Wireless Data Tutorial

Phil KarnSenior Staff Engineer

Qualcommkarn@qualcomm.com

http://people.qualcomm.com/karn

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

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

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

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 Internet’s

“grandfathers”

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

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

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 don’t 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

The Internet Reference Model

Application

Host-to-Host(end-to-end)

Internet

Subnet

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

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

The Major Internet Protocols

IP

ARP

Enet

PPP

TCP UDP

Telnet FTP

SMTP

POP

ICMP

DNS DHCPHTTP

Dial IS95 ISDN

Other subnetworks

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 doesn’t know about them facilitates scaling to very large networks

Service is usually best-effort Far easier to implement Standard examples: Ethernet, IP

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

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

The IP HeaderTotal LengthTOSVer IHL

Identification

Source Address

Destination Address

Header ChecksumTTL Protocol

0DF

MF

Frag offset

0

4

8

12

16

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

The UDP Header0

4

Source Port Destination Port

ChecksumLength

UDP Applications Short transactions

Domain Name System (DNS) Network File System (NFS)

Real-time applications Voice over IP

Multicasting Conferencing, broadcasting

TCP Connection-oriented Reliable

sequence numbering, retransmission Bi-directional

though many applications are unidirectional Featureless byte stream

records, messages, etc, imposed by application

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

Connections A socket is an {IP address, port} pair A connection is defined by a pair of sockets, I.e,

the 4-tuple:{{IP 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 isn’t exact This permits “well known ports” for servers

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

The TCP Header0

4

8

12

16

Source Port Destination Port

Sequence Number

Acknowledgement Number

Window

Checksum Urgent Pointer

offs flags

Wireless IP Considerations

Performance Reliability/availability

usually much lower than wired links Cost Routing/mobility Addressing Security

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

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

Transmission Control TCP - not the application - packetizes user byte

stream, deciding how much to send and when TCP’s name (“Transmission Control Protocol”)

emphasizes the importance of this function TCP’s rules:

A few big packets are better than many tinygrams Assume most timeouts are congestion-related

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 > 1

packet/round trip time

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

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 doesn’t have enough info in the header to

reliably distinguish ACKs for successive retransmissions of the same data

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

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 + 4*sdev 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

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, there’s no way to know if it’s 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

Van Jacobson Congestion Control (1988)

Limit effective transmit window to lesser of advertised receive window or local congestion window (cwind)

Cwind starts @ 1 packet, expands 1 packet for every packet acked called “slow start” - a misnomer since it’s

exponential over time! If a timeout occurs, assume congestion:

ssthresh = 1/2 cwind cwind = 1 packet

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

Radio Link ARQ

TCP (and other Internet transport protocols) designed for relatively low packet loss rates typically <1% 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!

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!

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

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

Mobility - Some Common Concepts Home agents

stationary systems that “own” mobile user’s 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

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

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

IP-in-IP Tunneling

Internet

HA

FA

Rest of Internet

Mobile user net

User

“owns” home netIP address block

ISP-assigned IP address

FA and HA can be Linux, BSD, NOS, etc

Tunnel

Tunneled Packet Format

Outer IPHeader

Src=HADst=FAProt=IP

Inner IPHeader

Src=CHDst=UserProt=TCP

(etc)

TCP/UDP

header(etc)

User data(if any)

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

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

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

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 couldn’t 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

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)

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)

Encryption and Security

Encryption is essential element in security but is not magic bullet

Can authenticate or provide confidentiality Governments don’t 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

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

PPP Frame Format

Flag PPPHdr Data FlagCRC

Flag: 0x7eHeader: 1-4 bytes (negotiable)CRC: 16 bits

PPP Framing Bit-synchronous channels

Synchronous modems, most leased lines Octet-synchronous channels

ISDN, IS-95 Asynchronous channels

Generic dialup modems

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

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 -> 0x7d, 0x5e [flag] 0x7d -> 0x7d, 0x5d [escape character]

other special characters can also be escaped as needed: 0x01 -> 0x7d, 0x21 [ascii control character] c -> 0x7d, (c ^ 0x20) [general rule]

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

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

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

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)

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

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

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

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

IS-95 CDMA Data Protocol Stack

IS-95Physical

RLP

PPP

IP

TCP/UDP

Appl

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

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

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

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

Cellular Data Overlays

Cellular Digital Packet Data (CDPD) Qualcomm HDR

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 @ 19.2 ks/s

usable throughput less due to (63,47) RS FEC Shared channel

busy/idle bits for contention

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

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 ms10 299.ATM7-0.XR1.LAX2.ALTER.NET (146.188.248.125) 499.837 ms 538.659 ms 499.821 ms11 195.ATM10-0-0.GW1.SDG1.ALTER.NET (146.188.249.65) 479.846 ms 498.681 ms 499.81 ms12 qualcomm-gw.customer.ALTER.NET (157.130.225.142) 490.27 ms 517.525 ms 519.817 ms13 storyprod.qualcomm.com (192.35.156.222) 529.863 ms 668.736 ms 519.828 ms

CDPD Traceroute

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

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

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

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

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

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

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

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

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

Hidden Terminals

A B C

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 B’s reception even though

it cannot hear A

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 they’re smaller than data messages

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 cellular’s market share