© 2004, r.c. levine page 1

© 2004, R.C. Levine

Digital Switching

EETS8304/TC-715N

SMU/NTU

Lecture Scheduled April 27, 2004

3G-2.5G Cellular/PCS Survey(print in PowerPoint slides format)


Why Third Generation?• Second generation digital cellular is already very

popular and successful, why further changes? What is lacking in the 2G cellular world?

• Lack of a single worldwide radio band– Requires smaller production runs and/or more complex

and lower performance multi-band handsets. These factors all lead to higher costs.

• Lack of a single worldwide technology standard– Same problems as above– Stubborn problem due to stubborn people!

• Circuit-switched service uses spectrum resources inefficiently for bursty data– Internet popularity motivates packet technology– Voice via packet transmission (VoIP) gaining interest


Technology Shares of ~470 Million Handsets Worldwide

Analog20%

IS-95 CDMA10%

TDMA (N.Amer)

7%

Other Digital12%

GSM51%

Source: Yankee Group(Wall St. J. Nov.18’99 p.B8)


Circuit Switched Data Services

• GSM and IS-136 were designed from the beginning to support circuit-switched data rates comparable to the Public Switched Telephone Network (PSTN)– 2.4 to 9.6 kb/s via inter-working modems located in

Mobile Service Switching Center (MSC)– FAX at 9.6 kb/s and lower bit rates

• Support of 14.4 kb/s and even higher bit rates is done via linking multiple time slots

• But these methods keep a radio channel locked to one user even when no data is transmitted at some times during a connection.


Packet Advantage• Packet transmission allows greater multi-user

capacity on radio channels– Unlike circuit-switched technology which reserves fixed

channel capacity whether instantaneously used or not– Facilitates unequal data rate in opposite directions by

individual user

• Direct fit with Internet TCP/IP and UDP/IP packet protocols– Focus of most data user’s needs today– Voice over IP (VoIP) is gaining interest in wired Internet,

and can also be supported on radio Internet– Also X.25 data packets as well (used more in Europe)


How Adequate is the Result?• The objective was universal mobile telecom

service (UMTS), the result was not quite universal

• Some progress regarding common radio band, but not enough:– European Union, Japan, Korea assigned common

IMT-2000 band• 60 MHz each for up- & down-link, centered near 1950 and

2150 MHz

– USA only permits new uses in existing 1900 MHz cellular band (partly overlaps IMT-2000 uplink)

• Maybe some shared worldwide use for TDD technologyKey: UMTS=Universal Mobile Telecommunication Service; IMT-2000= International Mobile

Telephony-2000


Too Many Technologies• Proponents of different technologies were unable to agree on one

universally compatible technology• 3GPP group issued standards for W-CDMA• 3GPP2, effectively a rival group, issued standards for cdma2000• Both W-CDMA and cdma2000 require new base radio equipment• A “3G-skeptic” group within 3GPP issued standards for 2+1/2 G

(also called 2.5G) in the form of GPRS and EDGE, packet systems with lower cost evolution from GSM

• 2.5G technologies require relatively simple, low cost upgrade of GSM base radio equipment (no change for some cases)

• All above systems require new mobile stations and packet network infrastructure

3GPP=Third Generation Partnership Project; W-CDMA=Wideband CDMA; GPRS=General Packet Radio System: EDGE=Enhanced Data Rates for GSM Evolution


W-CDMA vs. cdma2000

Notes: a. Up to 12 cdma2000 downlink carriers theoretically planned; b. TDD permits unequal total physical uplink vs. downlink data rates.

Aspect W-CDMA cdma2000

Evolution plan Easier to upgrade from GSM Easier to upgrade from IS-95

Carrier spacing 5 MHz up/downlink 1.25 MHz. Downlink uses three carriersa ; Uplink uses 1 carrier w/ 3.75 MHz b.w.

Uplink/Downlink Use Frequency Division Duplex or Time Division Duplex (TDD)b

Frequency Division Duplex

Share previous technology base radio equipment?

No: new base transceivers required. Transmitters can be shared; new receivers required

Pseudo-random code (CHIP) rate 3.84 Mc/s (changed from 4.096 after “discussions” with cdma2000 proponents)

1.2288 Mc/s each 1.25 MHz carrier; 3.6846 Mc/s on uplink carrier

Power Control to remedy “near-far” problem of cdma

1500 Hz sampling rate closed loop, up/downlink

850 Hz sampling rate closed loop, uplink

Base Station Synchronization Not required, but superior handover when used

Required

Data Rates supported Up to 2 Mb/s Up to 2 Mb/s

Data Format and Infrastructure Packet Switched; uses Internet infrastructure

Packet Switched; uses Internet infrastructure


Unpleasant Realities• The rivalry between long-time CDMA and TDMA opponents led

to two different CDMA-based 3G designs– cdma2000 proponents accused their opponents of offering

an incompatible 3G CDMA design for the primary purpose of having a standard not backward compatible with existing IS-95 CDMA... Viewed as an attempt to pre-empt the inventor’s own invention!

– But, CDMA in the field has yet to fulfill high-capacity promises although it has several beneficial properties

• Result is still two incompatible CDMA-based 3G air interface standards.– ETSI did modify the 3GPP CDMA “chip” rate to make

backward compatibility with IS-95 easier to design, but mainly different 3G planners only could agree to disagree.

CDMA= Code Division Multiple Access; 3GPP= 3rd Generation Partnership Project;ETSI= European Telecommunication Standards Institute


Positive and Negative Aspects of 3G

• CDMA (either version) requires costly upgrade of base radios

• Promised higher capacity of CDMA not proven after all these years; opinions about best future capacity are divided

• CDMA does allow mixture of data rates for various users, but so does packet transmission without using CDMA

• 2 Mb/s user data rate allows viewing entertainment; more than is necessary for most Internet uses


2+½ G• Some in the industry feel that an expensive new 3G

(2 Mbit/s-capable) infrastructure investment cycle with diverse air interfaces is not the solution– The price of such capability may repel its potential

customers.• Sometimes customers don’t want costly high tech!• The Iridium project is cited as an example.

– A lower cost intermediate capacity using existing infrastructure with GSM base radio hardware is proposed as an intermediate step, and possibly the true economic survivor

– The IS-95 CDMA evolution path also has an intermediate bit rate technology, sometimes loosely called 2+1/2G as well


2+½ G Radio Technologies

Notes: a. Sharing same carrier and base transceiver is economically important; b. Announced strategy of major US IS-136 carriers vacillates between this strategy (so-called EDGE COMPACT) and a straight-to-3G strategy.

Aspect GPRS EDGEEvolution from GSM Little or no GSM base

radio upgrade; requires packet switching infrastructure

Little or no GSM base radio upgrade; requires packet switching infrastructure

Evolution from IS-136 TDMA

Not planned. Install new EDGE base transceivers with existing IS-136 base transceivers b

Radio Modulation GMSK (exactly like GSM) 8PSK (triple GSM bit rate)

Uplink/Downlink Use Frequency Division Duplex Frequency Division Duplex

Share GSM time slots? Mix GPRS with GSM on same carrier frequency a

Mix GPRS with GSM on same carrier frequency a

Data Rates Supported Up to 14.4 kb/s per time slot; up to 128 kb/s per carrier

Up to 56 kb/s per time slot; up to 384 kb/s per carrier.

Packet Formats Supported IP (Internet Protocol) and X.25

IP (Internet Protocol) and X.25


Future RoadmapToday Intermediate Perhaps Future2G 2+1/2 G 3G

EDGE Compact,UWC136+

CDMAIS-95

IS-136

GSM GPRS

WCDMA

CDMA 2000

CDMAone

3GPP2

3GPP

1G=Analogcellular

EDGE

X X XX

X

Earlier plan to gradually phase in EDGE Compact in the 850 MHz band was later abandoned and replaced by migration of IS-136 to GSM, GPRS and EDGE (AT&T Wireless and Cingular)


2G TDMA Technologies• GSM 2G digital cellular uses gross 271 kbit/s

divided into 8 TDMA time slots (net 22.8 kbit/s/channel*) on 200 kHz bandwidth radio channel. Gaussian Minimum Shift Keying (digital GMSK FM) modulation.

• IS-136 digital cellular uses gross 48.6 kbit/s divided into 3 TDMA time slots (net 13 kbit/s/channel*) on 30 kHz bandwidth radio channel. Differential phase shift keying.

• Multiple TDMA channels may be used by one mobile unit for more total bit rate.

*Useable channel bit rate still less due to error-protection coding.


Important 2.5G/3G Features• 2.5G & 3G systems automatically vary the

amount of error protection coding used over the air interface, based on recent measured BER– Net data throughput thus varies also– Contrast to 2G, where amount of error

protection coding is fixed at design time

• 2.5G & 3G infrastructure systems are based on IP packet switching– Not on circuit switching as in 2G

BER=Bit Error Rate; IP= Internet Protocol


3G Infra-system StructureModule names are those for GPRS (likely earliest) implementation

MSC

Serves GSMVoice users.

BSC BTS

GGSN-(GatewayGPRS SupportNode) SGSN-

Serving GPRSSupport Node

Real system has multiple SGSNs, multiple BSS:BSC-BTS installations, not shown.

Uuto PSTN

MS/UE

BSS(other BSS installationsnot shown)

to Public

Internet }

This “private IP network” is called the“Backbone” IP network. It carries IP Packets to/from the Public IP net and the SGSN. IP packets, to/from thePublic Internet, “tunnel” inside of “envelope” IP packets.

In small 3G systems, the GGSN and theSGSN are the same switch.


Further 2.5G Details

• We show more details on 2.5 G: GPRS and EDGE than for W-CDMA or cdma2000.– The instructor knows more about 2.5 G;

it is fully documented and working in some prototypes

– Some parts, such as the packet switching infrastructure, are the same as 3G


EDGE-GPRS Protocols• IP packets are routed from Internet to SGSN via a

GGSN Gateway Node, then encapsulated in an IP Packet “Forwarding Envelope” for the trip via the SGSN to the mobile unit.– so mobile station appears to have a fixed Internet address

for duration of session, despite physical mobility and even possible handover from one SGSN to another.

• GPRS-EDGE can segment and re-assemble packets into blocks sized for radio channel transmission

• Net radio block size is dynamically adjusted from moment to moment, based on radio channel quality. Different modulation and error protection coding schemes give different useable net bit rate with same time slot duration.


EDGE Error Protection• EDGE supports 4 different convolutional codes; coding rates 1/2

or 2/3, or 3/4 or 1– Rate 1 (no redundant bit coding) is used only in benign

(interference free, fade free) channel conditions.

• Different combinations of data block size, coding rate and modulation produce 9 distinct Modulation & Coding Scheme (MCS) choices, depending on radio channel noise, interference and fading– Lower coding rate and GMSK give less throughput, but works

better (less bad packet retransmissions) for bad radio conditions

• Data Block size is always the same for each MCS choice• Data Blocks are used according to a TDMA channel assignment

schedule• Radio interface data block may be a small piece of an IP packet.


Example: GPRS-EDGE MCS-63 bits 34 bits 612 bits

USF RLC/MACHdr. HCS FBI Data: 74 octets=592 bits BCS TB

1392 bits total will be distributed over 4 bursts (GPRS Block) via interleaving (requires 8PSK modulation: EDGE)

36 bits 102 bits 1836 bits

SB=4 36 bits 102 bits 1250 bits

Puncturing usedto reduce databit field length,not USF or RLCbit fields.

Rate 1/3 convolutional code

USF=Uplink State FlagRLC/MAC Hdr.= Radio Link Control/Media Access Control HeaderHCS=Header Check SumFBI=Final Block IndicatorBCS=Block Check SumTB=Tail BitsSB=Which MCS indicator

USF is blockencoded rate 1/12 to ensure accuracy.USFvalue in downlinkcontrolswhich of 7mobilescan trans-mit on uplink on thattime slot.


GSM Frame and Slot

• Base Tx frame start is advanced 3 slots from logically corresponding Base Rx frame start– Mobile set using a designated slot first receives, then waits 2

slots, then transmits, then waits for 4 “idle” time slots, then repeats

– Mobile can do other things in 6 idle slots (see MAHO explanation)

– Mobile set does not transmit and receive simultaneously using only one channel (no radio duplexer needed).

– Mobile can make small Tx timing adjustments, in response to base commands, to adjust for 3.3 µs/km one-way (6.6 µs/km 2-way) radio signal delay

Tx=transmitter; Rx=receiver; Base Tx=Uplink; Base Rx= Downlink

5 6 7 0 1 2 3 4

0 1 2 3 4 5 6 7BaseTx

BaseRx

frame 4.615 ms

corresponding frame


Frame Sequence Channel Schedules• Since only one time slot is used for various scheduled channels, the following diagrams omit 7 of the 8 slots in each frame• You may visualize them as showing only the “front” time-slot in this “folded” TDMA frame sketch

frame 0 1 2 3 etc.• Other sources show a helical ribbon of time slots, rather than a folded sketch, to convey the same idea• The “front” slot represents the described traffic channel slot• Some connections use more than one slot (of the 8) per frame. This is not shown in these examples.• Numbering and counting of frames, etc. often begins with 0 (rather than 1) in GSM or GPRS documentation.• GSM TDMA frames are organized into 26 frame multiframes for user traffic or 51 frame multiframes for GSM control messages.


26-frame 2G GSM “Voice” Service Schedule• Any slot may use this schedule except those devoted to setup-related stuff• Downlink (full rate)

• Alternate (two half rate conversations)

• Uplink (full rate) (half rate not shown)

0123 …

0123 … 23 24 25

0123 … 23 24 25

23 24 25

Key to ColorsRed:Conversation 1 on full rate TCH(22.8 kb/s gross), or Conversation 1 on 1/2 rate TCH (11.4 kb/s).

Blue: Conversation 2 on half rate TCH

Yellow: SACCH for Conversation 1, Idle for 2

White: Idle time slot

Green: SACCH for Conversation 2

In half-rate situations, the SACCH for one is the Idle slot for the other.

120 ms

TCH=Traffic Channel; SACCH=Slow Associated Channel (call processing control signals)


GPRS Packet Schedules• May be used on any slot not assigned to 51-frame multiframes. GPRS and voice may use different slots on the same frequency.• Downlink

• Uplink

Block 0 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11

Key to ColorsSeveral colors represent packets from different subscribers. Note that the order and appearance is dynamically controlled by the amount of traffic, not by pre-assignment.

Corresponding up/down blocks not used by same subscriber.

Gold: PTCCH

White: Idle time slot

Block 0 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11

240 ms


Traffic Schedule Notes• One (or more) time slot per frame is used for

communication service.• “Idle” time slots (including some of the other 7

slots not used for service) are used by frequency-agile mobile Rx to measure signals from nearby cell base station Tx units, to gather data for Mobile Assisted HandOver (MAHO)

• Commands (downlink) and measurement reports for MAHO, etc.(uplink) transmitted via SACCH (for voice), or via PTCCH (for packets)


Voice and Data Handset Types• Various types of 2.5G or 3G handsets are

planned:– Voice and Data simultaneously using all-packet radio

transmission.(2.5G or 3G)– GSM circuit-switched voice and packet data

“simultaneously” using different time slots on the same carrier frequency (2.5 G, not 3G)

– Data-only, typically in the form of a PCMCIA card to use with a laptop computer or built into a palm-top device.(2.5G or 3G)

– Alternate voice or data (not both together) using dual mode 2.5G with IS-136 hardware.


Multiple Packet Access• Uplink and downlink uses of a particular slot by the same MS are

independent– Unlike voice application, where uplink & downlink have same

continuous bit rate.– Does not affect total uplink and downlink data rates of all users, but

facilitates unequal up/downlink rate by individual.

• When MS has packet data to send uplink, it acquires another slot via the slotted ALOHA procedure– Examine USF downlink transmission to find a slot/block which is

currently idle (code 0 indicates no traffic)– Attempt a transmission

• If distance to BS unknown, use shortened burst• Short burst prevents time overlap of adjacent time slot signals at base Rx

– Wait for acknowledgement and access grant, then continue to use that slot/block

• Downlink messages will contain time advance and power control signals


IP Packet Tunneling• To receive packets from public Internet, a Gateway “home” node

provides a stationary IP address for a wireless mobile, despite actual mobility– Meanwhile, wireless mobile uses a “foreign agent” in the visited IP

node to obtain a temporary IP address– Temporary internal IP address changes as mobile user moves from

one visited IP node to another– Mobile user appears to be fixed at GGSN as seen by the public IP

network

• Packets from GPRS gateway IP node to foreign agent are encapsulated in IP “envelope” addressed to foreign agent’s IP address– Unfortunately increases number of “hops” and packet transit delay

when mobile physical location is far from “home”

• Outgoing packets from wireless mobile node go directly to desired correspondent/destination, not necessarily using “home” node.


Prosaic Alternative• Why all the complication with tunneling, etc.?• Visited GPRS system provides Internet service access and

gateway.• Mobile user obtains dynamically allocated IP address from visited

GPRS Internet Service Provider– Like log-in to visited wired system in a visited city– Uses this to access smtp,pop servers of home ISP for e-mail, for

example– Uses a browser to surf the net

• Above are not the same capabilities as being a mobile IP node! • Using temporary IP address rather than “home” IP address

– This temporary address will change if mobile user moves from one serving GPRS IP node to another

– Mobile user will then need to again log-in at the new node.– Repeated log-ins not needed with 2.5G/3G Tunneling.


Abandoned EDGE “Compact” Plan• EDGE capability can be installed in existing GSM base stations

– Only some modulation circuit boards are needed in the base transceiver to support 8PSK in addition to GMSK, in some equipment no hardware changes

– The rest is digital software upgrades

• New base transceivers can be installed in existing IS-136 installations, supporting GPRS/EDGE on the nationally allocated radio bands– Connects to the MSC/Base Station Controller at one end, and the

antenna couplers at the other– Carrier frequencies are restricted to those licensed in North America– Some previous IS-136 carrier frequencies then unusable for IS-136 in

these cells (taken over by GSM/GPRS base stations)– Of course, this implies GSM voice support as well

• As of 2002, most plans for GPRS/EDGE no longer include the complicated “EDGE compact” frequency/time assignment plan, and instead use straightforward EDGE “classic” plan.


North American GPRS/EDGE• EDGE installations just described can support GSM-

only handsets where there is overlapping 850 MHz band coverage (simplest way).

• Multi-band GSM sets can be specifically made to utilize all world-wide GSM/EDGE frequencies (but more complex).– 850, 900, 1800, 1900, 1950 MHz bands

• IS-136 and GSM handsets with multiple digital modes/bands can work everywhere (but still more complex)

• GPRS data support is technologically uniform in digital format world-wide


Personal Conclusions• GPRS/EDGE is an achievable world-wide wireless packet data

network standard with strong North American presence already in place– Gives 56-384 kbit/s packet service to wireless mobile users– Main shortcomings are complexity (for GPRS designers!…

end users don’t see complexity) and possibly different frequency bands in different national areas

• But no other alternative is better in this regard• All proposals have complicated system designs, and no

common radio bands world-wide!

– Infrastructure upgrade costs for GSM base system are minimal, for IS-136 costs are higher but still very tolerable compared to 3G upgrade

• GPRS/EDGE, a 2+1/2 G technology, may yet have the best price/performance ratio of all the general purpose wireless data proposals


Further Personal Conclusions• 3G technology requires greater cost to upgrade base radio

equipment from 2G• Large amount of radio spectrum is used by 3G. Spectrum

must be purchased via auction at high prices.(Almost unbelievably high prices in e.g., UK)

• Consequently large cost to install and use• 3G provides higher bit rate than most customers need or want

– Frequently cited example: How many people need or want to view an entertianment motion picture on a portable handset?

• 3G may ultimately be installed and used only in selected locations– Example: near the stock exchange in downtown areas of a

few cities.

© 2004, r.c. levine page 1

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