W ireless Packet Networking

1Roland KempterUniversity of Utah, Department of Electrical Engineering, 2003

Wireless Packet Networking:

Background on state-of-the ar t wireless packet networking,novel system ideas

Roland Kempter University of Utah

Department of Electrical and Computer EngineeringEmail: [email protected]

Presented at iCORE, University of Alberta, 5.21.2003

by


Purpose of this presentation:

• Give you a basic understanding of the development of Wireless Communication Systems from the very beginning to the present

• Give you an overview over the most important technologies of Cellular and Wireless LANs

• Establish a logical link from today‘s state-of-the-art technology to our current research.

Preface


1. The history of Packet Based Wireless Networking

INTERMEZZO

2. Introduction, technical background of Packet Switched Networks

INTERMEZZO

3. State-of-theart in Wireless Packet Networking

INTERMEZZO

4. The futureof Wireless Packet Based Networking?

INTERMEZZO

5. What are „The Others“ doing?

Organization


1. History (overview)

• 1971 ALOHANET [21], the first wireless packet-switched network. - established by the University of Hawaii to connect the data centersof 7

campus sites located across 4 islands to the main data center without the use of phone lines. ALOHANET was connected to the ARPAnet on the mainland.

• In the 1980's, Amateur Radio Operators or " hams" , built terminal node controllers (TNC's) to connect their computers through their radio equipment.

• 1G networks- analog, ciruit switched, no „data mode“ , Systems like AMPS (Advanced

Mobile Phone Service)

• Ear ly 1990: Various Kinds of Satellite VSAT systems- capableof transmitting voice, fax and data- MF-TDMA and data packeting, (Frame Relay) [15]

[21] Abramson, N. "The ALOHA system, another alternative for computer communications," in Proc. 1970 Fall Jt Computer Conf, AFIPS Press, Arlington, Va., pp 281-285.

[15] www.ndsatcom.com, Satellite VSAT Systems, SKYWAN and SKYWAN DVB-IP


INTERMEZZO


Two terms joined together: Wireless and Packet (switched)

1) circuit switched:- there exists aswitched line from the user to the base station or receiver at all

times during the „call“ ,- permanently allocated network resources

2) packet switched:- splits data into packets and placespackets from multipleconnections on

shared physical circuits, - data is routed on a per-packet basis to its final destination, - bandwidth can beallocated dynamically

Basic Modesof Operation of WLANs:

1) infrastructuremode, from nodes to basestation2) ad hoc mode, from node to node

2. Introduction


Some Types of WLAN architectures

• wireless LANS, WLANs- most famous: IEEE 802.11x

• wireless WANs/MANsWide Area Networks, Metropolitan Area Networks, eg. - CDPD, Cellular Digital Packet Data, Standard for data transmission

using GSM , Global System for Mobile Communications, with up to 19.2 kpbs [17],

- GPRS, GSM Packet Radio Service [31]

• wireless PANsPersonal Area Networks, eg. - Bluetooth (IEEE 802.15)

lower data rates, low power consumption

2. Introduction

[17] „What is CDPD?“, http://www.novatelwireless.com/company/cdpd.html[31] „All about GPRS“, http://www.mobilegprs.com/home.htm


Some Types of WLAN architectures, cont‘d

• Satellite networks,- DVB-S(RCS) based networks, - VSat networks,

up/downstream up to 38Mbps (currently) MF-TDMA, big delays [15]

• (mobile) ad hoc networks,- Sensor Networks,- Emergency Relief,

multi-hop, infrastructureless, nodes (can also) act as forwarders/routers, network environment can be very difficult since routers can also be mobile!

2. Introduction

[15] www.ndsatcom.com, Satellite VSAT Systems, SKYWAN and SKYWAN DVB-IP


We heard about different Wireless LAN Technologies, but how do they work?

Understanding Packet Switched Networks:The OSI (Open Systems Interconnect) Layer Model

2. Introduction


Packet processing acc. to the OSI model

2. Introduction, The OSI Layer Model

Figure 1: OSI Layers



Figure 2: OSI Layers and Packeting



LAYER 1, PHY: The physical later is concerned with transmitting raw bitsover a communication channel.

Thedesign issues here deal largely with mechanical, electrical, and procedural interfaces and the physical transmission medium.

LAYER 2, MAC: Themain task of the data link layer is to take a raw transmission facility and transform it into a line that appears free of transmission errors in the network layer (error correction capabilities).

It accomplishes this task by having the sender break the input dataup into data frames (typically a few hundred bytes),transmit the frames sequentially, and process the acknowledgment frames sent back by the receiver.



LAYER 3: The network layer is concerned with controlling the operation of thesubnet. A key design issue is determining how packets are routed from source to destination.

Figure 3: IEEE 802.11 Packet format and Message Structure

The 802.11 Packet: A Sample Message Structure


2. Introduction, The IEEE 802.1 Standard and OSI Layers 1 and 2

Figure 4: IEEE 802.11 and the OSI Modell


So far, we’ve heard a lot about Networking Technology, but how do we use it in a multiple user scenario?

Deterministic and Random Channel Access Schemes

2. Introduction


• General Types of MACs– Multiple Access Protocols:

1) Deterministic channel access, some channel partitioning approaches:

- FDMA (and flavors): Frequency Division Multiple Access, every mobilenode has its own radio channel, channel is freed when a node finishes communication. Arbiter necessary.

- TDMA (and flavors): TimeDivision Multiple Access, resources (can be)divided into radio channels, further sub-divided into timeslots, node is assigned timeslot (and radio channel),multipleuserscan simultaneously share a radio channel. Arbiter necessary.

not very flexible, frame structure not easy to change dynamically, control loop over downlink with „channel information table“ necessary

2. Introduction, Deterministic MultipleAccess Protocols


1) Deterministic channel access, some channel partitioning approaches, cont‘d:

- Synchronous-CDMA (and flavors): CodeDivision Multiple Access, SpreadSpectrum.Examplesof SS: frequency hopping and direct sequenced, tell nodes which code to use orthogonal codes, global timing synchronous

PROs: - Capacity increases compared to „ real world“ TDMA systems,- improved call quality, - simplified system planning (frequency reuse), - better secur ity through scrambling, - improved coveragecharacteristics, - fewer cells (better diversity characteristics), - lower power consumption,- bandwidth on demand can be accomplished easily by changing

the length of thespreading sequence

2) Demand Assignment Protocols: Polling, Token Passing

2. Introduction, Deterministic MultipleAccess Protocols


3) Random multiple AccessSchemes, wireless networks:

- (Pure) ALOHA [21], simplest possible protocol, a station with a message simply transmits it to completion. If no collision occurred, message gets through, otherwise wait random time and retransmit.

CONTRA: works for when transmissions are rare; quickly degenerates as load increases. Performance analysis based on assumed Poisson distribution shows max throughput of 18%. [20]

- Slotted ALOHA: divide time into slots and restrict transmissions to time slots, station waits until next time slot to transmit but slots must be synchronized, max. throughput of 36% [20]

- ALOHA with 2 different power levels (transmit either with high or low power), improves max. throughput of pure ALOHA to 26% and slotted to 52% [20]

2. Introduction, Random Multiple Access Protocols

[21] Abramson, N. "The ALOHA system, another alternative for computer communications," in Proc. 1970 Fall Jt Computer Conf, AFIPS Press, Arlington, Va., pp 281-285.

[20] www.cs.nps.navy.mil/people/faculty/ baer/N3502/Lans-1.ppt


2. Introduction, Random MultipleAccess Protocols

4) Random Channel Access:

- (totally) Asynchronous-CDMA: nodes choose their spreading sequence randomly

PN (pseudo-noise) sequences, no orthogonal user channels but channels separated by a given processing gain N, users can be totally asynchronous

no global network time

There are many additional intermediate formats!


2. Introduction, Random Multiple Access Protocols

5) Random Multiple Channel Access, wired networks, Ethernet:

- With collision detection:

CSMA/CD (Carrier Sense Multiple Accesswith Collision Detection),

- in Ethernet, in case of a collision, voltage levels change to indicate that a collision has occurred

BUT: Not feasible in wireless networks, It is not possible to detect if acollision has occurred


6) Random multiple access, IEEE 802.11x based networks, only most relevant

• With collision avoidance: CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance):


t

medium busy

DIFSDIFS

next frame

contention window(randomized back-offmechanism)

slot timedirect access if medium is free ≥ DIFS

Figure 5: CSMA/CA – basic operation [32]

• Channel idle during DIFS (Distributed InterframeSpace), transmit frame

• If the medium is busy, wait for a free DIFS and a random back-off time

• If another station uses the medium during the back-off time of the station, the back-off timer stops (fairness)

[32] Andrzej Duda, Laboratoire LSR, ``Performance anomaly of 802.11b``


6) Random multiple access, IEEE 802.11x based networks, only most relevant, cont‘d

• With collision avoidance: CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance):

- In contrast to CSMA/CD (-Collision Detection) for Ethernet. - CSMA/CA is used to reduce network collisions by listening to the network

before broadcasting.

CONTRA: All transmitted frames have to be acknowledged

After transmission of a packet wait until SIFS (Short Interframe Space) is up, send an ACK

increases traffic, hidden node/exposed node problem(no „orthogonal“ user channels).



2. Introduction, Random MultipleAccess Protocols, Hidden Node Problem

• Hidden NodeProblemCOLLISION

A B C

- A talks to B - C does not receive A so C thinks the channel is free (out of range)- C talks to B- at B, the signals from A and C collide

Collisions,Resources wasted

Figure 6: Hidden Terminal Problem


2. Introduction, Random MultipleAccess Protocols, Exposed Node Problem

- B talks to A- C wants to communicate with D- C thinks channel is busy- C stays quiet (when it could have transmitted)

• Exposed Terminal Problem

Underutilization of channel,Lower effective throughput, channel capacity wasted

A B C

NOT POSSIBLE

D

Figure 7: Exposed Terminal Problem


6) Random multiple access, IEEE 802.11x based networks, only most relevant, cont‘d

- Even more overhead imposed by the MACA (Medium Access with Collision Avoidance), and MACAW (Multiple Access with Collision Avoidance for Wireless [11]) extensions (MAC Layer signalling)

RTS-CTS-ACK : message transmissions controlled by Request-To-Send and Clear-To-Send messages with acknowledgement, send a short RTS-packet by setting the NAV-field (Network Allocation Vector) inside the RTS-frame to the duration of the transmission, thereby trying to avoid the “Hidden Node Problem”

try to exclusively reserve the channel.

- Fairness of access provided by 4 different inter-frame spacings and an optional PCF(Point Coordination Function) for time critical messages

only basic QoS implementation


[11] V. Bharghavan, A. Demers, S. Shenker, and L. Zhang, “MACAW: A Media Access Protocol for Wireless LANs,” in Proc. of ACM SIGCOMM ’94, 1994.


Questions for channel access method design:

Exploit thenature of the PHY-Layer

2. Introduction, Random Multiple Access Schemes

Questions for MAC Layer design (typically):Fairness of access, overall- and per user throughput


INTERMEZZO

The history of Wireless Communications is the

r ise of Digital over Analog Communications

as it emerges from Circuit- to Packet Switching


If we talk about Wireless Networking, we mostly refer to two basic technologies:

1) Wireless LANs, like 802.11x

2) Cellular Wireless Communication Systems

Let’s talk about Wireless LANs, respectively about 802.11x first…

INTERMEZZO


3. State-of-the Art Wireless (Packet) Networking, IEEE 802.11x

• In 1997, the IEEE drafted the 802.11 standard for wireless local area networking [9].

Since then, the 802.11 standard hasbeen extended to the 802.11a, b and g flavors in order to increase throughput by defining new PHY layers but basically using thesame MAC layer asdefined in the original 802.11 document [24]

- 802.11: up to 2 Mbps [9]

- 802.11b: up to 11 Mbps, compatible to 802.11[8]

[24] The IEEE 802.11 Tutorial, Doc. Number IEEE P802.11-96/49B[7] IEEE Std. 802.11a-1999 ”Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)

specifications, High Speed Physical Layer Extension in the 5 GHz band” , Pages 1–45.[8] IEEE Std. 802.11b-1999 ”Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)

specifications, High Speed Physical Layer Extension in the 2.4 GHz band” , Pages 1–58.[9] ANSI/IEEE Std. 802.11, 1999 ”Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer

(PHY) specifications” , Pages 1–185.


3. State-of-the Art Wireless (Packet) Networking, back to IEEE 802.11x

802.11x standards, cont‘d

- 802.11a: up to 54 Mbps in the 5.2 GHz band [7]

- 802.11g: up to 54 Mbps in the 2.4GHz band [18]

...and many others like802.11f for Inter Access Point Communication that areout of scopeof this presentation.

[18] www.apple.com, „Airport Extreme“


Figure 8: Overview over the different IEEE 801.11 flavors, [5]


[5] Texas Instruments, IEEE 802.11g, „New Draft Standard Clarifies Future of Wireless LAN“



• PHYsas defined in 802.11x [12]:

– IEEE 802.11b: 2.4 GHz-2.4835 GHz DSSS (Direct Sequence Spread Spectrum) or FHSS (Frequency Hopping Spread Spectrum) with CSMA/CA

– IEEE 802.11a: 5.725 GHz-5.850 GHz OFDM (Orthogonal Frequency Division Multiplexing) with CSMA/CA

– IEEE 802.11g: 2.4 GHz-2.4835 GHz OFDM (Orthogonal Frequency Division Multiplexing) with CSMA/CA

for all 802.11x type networks:

Random Channel Access with CSMA/CA

[12] IEEE, “ IEEE Standard for Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications,” 1997-.



The channel access method as defined in

IEEE 802.11x does not exploit the nature

of the different PHY-Layers for 802.11x


3. State-of-the Art Wireless (Packet) Networking3. State-of-the Art Wireless (Packet) Networking

We just talked about 802.11x…

1) Wireless LANs, like 802.11x

2) Cellular Wireless Communication Systems

Let’s move on towards Cellular Wireless Networks…


3. State-of-the Art Wireless (Packet) Networking, Evolution of Mobile Communic.

Figure 9: Evolution of Wireless Mobile Networking, [10]

[10] www.prism.gatech.edu/~gtg221d/lecture5.pdf


3. State-of-the Art Wireless (Packet) Networking, 2nd Generation Wireless

• 2G Networks, [31] :

– GSM: TDMA/FDD, circuit switched, [29]

– HSCSD (High Speed Circuit Switched Data): combination of multiple GSM channels, circuit switched, data rates up to 43.2 kbps, [30]

– IS-95: CDMA standard, used in US/Asia,

– IS-54: TDMA/FDD, used in the US, dual mode (analog and digital, in analog mode AMPS compatible)

– IS-136: Upbanded IS-54, used TDMA even in control channel, evolution towards packet switched, referred to as „The TDMA standard“

[29] „What is GSM?“, http://shoshin.uwaterloo.ca/~jscouria/GSM/gsmreport.html[30] NOKIA, „What is HSCSD?“, http://www.nokia.com/nokia/[31] „Overview over Wireless Communication Standards“,

http://www.mobileinfo.com/Wireless_Networks/Network_standards.htm


3. State-of-the Art Wireless (Packet) Networking, 2.5nd Generation Wireless

• 2.5G Networks,

packet based networking based upon GSM

• GPRS (General Packet Radio Service)- overlaying a packet based air interface on the existing circuit switched,

GSM network. - „Always-on“ , - optimization of network resources, Data rates up to 171.2 kbps [31]

• EDGE (Enhanced Data rate for GSM Evolution)- new PHY, new modulation technique and new coding scheme, TDMA, [16]

- data rates up to 384 kbps,

[16] Ericsson, „EDGE, Introduction of High Speed Data in GSM/GPRS networks“[31] „All about GPRS“, http://www.mobilegprs.com/home.htm


3. State-of-the Art Wireless (Packet) Networking, 3rd Generation Wireless

• 3G Networks,Nine W-CDMA proposals have been submitted, they can be represented by two general approaches:

1. CDMA2000: - synchronous, - Walsh-sequences (Wideband CDMA acc to IS-95); - data rates: 144– 384- 2000 kbps „ indoor“ or later higher, - Multi-Carrier [26] , slotted sync channels, paging channels,

2. UTRA: - sync. channels (code assignment etc..), - timing does not need to be as accurate as for CDMA2000

but a lot of overhead due to power control and embedded pilot transmission,

- Single-Carrier W-CDMA,

Synchronized CDMA (code assignment/timing), Scheduling Overhead, for more info refer to [27]

[27] R. Z. Ziemer, „3G CDMA –WCDMA and cdma2000“, http://www.ieee.or.com/Archive/diversity_in_3g/diversity_in_3g.pdf


INTERMEZZO, Recap

- Controlled, (quasi-) synchronous, (quasi-) orthogonal,power and bandwidth controlled networks or sub-optimal [26], [22], [24]

- Overhead due to scheduling

- Bandwidth resource scheduling not optimal (very complex)

- Node and basestation can be very complex,thus increasing power consumption and cost of the node

3G/802.11x Wireless Networking means:

[26] The CDMA2000 standard, http://www.3gpp2.org/Public_html/specs/C.S0002-A_v6.0.pdf[22] www.itu.int[24] The IEEE 802.11 Tutorial, Doc. Number IEEE P802.11-96/49B


INTERMEZZO, Recap

So far so good, but how can we improve this? And can it be improved?

The next part of the presentation will:

1) Summar ize the problems of state-of-the-ar t technology

2) Give an overview over how we think the situation can be improved


• In state-of-the-art wireless packet switched networking, issues involving transmissions from the nodesarebeing addressed by somehow arbiting the channel

Increased overhead, lower user bandwidth, unneccesary complexity

• In solutions like MACA, MACAW for 802.11x networks and sophisticated ACK schemes in general, issues regarding transmissions from the nodes are reduced with the cost of high complexity/high level of overhead but the basic issues are not solved!

Example: - RTS (request to send) messages can still collide.- even with separate paging channels, the problem can only be reduced

The Problems are not solved but only miminizedat a high pr ice!

4. The Future of Wireless Networking, The Problems of State-of-the-Art Technology


One Example for these overheads:

• SatelliteNetworks, ASTRAnet BBI © :- fully utilized MF-TDMA DVB-S transponder on

forward link (38Mbps),- DVB-RCS on the return link, - 2000 user terminals:

overhead due to scheduling tables for the return link in the forward link: up to 40%

4. The Future of Wireless Networking, The Problems of State-of-the-Art Technology


Use fully asynchronous, random CDMA, simplify the mobile node!

• Random Sequences can deliver the same System Capacity as Orthogonal Sequences [25]

• Power control at the chip level is unneccesary [28]

The Future?

4. The Future in Wireless Packet Networking?

[25] Alex J.Grant, P.D. Alexander, „Random Sequence Multisets for Synchronous Code-Division Multiple-Access Channels“ , IEEE Transactions on Information Theory, Vol. 44, NOV. 1998

[28] S.Verdu, S. Shamai, „Spectral Efficiency of CDMA with Random Spreading“, IEEE Transactions on Information Theory, Vol. 45, NO. 2, March 1999


The Future!

...but is this feasible?



Problem (part 1):

How to detect an ongoing transmission for such an asynchronous system without pre-determined random spreading sequences?

Checking for all possible randomly chosen spreading sequences at the base station at any time quickly becomes impossible as the number of users increases.

Answer (part 1):

Intruduce a novel packet format on the PHY, use a common access preamble (known spreading code) in order to indicate a transmission (the SHORT header is then spread ALOHA limited).

Inside the preamble, tell the receiver the codesequence that hasbeen chosen (a different one) for the data portion not spread ALOHAlimited anymore!

4. The Future in Wireless Packet Networking, The Solution?



Figure 10: Packet Format as proposed in [1]

- Common AccessPreamble for all nodes in the network,- Different spreading code for data portion, Code IDis the spreading codeof the data portion,

- Timing can be recovered using the Acc. Preamble- Packet carr ies all info necessary for decoding ITSELF- We can use a joint detector for decoding

[1] P. Kota and C. Schlegel, “A wireless packet multiple access method exploiting joint detection”


4. The Future in Wireless Packet Networking, Some Results

Figure 11: Packet Arrival Rate vs. Probability of Packet Loss, Ld/Lh=100 according to [1]

As the Multiuser Detector capability K increases, the prob. of packet loss decreases dramatically



4. The Future in Wireless Packet Networking, Some Results

Figure 12: Throughput vs. Arrival Rate, Ld/Lh=100 according to [1]

As the Multiuser Detector capability K increases, throughput increases dramatically



BUT (part 2):

Most current Wireless Networksare „ controlled“ networks:

a base station (or a Master node) synchronizes and controls communications of nodes within its radio range, that‘s what its MAC layer has been designed for

Our future system will be asynchronous, use random spreading and transmit at a random power level.

In this case, do westill need a MAC Layer comparable and as complicated to the one used in 802.11x or in any of the 3G systems?

(although not presented in detail here, they have similar limitations)


?



Answer (part 2):

NO!- We might not need an ACK-scheme on the lower layers

- We want to reduce protocol overhead of „classical“ Wireless Networks such as 2/3G CDMA systems, (MF)TDMA satellite networks and wireless networks of any 802.11 flavor, thus increasing payload bandwidth

- We want to reduce complexity at the transmitter (mobile node) as far as possible and thereby reduce power consumption at the transmitter

- We want to introduce QoS (defined as capacity per user at a given time and reliable communication) to asynchronous CDMA type networks on the lowest Layer possible – if not in a different way.


What needs to be done?:

- find a mathematical model for such a CDMA system (how to handle the capacity „adjustments“?) What is the capacity of such a system?

- define multiplescenar ios (eg. WLAN, mobile communications, satellite communications)

- Implement it

Definea „ SHOOT AND FORGET“ network but don‘ t forget QoS aspects!



1st Step: What are the impacts on per user capacity if the number of users within the system changes randomly?

find a realistic traffic model

2nd Step: What are the additional impacts on system performanceof random power and a realistic channel model?

find a realistic channel model

3rd Step: Instead of handling QoS aspects on the protocol layers, can we define QoS as a geographical expression?

4. The Future in Wireless Packet Networking, next steps in our research


4. The Future in Wireless Packet Networking? - Summary

Up to this presentation:

Define a MAC, adopt different PHYs

After this presentation:

Model the Network after the Channel


INTERMEZZO – Are We the Only Ones?


• Commercial Products:3G networks like CDMA2000 and UMTS do not exploit the nature of CDMAup to the link layer and/or the possible asynchronicity of a CDMA wireless network

The resource to bemanaged in systems like UMTS, cdma2000 is the number of available spreading codes and power levels (users can share the same timeslots and bandwidth).

5. What are „The Others“ doing? - Commercial Products


• Commercial Products, cont‘d:

The ISMA (Inhibit Sense Multiple Access-) protocol reduces the randomness in channel access by broadcasting in the downlink information about the uplink(indicate status of the available speading code sequences) [14]

No random channel accessbut asynchronicity, introduces some kind of delay to any transmission (listen for code information on the broadcast channel), create a control loop over mobilenode(s) and base station(s).

This Protocol delivers access regulation (fairness of access) and can satisfy QoS and adaptive data rate needs (as number of users increases) by varying the spreading factor with overall system load (total number of users)

5. What are „The Others“ doing? - Commercial Products

However : System Capacity has to be predefined (N users can share a number of K spreading codes)

[14] J.Perez-Romero, R.Agusti, O.Sallent, „An Adaptive ISMA-DS/CDMA MAC Protocol for Third-Generation Mobile Communications Systems“ in IEEE Transactions on Vehicular Technology, Vol. 50, No.6, November 2001


• Lizhi CharlieZhong, Berkeley is working on CDMA sensor networks and MAC layers. [2]

• Georgia Tech is working on sensor networks as well as 4G mobility management and 4G wireless systems, next generation wireless internet, IP QoS in Next Generation Internet systems, Wireless LANs and Satellite IP networks. [3]

5. What are „The Others“ doing? – Research, just a few

[2] http://bwrc.eecs.berkeley.edu/People/Grad_students/czhong/[3] http://www.ece.gatech.edu/research/labs/bwn/CurrentProjects.html

…and many many more


Thank you for your attention,

Questions are welcome!

THE END

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