean round trip time
• combination of high loss and variable delay is
  bad for throughput

34
Filtering Round Trip Time Measurements

• The TCP header has no way to distinguish a
  retransmitted segment from the original
• If the sender gets an ACK for a retransmitted
  packet, theres no way to know if its for the
  original transmission or a retransmission
• I.e., the RTT measurement is unreliable
• Therefore, only RTT measurements on segments that
  were ACKed the first time are used
• Also, the RTO backoff is clamped for the next
  packet after a retransmitted one
• avoids stable collapse state

35
Van Jacobson Congestion Control (1988)

• Limit effective transmit window to lesser of
  advertised receive window or local congestion
  window (cwind)
• Cwind starts _at_ 1 packet, expands 1 packet for
  every packet acked
• called slow start - a misnomer since its
  exponential over time!
• If a timeout occurs, assume congestion
• ssthresh 1/2 cwind
• cwind 1 packet

36
VJ Congestion Control - 2

• After recovery, slow start continues until cwind
  ssthresh
• Then cwind increases by 1/cwind on every ack
• this tests the waters to see if the path can
  support more traffic

37
Radio Link ARQ

• TCP (and other Internet transport protocols)
  designed for relatively low packet loss rates
• typically lt1 or less than one packet/RTT
• Most mobile wireless channels have higher loss
  rates even with coding and power control
• A link-level RLP can lower the loss rate to a
  range that can be adequately handled by TCP
• The RLP does not have to be perfect
• just good enough!

38
Other Approaches

• Proxying/spoofing
• TCP ACK snooping/spoofing
• Protocol translation (e.g., WAP)
• All violate end-to-end principle
• less robust
• complicates security
• Just say no!

39
Intermittent Connectivity

• Already common on wired networks
• dialups
• roving laptops
• Generally handled at the application layer
• e.g., Post Office Protocol (POP) for email
• Experimental proposals for TCP
• ICMP reachable message

40
Mobility

• Allowing a user to keep a fixed address (at some
  level) when changing attachment points to a
  topologically-routed network
• both the PSTN and the Internet are topological
• Roaming cell phones and Internet users are very
  similar in this respect

41
Mobility - Some Common Concepts

• Home agents
• stationary systems that own mobile users
  address and accept traffic on behalf of mobile
  user
• analogous to cellular HLRs
• Foreign agents
• provide service to mobile user
• analogous to cellular VLRs
• Registration
• mobile users communicate back through serving
  system to home agents to indicate current location

42
Multi-Layer Mobility

• Mobility can be provided at several different
  layers with different advantages/disadvantages
• IP level (Mobile IP)
• Domain Name System (DNS)
• Application-level
• Post Office Protocol (POP)
• various Internet telephony directory servers

43
Mobility at the IP Layer

• Mobile user keeps fixed IP address
• IP packets to the mobile user are received by the
  home agent and tunneled to his current location
• The most transparent form of mobility
• everything works as if the host were fixed
• TCP connections stay open when host moves

44
IP-in-IP Tunneling
Tunnel
HA
owns home net IP address block
Internet
FA
Rest of Internet
User
ISP-assigned IP address
Mobile user net
FA and HA can be Linux, BSD, NOS, etc
45
Tunneled Packet Format
Outer IP Header SrcHA DstFA ProtIP
Inner IP Header SrcCH DstUser ProtTCP (etc)
TCP/ UDP header (etc)
User data (if any)
46
Problems with Mobile IP

• Mobile IP is elegant, but it comes at a price
• Increased per-packet overhead for tunneling
• Non-optimum routing
• increased delay, lowered reliability
• can be serious over wide areas

47
Mobility in the DNS

• The DNS provides a layer of indirection that can
  be used to provide mobility
• When a mobile host moves, it obtains a new IP
  address and registers it with the server for his
  zone
• Requires short DNS TTLs if the host moves
  frequently
• Existing TCP connections break when moving
• Advantage of much more efficient routing
• no need to tunnel every user packet through home
  agent

48
Application Mobility

• Certain important applications have protocols
  specifically designed to support mobility
• Best example email
• SMTP implies ability to listen continuously at a
  fairly stable IP address for incoming mail
• TURN command never implemented
• POP allows user to pull mail from a relay server
• mail server plays role of home agent
• POP is the registration protocol

49
Is Mobile IP Really Needed?

• Most mobile hosts function only as clients
• HTTP, SSH/Telnet, FTP
• SMTP (for sending mail)
• POP (for fetching mail)
• Most couldnt run servers anyway
• intermittent operation on battery power
• connectivity limits (e.g., air travel)
• Most transactions are very short-lived
• but not all
• Dynamic addressing has served the dialup ISP
  market well

50
Addressing

• IP addresses are an increasingly scarce resource
• 232 used to seem like such a large number
• IP does use space more efficiently than PSTN
• Long term solution IPv6
• 2128 still less than number of atoms in universe
• Short-term fixes have been effective
• dynamic address allocation (PPP, DHCP)
• CIDR
• NATs, private address blocks (e.g., 10.x.x.x)

51
Security

• General Internet problem, not just wireless
• security issues only more obvious on wireless
• Worthy of an entire tutorial by itself
• General principle place security mechanisms
  close to entity being protected
• Different mechanisms for different needs
• link resource (e.g., theft of carrier service)
• host computers (end-user privacy)

52
Encryption and Security

• Encryption is essential element in security
• but is not magic bullet
• Can authenticate or provide confidentiality
• Governments dont like confidentiality
• export controls used to thwart widespread use
• Carriers not motivated to protect users privacy
• and pressured by CALEA to do opposite
• Ergo, user-provided end-to-end encryption
  essential

53
Point-to-Point Protocol (PPP) - RFC1661

• Carries IP over generic point-to-point link
• Dialup modems
• ISDN
• Leased lines
• IS-95 CDMA traffic channels (above RLP)
• Type field for non-IP protocols
• Configuration negotiation
• addresses, max sizes, etc
• Authentication at link setup
• No retransmission

54
PPP Frame Format
PPP Hdr
Flag
Flag
Data
CRC
Flag 0x7e Header 1-4 bytes (negotiable) CRC 16
bits
55
PPP Framing

• Bit-synchronous channels
• Synchronous modems, most leased lines
• Octet-synchronous channels
• ISDN, IS-95
• Asynchronous channels
• Generic dialup modems

56
PPP on Synchronous Channels

• Conventional HDLC framing
• opening, closing flags
• 0-bit stuffing of data for transparency
• 16-bit frame CRC
• no link-level retransmission (framing only)
• functionality in chips like Z8530 SCC

57
Octet-Synchronous PPP

• Some channels (ISDN, IS-95) provide PPP with a
  synchronous octet (byte) stream
• No need for bit stuffing (physical layer
  maintains byte alignment)
• Still need frame delimiters and CRCs
• byte stuffing to protect special chars
• 0x7e -gt 0x7d, 0x5e flag
• 0x7d -gt 0x7d, 0x5d escape character
• other special characters can also be escaped as
  needed
• 0x01 -gt 0x7d, 0x21 ascii control character
• c -gt 0x7d, (c 0x20) general rule

58
Asynchronous PPP

• Universally used on dialup modems
• Like octet-synchronous except arbitrary idle time
  between bytes
• Still need frame delimiters, CRCs, byte stuffing
• same escape sequence procedure for special chars
• Replaces earlier non-standard SLIP (Serial Line
  IP) protocol
• IP only
• no negotiation facilities
• no frame CRC

59
PPP Link Configuration Protocol (LCP)

• Runs when link first brought up
• Negotiates link-level parameters
• max frame size
• special characters to be escaped (besides flag
  escape)
• use of abbreviated PPP frame headers
• default has address control 2 byte type to
  look like standard HDLC UI-frame
• most links negotiate to omit address control
  and to use 1-byte type field

60
PPP IP Configuration Protocol (IPCP)

• Establish IP address of client
• PPP server allocates temporary address, or
• client notifies server of fixed address
• Negotiate use of VJ TCP/IP header compression

61
Data on Digital Cellular Channels

• IS-95 CDMA
• IS-707 data standards
• No modifications required to BTS
• major advantage given widespread IS-95 deployment
• Globalstar
• very similar to IS-95 wrt data
• GSM
• circuit switched
• General Packet Radio Service (GPRS)

62
The IS-95 Channel

• Semi-connection-oriented
• hardware allocated to call, but air resource is
  dynamically shared
• Designed for variable-data-rate vocoder
• Frames sent at constant 50 Hz (20ms) rate
• Four fixed-size frames with raw sizes
• Rate set 1 ("9.6") 24, 48, 96, 192 bits
• Rate set 2 ("14.4") 36, 72, 144, 288 bits
• Viterbi decoder tails and CRCs of varying sizes
  reduce usable payload

63
Data on the IS-95 CDMA Channel

• The IS-95 physical channel was designed for
  voice data was an afterthought
• Voice delay considerations limit frame size
• limited interleaving for slow fading
• power control helps
• Typical frame loss rates 1-2
• acceptable for voice
• unacceptable for data

64
Performance Without RLP

• 1500 byte IP/PPP packet, IS-95 Rate Set 1
• 1500 bytes/22 bytes/frame 68 frames
• For FER.01, probability of packet success
  is(1-.01)68 0.505 (pretty bad)
• For FER.02, probability of packet success
  is(1-.02)68 0.253 (even worse)
• TCP can only recover by resending entire packet
• selective link-level retransmission clearly needed

65
Packet Data over IS-95 CDMA

• IS-99/657/707 define a Radio Link Protocol for
  sending packet data over IS-95 CDMA
• RLP breaks variable-length PPP packets into one
  of the 4 frame lengths supported by IS-95 Rate
  Set 1 or 2 traffic channels
• RLP senders add sequence numbers to frames
• RLP receivers NAK missing frames and the senders
  retransmit them
• RLP is mostly reliable it does not try to
  provide perfect reliability

66
IS-95 CDMA Data Protocol Stack
Appl
TCP/ UDP
IP
PPP
RLP
IS-95 Physical
67
Quick Net Connect

• Original concept IP packet data support with
  dormant mode
• similar to demand-dialed ISDN
• Political obstacles to CDMA packet data
• lackluster carrier interest
• vendor resistance (CDPD competition?)
• inability to appreciate importance of Internet
• some telcos still think data modems
• Asynch data/fax service based on TCP/IP
• this was the hook for QNC

68
MDR

• Multiple IS-95 channels associated with single
  user data stream
• conceptually similar to ISDN B-channel bonding
• Variable-rate CDMA channel lessen need to
  deallocate unused channels quickly
• hardware is dedicated to call, but channel
  resource is dynamically shared

69
GSM

• Time-division multiple access channel
• Burst rate 270.833 kb/s
• 8 timeslots/channel
• 182.4 kb/s/channel (including FEC)
• Widespread in Europe, less so in US
• Circuit-switched data already deployed
• 9.6 kb/s (sometimes 14.4 kb/s with less FEC)
• dedicated air resource during call, wasteful for
  bursty packet traffic
• no direct ISP connection, must dial modem pool

70
GPRS

• Medium-speed packet mode extension to GSM
• similar to CDMA MDR
• FEC rates 1/2 to 1
• 9.05 to 21.4 kb/s/timeslot
• Likely peak usable throughput 60 kb/s
• Can use up to 8 timeslots at once
• dynamically allocated
• Link ARQ with LLC
• HDLC and LAPD-like

71
Cellular Data Overlays

• Cellular Digital Packet Data (CDPD)
• Qualcomm HDR

72
Cellular Digital Packet Data (CDPD)

• Packet data overlay on AMPS
• connectionless (simpler than IS-95)
• Requires dedicated equipment in each cell
• only shares spectrum, antennas power
• limited coverage, high costs prices
• RF channel compatible with AMPS (30 KHz)
• GMSK modulation _at_ 19.2 ks/s
• usable throughput less due to (63,47) RS FEC
• Shared channel
• busy/idle bits for contention

73
CDPD Network Architecture

• Backbone network based on OSI
• defacto obsoleted by Internet protocols
• Static IP addresses
• can carry between serving systems
• inefficient wide-area Internet routing

74
CDPD Traceroute
traceroute to storyprod.qualcomm.com
(192.35.156.222), 30 hops max, 40 byte packets 1
san-diego-114.wireless.gte.net (198.226.11.26)
354.057 ms 347.197 ms 369.513 ms 2
198.226.23.161 (198.226.23.161) 389.023 ms
417.724 ms 419.519 ms 3 s11-0-0-18.houston1-cr1
.bbnplanet.net (4.0.248.133) 499.053 ms 438.012
ms 439.506 ms 4 h3-0.dallas1-br2.bbnplanet.net
(4.0.2.37) 439.056 ms 457.525 ms 429.508 ms 5
a4-0-1.atlanta1-br1.bbnplanet.net (4.0.3.237)
439.066 ms 417.797 ms 459.476 ms 6 4.0.2.142
(4.0.2.142) 479.025 ms 458.099 ms 459.846 ms
7 104.ATM2-0.XR1.ATL1.ALTER.NET (146.188.232.50)
479.854 ms 438.699 ms 429.833 ms 8
195.ATM3-0.TR1.ATL1.ALTER.NET (146.188.232.86)
839.835 ms 458.743 ms 459.819 ms 9
109.ATM6-0.TR1.LAX2.ALTER.NET (146.188.136.50)
499.84 ms 488.663 ms 529.831 ms 10
299.ATM7-0.XR1.LAX2.ALTER.NET (146.188.248.125)
499.837 ms 538.659 ms 499.821 ms 11
195.ATM10-0-0.GW1.SDG1.ALTER.NET (146.188.249.65)
479.846 ms 498.681 ms 499.81 ms 12
qualcomm-gw.customer.ALTER.NET (157.130.225.142)
490.27 ms 517.525 ms 519.817 ms 13
storyprod.qualcomm.com (192.35.156.222) 529.863
ms 668.736 ms 519.828 ms
75
HDR

• High speed wireless packet data system under
  development at Qualcomm
• Physical layer borrows from IS-95, but redesigned
  specifically for packet data
• will require BTS overlays (like CDPD)
• 1.2288 MHz spread BW (same as IS-95)
• Semi-connection-oriented (like IS-95)
• Throughput depends on loading and distance
• somewhat like ADSL

76
HDR Forward Link

• Single stream of 128-byte frames
• somewhat like ATM
• Fixed symbol rate
• Modulation alphabet and FEC code rate determine
  user data rate
• Constant transmit power
• Data rate controlled by mobile request
• 38.4kb/s up to 2.4Mb/s
• rate depends on SNR

77
HDR Reverse Link

• Fixed-time 53ms frames
• Pilot subchannel
• Data rate varies from 4.8kb/s - 307kb/s
• depends again on link margin
• Closed loop power control
• similar to IS-95

78
Speed Considerations

• The higher the data rate, the slower the relative
  fading
• larger packets are good
• higher data rates are bad (unfortunately)
• Ergo, ARQ link protocol still required
• HDR RLP similar to IS-707/IS-95
• byte-numbered vs frame-numbered

79
Cellular Data Summary

• Wireless systems discussed so far are
  cellular-based
• asymmetric fwd rev links on different
  frequencies
• no direct mobile-to-mobile communication
• systems centrally managed
• Service model telephone company or ISP

80
Ad-Hoc Packet Radio

• Original model for DARPA work
• Single frequency, symmetric modulation
• permits direct peer-peer communication
• Self-organizing topology
• Decentralized control
• Well suited to unlicensed bands (Part 15)
• Service model UseNET, Internet backbones

81
Examples of Ad-Hoc Nets

• DARPA SURAN
• Pioneering work in 1970s-1980s
• Amateur (ham) packet radio
• early 1980s-present
• Part 15.247 devices
• Many proprietary designs
• IEEE 802.11
• Metricom

82
Advantages of Ad-Hoc Networks

• Lower getting-started costs
• no need to install base stations
• easier temporary setup
• Well suited to free unlicensed spectrum
• significant savings given typical auction prices
• Inherent scalability
• with power control cooperative relaying, each
  user contributes to network capacity

83
Challenges of Ad-Hoc Networks

• Hidden terminal problem
• with every terminal transmitting on the same
  channel, stations can interfere with others it
  cannot hear
• addressed with MACA protocol in 802.11
• Power control necessarily more coarse than on
  full-duplex IS-95 or HDR channel

84
Hidden Terminals
A and B can hear each other B and C can hear each
other A and C cannot hear each other
If C transmits while A is transmitting to B, C
will interfere with Bs reception even though it
cannot hear A
85
MACA

• RTS/CTS handshake to reduce chances of hidden
  terminal collision
• Sender sends brief Request-to-Send (RTS) giving
  data length
• Receiver returns Clear-to-Send (CTS) echoing data
  length
• All other transmitters stay off channel long
  enough for sender to finish
• Collisions can still occur on RTS messages
• but theyre smaller than data messages

86
Conclusion

• Roles exist for both cellular and ad-hoc data
  networks
• cellular provides common-carrier service
• ad-hoc provides flexibility
• Will be interesting to see if/how ad-hoc networks
  take cellulars market share