Author: Martin

20 Years Ago The GSM MoU Was Signed

Both a long time and a short time, but 20 years ago on the 7th of September 1987 the GSM Memorandum of Understanding (MoU) was signed by 12 European nations on September the 7th. It took a bit after that but three and a half years later the first GSM network, today known as Elisa in Finland, opened its doors. Since then the mobile networks are constantly evolving and activities are still accelerating rather than slowing down.

For a long time, mobile networks were considered voice only networks and even SMS was only added to networks at the end of the 1990’s. Since then mobile data services have evolved in only a few years from speeds of a couple of bits per second to multi megabit 3.5G highspeed. Nevertheless, everything is still based on the GSM standards from back then, or 3GPP standards as they have been renamed to in the meantime.

The telecoms industry had a tremendous roller coaster ride during that time. For many people working in the industry the ride did not always go to the better side. After the .com burst at the beginning of this decade, tens of thousands of people in the telecom industry have lost their jobs. Today, unlike the IT industry which seems to have picked up steam again, the telecoms industry with some exceptions is still struggling and the turbulences are far from over. A challenging time for everyone in the industry which takes true determinism.

Looking at the user side it’s incredible to see the changes mobile networks have brought to the life of people in both rich and poor countries. Voice was the first revolution and today few people can still imagine a life without cell phones. Generation-C can probably not imagine it at all. After a number of iterations I can see mobile Internet access now also picking up with people around me and even some skeptics of yesterday are now using a Blackberry or HSDPA mobile data card in their notebook. Generation-C is next, pricing levels are close to become affordable for them!

Good sides, bad sides, but no matter happy birthday GSM!

Femtocell Thoughts – Part 1

There is currently a lot of hype around Femtocells, tiny user installable 3G cells for homes and offices. Surely an interesting technical concept but still with many question marks attached such as why would users want a 3G cell at home or at the office and what the benefits are for the operator. Here’s what I think:

Operator Benefits

3G networks are operated on the 2100 MHz frequency band in Europe and Asia and in the 1900 MHz band in the U.S. which is far from ideal for in-house coverage. Even in cities it can be observed that dual mode 2G/3G mobiles frequently attach to the 2G network because many GSM operators use the 900 MHz band in Europe which is much better suited for in-house coverage as lower frequencies penetrate walls much better. Some proponents of Femtocells claim that in-house coverage for voice calls are greatly improved by Femtocells. In cities however, this benefit is rather small since GSM in-house coverage is usually not an issue. The user on the other hand does not really care if his voice call is handled by the 2G or 3G network.

An improvement could be seen however in cases where the mobile can’t decide to stick with either the 2G or the 3G network due to changing 3G signal levels. This creates small availability outages while the mobile selects the other network type. During these times, incoming voice calls are either rejected or forwarded to voicemail.

Also, it can often be observed in practice that a mobile device with weak 3G in-house coverage changes to the 2G network once a connection to the Internet is established (e.g. to retrieve eMails or to browse the web on the mobile phone) and sometimes changes back to the 3G network during the connection. The reason for these ping pong network selections are the changing reception levels due to the mobility of the user and changing environmental conditions. Such network changes result in outage times which the user notices since an eMail takes longer to be delivered or because it takes a long time for a web page to be loaded.

Another solution to the issues described above is the use of the 900 MHz frequency band for 3G in Europe and Asia and the 850 MHz band in the U.S. It is likely that this will happen over the next few years since regulators more and more tend to open the 900 MHz band for 3G networks in Europe. It will take a number of years however before network operators will have deployed their 3G networks in those lower frequency ranges and until devices for these bands are available. It’s also likely that 900 MHz cells would first be used to cover rural areas instead of enhancing coverage in areas already covered by 3G in the 2100 MHz band. In the meantime, Femtocells definitely have the advantage.

As the above weaknesses of 3G in higher frequency bands show, femtocells can increase customer satisfaction. Putting a femtocell in the user’s home would have the additional advantage for network operators of reducing churn, i.e. customers changing contracts and changing the network operator in the process. Customer retention is all the more reinforced if the Femto comes in a bundle with DSL access as further described below since changing wireless contracts also has consequences for the fixed line Internet access at home.

Another advantage of femtocells is to reduce the gradual load increase on the 3G macro network as more people start using 3G terminals for voice and data connections. This could result in a cost benefit since if the right balance of macro and femtocells are reached, fewer expensive macro cells would be necessary to handle overall network traffic.

The question is how much these advantages are worth to a network operator since Femtocells do not come for free!?

More to come

So much for now. Part 2 to come soon deals with why users would put a 3G femotcells into their home and part 3 will look at the technical background and hurdles for femtocells.

As always, comments are welcome!

More On User Installable Certificates for Wifi (And WiMAX)

Screenshot0011
Some days ago I have speculated how a WiMAX world might look like without SIM cards. It’s likely that certificates are going to be used to authenticate terminals and users to the network (and vice versa). The big open question is whether these certificates are pre-installed and can not be changed, i.e. the device is locked to the network, or if they can be installed by the user. This would have the advantage that a device can be used in any network that allows user installable certificates.

Wireless LAN already uses user installable certificates in WPA-enterprise mode and larger organizations are already making use of this. Here’s an example from a university that uses PEAP. To my surprise, my Nokia N93 phone already seems to support a large number of different EAP authentication methods today, see the picture on the left.

What’s worrying me a bit is the many different types of EAP methods. That’s going to create the heck of an issue for non-tech end users. Also, how do you keep a (software) certificate save? With a SIM card that’s much easier since it is a piece of hardware.

A Yoigo Weekend

I’ve been in Spain for a couple of days and I am glad that a number of people have told me about Yoigo, a new mobile operator in Spain,  which offers Internet access via Prepaid SIMs. So here’s a report from my weekend with Yoigo and Madrid:

How To Get The SIM Card

Once in Madrid the first mission was to get a SIM card. According to the Wiki, Yoigo SIM cards are sold in "The Phonehouse" shops. So before my departure I checked the web page and located the shop closest to my hotel. Buying the SIM card only took a couple of minutes. The SIM cost €20 euros and included €20  worth in phone calls and Internet connectivity. The price per day for Internet connectivity is €1.20 (€1.36 with taxes) for 2G and 3G access. It doesn’t sound like a lot but if used every day it amounts to about €40 a month which is on par with what other operators offer as well. However, charing daily certainly offers the door to new market segments.

Some people reported that Yoigo only covers a few cities with 3G so far, but Madrid was fortunately part of the list.

Mobile Phone And Notebook Use

Screenshot0013
The first megabyte is billed by the kilobyte and all data traffic afterwards during that day is free. After each data session or phone call a USSD message reports usage and remaining credit. The picture on the left shows the message which reports a cost of €0.00 of a data session established after the first megabyte has been used. Previous reports were not quite clear if Yoigo requires the use of a proxy and otherwise blocks all other ports or if the connection is open and other services like eMail, etc. can be used as well. I can definitely confirm that the connection is open and I used the SIM for both web browsing, eMail and Shozu with the mobile phone and with the notebok to browse the web, check my eMails, Yahoo messenger, for IPSec VPN connections, etc.

Network Performance

Yoigo_throughput
Yoigo does not seem to have 3.5G HSDPA yet, as my phone just established plain UMTS 3G connections. While 3G comes nowhere near 3.5G HSDPA speeds, it’s nevertheless fast enough for most activities (384 kbit/s which equals about 45 kBytes/s). Unfortunately, Yoigo seems to have similar problems like Wind in Italy since I had a lot of IP layer retransmissions due to ‘duplicate acknowledgments" which indicate packet loss. I tried during different times during daytime and also nighttime but the problem persisted. Thus, it’s not a busy hour problem. I can also rule out terminal incompatibility as I saw the same behavior with a Motorola V3xx and a Nokia N93 in combination with a notebook. As a consequence web pages take somewhat longer load and file download performance is around 12 kByte/s instead of 45 kByte/s. Throughput peaks were at around 45 kBytes/s which indicates I got a 384 kbit/s bearer, while the low overall throughput is caused by the frequent retransmissions. The second picture on the left shows a pretty disastrous throughput graph of a file download.

I can’t say if this is a temporary problem or not since I’ve only been in Spain for a couple of days. If I lived in Spain, however, and the problem persisted it would definitely make me go to another operator. So I hope Yoigo takes a closer look and fixes the issue. If you like Wireshark traces of the issue, let me know 🙂

Summary

Despite the less than optimal performance I was quite happy with my Yoigo weekend experience in Madrid. In total I transferred about 60 MB during 4 days which is not much, but I have been on vacation after all 🙂 There are still about €13 of the initial €20 left on the SIM card which will probably be eaten up by the €6 per month minimum usage fees over the coming months. However, I hope that the SIM stays active till next February when I will probably come back to Spain for the Mobile World Congress (formerly the 3GSMWorldCongress) in Barcelona. Until then I hope Yogio will also have upgraded to HSDPA.

Nokia’s Ovi: Let’s See How The Key Looks Like

This weeks announcement by Nokia of the launch Ovi (Finish for ‘door’) to bundle and extend their mobile services has certainly caught the attention of a couple of people in the industry, even though many saw it coming for a while. For now the announcement did not go into the details of the who, when, where and what but that’s certainly going to change in the next months. I think it is a smart move that comes at the right time. Networks, devices, price plans and applications are close to having reached a level of maturity in many countries to offer an easy, affordable and fun way for people to extend their reach and to discover the world in new ways with the mobile Internet.

Nokia’s Advantages Over Network Operators

Nokia has a number of advantages when it comes to mobile services over network operators. First, they develop the devices that the services run on themselves. Thus they don’t suffer from the fracture network operators have to deal with since their strategy is that the services they offer must run on devices of different manufacturers. Take a look at Vodafone Live! for example and the constant struggle to make the menu structure, icons, etc. of phones of different manufacturers look alike. On top, Nokia has a great mobile platform with S60 that is far more capable than the common minimum denomiator of a multi vendor terminal approach. This makes it a lot easier to develop compelling and easy to use services such as NokiaMaps, which will become a part of Ovi.

Beyond the software platform, Nokia has launched great mobile phone hardware in the past 18 months with their N-series and E-series product categories. Most of these devices now include Wifi which I think is key to delivering services independently of a mobile operator. With Wifi the mobile phone, or rather the ‘multimedia computer’ to use a Nokia term, easily integrates into the users home and office environment.  The devices now also include a SIP client which (I think) can become active automatically when the correct WLAN is detected which again circumvents the mobile network operators monopoly and gives users more choice. Nokia didn’t say if VoIP will be part of Ovi but I think it would be a smart move for reasons discussed further below. Integrating into the users home and office environment with Wifi will go far beyond VoIP in future though, as first attempts with UPnP and the Nokia Mobile Web Server already demonstrate today. With the Wifi integration the device can interact with other local devices such as the TV, the computer, the hifi stereo, the network storage, etc. which is not possible at all in a 3G only strategy. Nokia’s cooperation with Microsoft might help further develop this opportunity in the future, too, if Microsoft is willing to cooperate despite their own ambitions with their Microsoft Mobile (Pocket PC) OS.

Economy of scale is also very important. Again, this is an advantage for Nokia since their platform embraces the Internet and can thus be available anytime anywhere. This is also a great tie-in with Wifi integration as some services might only be usable in some countries with still high 3G prices via Wifi at home due to the large amount of data that needs to be transferred (e.g. song downloads). Also, for travelers like me who are used to changing SIM cards several times a month, access to services is independent of operator and place. Mobile network operators on the other hand continue to struggle to recover their investments into their service portals and device adaptations since their user base is considerably smaller. Also, I think their innovation cycles must be a lot longer than what Nokia can do with Ovi since Nokia has to do operator specific adaptations of their software for a myriad of different operators which surely has a huge impact on number of people required and turnaround times.

I think Ovi comes a the right time because 2007 is definitely the year in which the mobile Internet has finally become affordable in many countries. On top, mobile Internet access is now also possible via pre-paid SIM cards in many countries. Early adopters like young people, students, hi-tech travelers etc. have waited for this for years. For Nokia this means that they no longer have to necessarily partner with mobile operators to deliver their services. Users can use pre-paid SIMs or Wifi at home. Excellent and almost unheard of only one and a half years ago! If I were Nokia, I would consider selling such prepaid offers in Nokia stores and on the web as a further kick-starter for devices and services.

The Bitpipe Clash

Inevitably this course brings Nokia even more head on with mobile network operators than before as many still have an interest on doing services themselves. Some might see the change coming and start partnering with Nokia. ‘3’ has made a similar move a couple of months ago and their X-series brand in essence is a co-operation with Internet service providers such as Skype and Co. With Nokia’s Ovi that could be much simpler in the future since an operator would only have to talk to Nokia and not to a myriad of different companies.

How Does The Key Have to Look Like?

I am looking forward very much at how Ovi is going to look like. Having good services is one thing, easy inital configuration is another. Ideally services are both usable from the mobile devices as well as from a PC. It should be possible to configure everything on the PC which is then pushed to the mobile device. Certainly not easy to do in a very very very user friendly way but not impossible. One could for example use SMS push,  Wifi or a plug-in for the Nokia PC suite. Personally I’d prefer web based services with SMS push configuration. No software installation would be required and it would be platform independent (think Windows, Mac OS, Linux,…)

And finally, I hope Nokia will stay committed to the S60 motto of ‘open to new features’ and follow a dual strategy of Ovi as a platform for their services alongside an open mobile OS platform for third party applications that can do the same and other things without the neat integration into one package.

Continuous Packet Connectivity (CPC) Is Not Sexy – Part 3

In a previous post I’ve given a broad overview of a 3GPP release 7 work item called "Continuous Packet Connectivity" (CPC).
This feature or rather this set of features aim to improve user
experience by enhancing battery lifetime, reaction time after idle
times and to increase network bandwidth in situations with many
simultaneous voice over IP and other real time service users. Rather
than introducing a bold new concept, CPC very much works "under the
hood" by improving functions that are already present. Part 2 of this mini series has started to look at a first set of features and this part now finishes by looking at the remaining ones:

Discontinuous reception (DRX) in Downlink at the UE (based on section 4.5 of 3GPP TR 25.903):

While a mobile is in activate high speed (HSDPA) mode it has to monitor one or more high speed shared control channels (HS-SCCH) to see when packets are delivered to it on the high speed shared channels. This monitoring is continuous, i.e. the receiver can never be switched off.

For situations when no data is transmitted or the average data transfer rate is much lower than what could be delivered over the high speed shared channels, the base station can instruct the mobile to only listen to selected slots of the shared control channel. The slots which the mobile does not have to observe are aligned as much as possible with the uplink control channel gating (switch off) times. Thus there will be times when the terminal can power down the transmitter unit to conserve energy.

Once more data arrives from the network than what can be delivered with the selected DRX cycle the DRX mode is switched off again and the network can once again schedule data in the downlink continuously.

HS-SCCH-less operation which includes an HS-SCCH less initial transmission  (based on section 4.7 and 4.8 of TR 25.903):

This feature is not intended to improve battery performance but to increase the number of simultaneous real time VoIP users in the network.

VoIP service e.g. via IMS requires relatively little bandwidth per user and thus the number of simultaneous users can be high. On the radio link, however, each connection has a certain signaling overhead. Thus, more users mean more signaling overhead which decreases overall available bandwidth for user data. In the case of HSDPA, the main signaling resources are the high speed shared control channels (HS-SCCH). The more active users, the more they proportionally require of the available bandwidth.

HS-SCCH-less operation aims at reducing this overhead. For real time users which require only limited bandwidth, the network can schedule data on high speed downlink channels without prior announcements for the terminal on a shared control channel. This is done as follows: The network instructs the mobile not only to listen to the HS-SCCH but in addition to all packets being transmitted on one of the high speed downlink shared channels. The terminal then attempts to blindly decode all packets received on that shared channel. To make blind decoding easier, packets which are not announced on a shared control channel can only have one of four transmission formats (number of data bits) and are always modulated using QPSK. These restrictions are not an issue for performance since HS-SCCH-less operation is only intended for low bandwidth real time services.

The checksum of a packet is additionally used to identify for which terminal the packet is intended for. This is done by using the terminal’s MAC address as an input parameter for the checksum algorithm in addition to the data bits. If the terminal can decode a packet correctly and if it can reconstruct the checksum the packet is intended for the terminal. If the checksum does not match then either the packet is intended for a different terminal or a transmission error has occurred. In both cases the packet is discarded.

In case of a transmission error the packet is automatically retransmitted since the terminal did not send an acknowledgement (HARQ ACK). Retransmissions are announced on the shared control channel which requires additional resources but should not happen frequently as most packets should be delivered properly on the first attempt.

And for more on HSDPA and HSUPA…

I hope that this introduction to Continuous Packet Connectivity (CPC) answers more questions than it raises. In case some fundamental things remain unclear consider taking a look at my book on mobile communication systems which covers HSDPA and HSUPA from the ground up.

External Antenna For 3G Internet Access Saves Me Again

Hsdpa_antenna_small
I like the countryside, much more quiet and relaxing than cities, especially for vacation. Slight disadvantage one has to live with, though, is that 3G signal quality is often not very strong. I am at the Côte d’Azur at the moment in between Monaco and Menton in a house outside the small villages along the coast. As in previous cases, 3G coverage is non existent at desktop level but quite o.k. only a few feet above the desk. I am glad that instead of a PC card I took my Motorola V3xx HSDPA mobile with me which can be nicely attached to the window. The USB cable makes all the difference in reception quality. Data rates are excellent (*) even in such a cell edge case.

For worse conditions I have a 2m (6 feet) USB extension cable 🙂 For PC card owners I strongly recommend buying an external antenna with a similar cable length before going on vacation.

(*) My connection speed is around 400 kbit/s but not because the radio link could not support more but because my connection is throttled by the operator.

Bluetooth Gets A Major Security Overhaul In Version 2.1

The Bluetooth Special Interest group has recently published version 2.1 of their specification. While the press mostly writes about the new easy pairing mechanisms, 2.1 contains a complete security overhaul.

It looks like this has become quite necessary since two attacks are now known that can break the security of the pairing process. While encrypted Bluetooth transmissions are still secure, intercepting the initial pairing sequence allows a passive attacker that just collects packets to compute the PIN used by both devices. A further attack for which specialized equipment is required can force a Bluetooth device into re-pairing so an attacker does not have to be present during the initial pairing. In practice, re-pairing means entering the PIN again. Users thus do get a warning when somebody tries such an attack.

BT 2.1 changes the pairing process fundamentally and now offers the following modes in what the standard referrs to as "Secure Simple Paring":

Numeric Comparison Protocol: The major difference of this pairing scheme compared to what has been done before is that instead of a PIN, a pair of public/private keys is used by each device together with the Elliptic Curve Diffie-Hellman one way cryptography algorithm. Each device sends its public key to the other side which then uses it to encrypt a secret to be returned. Once the encrypted secret is received, it is decrypted by using the private key. The encryption/decryption only works one way so an attacker intercepting the communication cannot decrypt the secret and thus cannot generate the link keys that identify the devices later on. This kind of authentication and ciphering key generation is very similar to what is done today to protect web pages with SSL/TLS and certificates. To protect against a man in the middle attack the pairing procedure then continues and a 6 digit number is then calculated on both sides and shown to the user. The 6 digit number will only be identical on both sides if the conversation has not been tampered and hence ensures that no third device has intercepted and modified the pairing messages. (Note that a man in the middle attack is not possible in SSL/TLS since it uses a certificate authority and trusted certificates in the browser)

Just Works Protocol: The same as the Numeric Comparison Protocol described above but the 6 digit number calculation process is skipped. While offering no protection against an active man in the middle attack some devices do not have a display to show a 6 digit number. Thus, such a pairing should only be performed when the user can be reasonably certain that no attacker can be close by. The encryption whenever a connection is established later on, however, is not impacted by this weakness and thus even this protocol offers enough protection for most applications. It’s also important to point out that the attacker has to be present each time the two devices start to communicate with each other as otherwise the connection establishment will fail.

Passkey Protocol: In this protocol a passkey (PIN) which has been entered by the user in both devices is used just as in the original Bluetooth specification. The way the PIN is used during the pairing process, however, is fundamentally different. Again public/private key pairs and random numbers are used by each device to generate the link keys. The PIN is just used to  prevent a man in the middle attack as follows: For each bit a commitment message is generated by both devices by using a one way algorithm that takes the public keys of both sides, a random number generated individually by each side and the bit of the PIN to be committed as input parameters. Afterwards the commitments are exchanged between the devices. Once this is done, device A then sends the random number it has used to generate the commitment. This allows device B to verify that the commitment message was not tampered with. If the message was correct, device B sends its random number to device A so it can also check that the message was correct. For the following bit the order in which the random numbers are exchanged is reversed, i.e. device B has to send the random number first. A device in the middle can thus not forge commitments since it does not know the PIN and the random numbers are only exchanged after the commitment. Since commitments are given in an alternating fashion a device in the middle can only get one bit of the PIN from each side before it has to start guessing the value of the bit.

Out Of Band Protocol: And finally, Bluetooth 2.1 also allows using out of band exchange of security information such as via NFC (Near Field Communication) during the pairing process. This makes things even easier and more secure by only requiring two devices to come very close during the paging process. Both active and passive NFC is supported. In active mode, the NFC device is connected to the Bluetooth chip and can both transmit and receive authentication information. Some devices such as headsets do not have room for the extra NFC hardware. In such a case a passive NFC tag is used which could be attached to the manual or the box of the device. Pairing is then initiated by a device with an active NFC component which is held close to a passive NFC tag. The passive NFC tag then transmits all information required to start the pairing process.

Here are some links for the technical deep-dive:

Continuous Packet Connectivity (CPC) Is Not Sexy – Part 2

In a previous post I’ve given a broad overview of a 3GPP release 7 work item called "Continuous Packet Connectivity" (CPC). This feature or rather this set of features aim to improve user experience by enhancing battery lifetime, reaction time after idle times and to increase network bandwidth in situations with many simultaneous voice over IP and other real time service users. Rather than introducing a bold new concept, CPC very much works "under the hood" by improving functions that are already present. Part 2 and 3 of "CPC Is not sexy" now take a closer look at the individual features:

A new UL DPCCH slot format configurable by Layer 3 in a semi-static way (Section 4.1 of 3GPP TR 25.903):

In UMTS networks, information is sent in both uplink and downlink in virtual channels. For a connection several channels are used simultaneously since there is not only user data sent over a connection but also control information to keep the link established, to control transmit power, etc. Currently, the radio control channel in uplink (the Uplink Dedicated Control Channel, UL DPCCH) is transmitted continuously even during times of inactivity in order not to loose synchronization. This way, the terminal can resume uplink transmission of user data without delay whenever required.

The channel carries four parameters (for details, see 3GPP 25.211, chapter 5.2.1):

  • Transmit power control (TPC)
  • Pilot (Used for channel estimation of the receiver)
  • TFCI (Transport Format Combination Identifier)
  • FBI (Feedback indicator)

The pilot bits are always the same and allow the receiver to get a channel estimate before decoding user data frames. While no user data frames are received, however, the pilot bits are of little importance. What remains important is the TPC. The idea behind the new slot format is to increase the number of bits to encode the TPC and decrease the number of pilot bits while the uplink channel is idle. This way, additional redundancy is added to the TPC field.

As a consequence the transmission power for the control channel can be lowered without running the risk of corrupting the information contained in the TPC. Once user data transmission resumes, the standard slot format and higher transmission power is used again.

UL HS-DPCCH gating/discontinuous transmission (DTX) in 2 cycles (based on section 4.2 of TR 25.903 ) connected with a F-DPCH gating in DL and an implicit CQI reporting reduction in UL (see section 4.4 of TR 25.903)

CQI reporting reduction: To make the best use of the current signal conditions in downlink, the mobile is required to send information back to the network about how well a transmission was received. The quality of the signal is reported to the network with the Channel Quality Index (CQI) alongside the user data in uplink. The proposed concept has the goal to reduce the transmit power of the terminal while data is transferred in the uplink but not in the downlink by reducing the CQI reporting interval.

UL HS-DPCCH gating (gating=switch off): When no data is transmitted in both uplink and downlink the UL DPCCH for HSDPA is switched off. Periodically it is switched on for a short time to transmit bursts to the network in order to maintain synchronization. This improves battery life for applications such as web browsing. The solution can also improve battery consumption for VoIP and reduces the noise level in the network (i.e. more simultaneous VoIP users)

F-DPCH gating: Terminals in HSDPA active mode always receive a Dedicated Physical Channel in downlink in addition to high speed shared channels which carries power control information and Layer 3 radio resource (RRC) messages, e.g. for handovers, channel modifications etc. The Fractional-DPCH feature puts the RRC messages on the HSDPA shared channels and the mobile thus only has to decode the power control information from the DPCH. At all other times the DPCH is not used by the mobile (thus it’s fractional). During these times, power control information is transmitted for other mobiles using the same spreading code. Consequently, several mobiles use the same spreading code for the dedicated physical channel but listen to it at different times. That means that fewer spreading codes are used by the system for this purpose which in turn leaves more resources for the high speed downlink channels.

Your head is still not spinning? Great, then watch out for part 3 of this mini-series which explains UE DRX and HS-SCCH-less reception!