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Archive for November, 2007

Just two weeks ago Zahid Ghadialy uploaded a post on Carrier Ethernet Transport (CET). It was a basic introduction which had to be supplemented by my own research to understand what CET actually meant. Part of the difficulty is the “collision” of terms. For example, the interview of a Nokia-Siemens manager mentions the term “transport layer”. This can be confusing if it is taken in relation to the OSI model. Transport layer within the context of CET is quite different from the transport layer that is TCP or UDP. We will revisit this later.

So what is CET and how does it fit into the world of mobile and wireless? Meriton Networks defines CET as,

an architectural approach for building scalable transport infrastructure for supporting Ethernet and the evolution to NGNs.

This is a good definition because we can learn much about CET from the definition alone. The following points can be made:

  1. CET is not Ethernet itself but an architecture that enables wide scale deployment of Ethernet from its so far traditional use.
  2. Current transport networks are somehow not scalable and CET with the use of Ethernet can provide that scalability.
  3. NGNs require QoS support, high bandwidth, transport efficiency and scalability. These can be met with CET.

I got interested in CET because of a recent press release from India’s cellular market leader Airtel [Shockingly Fast Broadband]. The article claims that Airtel can now provide 8 Mbps broadband to its customers, the first provider in India to reach such a high bandwidth. Their transport architecture that makes this possible is CET which has been deployed in Bangalore, Chennai and Delhi. In other areas, CET will be introduced slowly. Meanwhile SDH has been upgraded to support 8 Mbps. Airtel claims that its infrastructure is now IPTV ready.

Ethernet is defined in IEEE 802.3 and relates to Data Link layer of the OSI model. The physical media can be anything. Traditionally, coaxial cables were used. Today, most networks use twisted pairs. Those with high bandwidth requirements such as Gigabit Ethernet may use optical fibres. Ethernet has been used in LANs widely but it do not scale to MANs and WANs. For example, spanning trees do not scale to large networks. Thus Ethernet have rarely been considered for carrier transport. Moreover, SONET has consistently offered higher bandwith than Ethernet and sub-50 ms resilience, so that most transport networks today use SDH/SONET architecture. In a wireless environment, data link layers are usually different and do not conform to the IEEE 802.3 Mac data frame format.

The advantage that Ethernet brings is its ease of implementation and low-cost when compared to SONET. Ethernet is essentially a connection-less protocol. It enables multiple access but has no functionality for providing QoS. Yet CET is attempting to get more out of Ethernet by using it with other technologies/layers.

There was a time when end-to-end packet service was essentially connection-less using IP. Routing was performed at IP (Layer 3). With MPLS, label switching was done (Layer 2). MPLS introduced a connection-oriented virtual paths based on labels. This enabled traffic engineering and QoS provisioning. It was widely deployed in transport networks. With the higher bandwith requirements and greater flexibility demanded by NGNs, this task is being pushed from Layer 2 to Layer 1. Ethernet enables a simple and uniform interface while the underlying transport could be any suitable physical layer that has some switching functions as well. CET, for the sake of system performance, blurs some of the traditional boundaries between layers as defined in the OSI model.

Ethernet data frames can be carried across the transport network. Switching is likely to happen at the optical domain. Wavelength switching, sub-wavelength switching or Ethernet tunnel switching are possible. Using just enough Layer 2 functionality at the optical layer, CET enables a clear separation between service layer and transport layer. The latter refers to the mechanism by which data is transported within the network. It does not refer to the transport layer of OSI model. Switching happens within the transport layer. Carriers can concentrate on their services rather than the transport mechanisms because these two are no longer closely coupled. This is represented in Figure 1 (from Meriton Networks).

Figure 1: CET Separation of Service and Transport Layer
CET Service and Transport Layers

Issues of scalability and QoS are addressed using Provider Backbone Bridging with Traffic Engineering (often called Provider Backbone Transport) PBB-TE/PBT. Spanning trees have been replaced with GMPLS to create bridging tables. It will be apparent from these considerations that Carrier Ethernet is a lot more than just Ethernet. Carrier Ethernet is defined in relation to services on provider networks. It is partly about the transport infrastructure but also about the service delivery aspects. Thus, we have two key aspects of Carrier Ethernet – Carrier Ethernet Service and Carrier Ethernet Solution/Transport.

Many organizations are involved in the standardization of Carrier Ethernet (IEEE, IETF, ITU, MEF). Among these Metro Ethernet Forum (MEF) is the primary organization. The standard is by no means complete. Carrier Ethernet is the future and it is still in its infancy.

For further reading:

  1. Tom Nolle’s Blog
  2. Comparing Alternative Approaches for Ethernet-Optical Transport Networks
  3. Ethernet Technologies, Cisco documentation.
  4. Abdul Kasim, Delivering Carrier Ethernet: Extending Ethernet Beyond the LAN, McGrawHill, 2007.
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Yesterday at MoMo Bangalore’s November event, it was yet another session of sensible arguments, discussions and insights which I have come to believe characterize every MoMo event. In this community, ideas are shared openly. Suggestions are welcomed and analyzed. In a world in which knowledge is power, the people of MoMo community realize there is a greater power in sharing knowledge. Yesterday’s session had a high level of interactivity and this in part must be credited to Deepak Srinivasan, the CEO of Mobiance Technologies, who opened the floor for Q&A from the outset of his presentation.

Mobiance is in the business of mobile Location Based Services (LBS). Unlike the usual method of using GPS for delivering LBS, Mobiance has adopted a different approach based on a patent pending technology. The technology involves locating the mobile using cell triangulation. In other words, a mobile’s location can be identified by it’s distance from at least three base stations. This is done using Timing Advance. I believe (the discussions did not get into details of the technology), no extra signalling needs to be generated on the air interface for this purpose because such data is usually readily available with the network. All signalling is at SS7 level between Mobiance system and MSC, BSS and OMC. Data from the signalling network is filtered and processed intelligently to keep Mobiance generated traffic to a minimum.

Coming from an engineering background, I know that none of this sounds difficult. In fact, there is nothing to be solved here. GSM as a standard has evolved over a few decades. It has solved the problems of mobility management to a large extent. The value that Mobiance brings is in applying the fundamentals effectively for services that matter and marketing them at the right level. For example, Mobiance claims that their method of using timing advance brings greater accuracy. In Metros, an accuracy of 150-200 m is possible; 250 m in other cities that include Bangalore, Ahmedabad and Hyderabad; 2.5-3 km in rural areas and highways. The latency of locating a mobile is only 3 seconds. Locations can be updated once a minute or once in fifteen minutes or even once in an hour. This flexibility is in-built into their platform that allows configurability to administer QoS policies. Some solutions may require only a location displayed as a text; others may require the location displayed on a map that delineate taluk boundaries. Such flexibility of solution delivery exists with Mobiance. The technology does not need changes to base stations or mobiles. Even a low-end handset can get services enabled by this technology.

What are the services that can be deployed on this platform? Most of us will be familiar with services for consumers. These are generally services available in the vicinity – ATMs, restaurants, theatres, internet cafes. Then there are navigation services that tell drivers/pedestrians how to get from point A to point B. For enterprises, LBS can be used to deliver fleet management systems or verifying if their distribution network is working as planned.

The company was started in October 2004. In March 2007, they signed a contract with Airtel. In July 2007, their Enterprise LBS solution was launched. The challenge for Mobiance is to bring in more services, use more accurate maps and GIS data, partner with more cellular operators and continue to build on the partnerships that they currently have. As I listened to the presentation with interest, I can attribute their success to these key factors:

  1. Local solutions for local needs: India has many languages. People communicate differently. Directions are given differently – “next to”, “opposite to”. Any LBS enabled service should consider the cultural environment in which it is deployed.
  2. A model of effective partnership: Mobiance has recognized that solutions that work in a Western context may not work in India where partnerships are important. While in the West one may license Navteq maps, in an Indian context what is needed is a strong ecosystem of providers, partners and users in which everyone benefits and progresses in tandem. Thus Mobiance partners with device vendors, cellular operators, GIS (Geographic Information System) and map providers, and the like.
  3. Do what you do best: Mobiance has created the platform for LBS. This platform can be used by partners in developing services. Mobiance is not into creating these services. They concentrate on getting the platform right leaving the creation of services to others. Their platform API is currently based on XML-RPC.
  4. Initial focus: one common reason why startups fail is that they try to do too many things from the start. While the host of services that LBS can enable is many, Mobiance is focusing only on its enterprise solutions (fleet management, verification, awareness). A success in these can lead to more services in various other segments.

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Yesterday I visited a bookshop on Bangalore’s M.G. Road. I came across this classic – John G. Proakis, Digital Communitations, Fourth Edition, McGraw-Hill, 2001. I was overcome with a wave of nostalgia. This is a book I had used in engineering course in college some twelve years ago. I must have used the second edition. As a student, with limited financial resources, I had never bought a single technical book. The library supplied all that I needed. But yesterday I bought my first technical book. At $10.50, it was cheap by international standards and reasonably priced for India.

It has been many years since I read a book like this. The equations looked alien to me. I could not recognize the Fourier Transform. The complementary error function erfc(x) meant something but I had only a vague idea of its definition. The famous theorems of Shannon were somehow familiar but still a distant memory. Nonetheless, it was exciting to get back to the basics of digital communications.

In this post, I shall try to touch upon three things from this book and look at their relevance from the perspective of GSM/UMTS.

Noisy Channel Coding Theorem

This theorem from Shannon is stated as [1]:

There exist channel codes (and decoders) that make it possible to achieve reliable communication, with as small an error probability as desired, if the transmission rate R < C, where C is the channel capacity. If R > C, it is not possible to make the probability of error tend toward zero with any code.

A related equation to this defines the upper bound for the capacity C of a band-limited AWGN channel whose input is band-limited and power-limited [1]:

C = W log2 (1 + Pav/WNo) where

Pav is the average signal power
W is the signal bandwidth
No is the noise power spectral density

So for higher normalized capacity C/W (bits/s/Hz), power has to increase. So if we increase the modulation from QPSK to 16QAM as in HSDPA we get a higher normalized capacity. At the same time, we tighten the constellation if we keep the average signal power the same. A tighter constellation leads to higher BER. To obtain the same BER, we need to increase signal power. So although 16QAM gives us higher bit rate, this must be accompanied by higher power as indicated by the capacity formula. Another way to look at the same thing is to say that by moving to 16QAM we have more bits per symbol. Given that the energy per bit is the same, we require more power. This is understood by the following equation:

Pav/WNo = (Eb/No)(C/W) by substituting EbC for Pav.

On the other hand, for a fixed power, only an increase in bandwidth will give us higher capacity. This is true if we compare GSM against UMTS. The former has a bandwidth of only 200 kHz while the latter has one of 5 MHz.

A comparison of Shannon’s limit against the performance achieved by some standards is given in Figure 1 [2]. We can note that HSDPA is within 3 dB of the limit which is pretty good. We must remember that Shannon’s limit is for an AWGN channel, not for a fading channel (slow or fast). Yet fading characteristics can only make the performance worse, so that Shannon’s limit is still a definite upper bound on capacity. The fact that HSDPA can come within 3 dB of the limit is a tribute to advance receiver techniques. The Rake receiver is a key element in UMTS that makes the best of multipath diversity. Likewise, fast power control mitigates fades and improves BER performance. Without fast power control, the average SNR would be significantly higher to maintain the same BER.

Figure 1: Performance Relative to Shannon’s Limit
Performance against Shannon Limit

In a future post, I will look at MIMO in relation to Shannon’s limit.

Channel Coherence Bandwith

In a multipath channel, delayed versions of the signal arrive at the receiver with different delays, amplitudes and phases. A multipath intensity profile is the delay power spectrum of the channel in which most of the signal power arrives together with low delay and tapers off towards higher delay. The range of values over which this profile is non-zero is called the multipath spread or delay spread (Tm) of the channel. In practice, a certain percentage may be used to define the multipath spread, i.e. 95% of total power is within the multipath spread. In the frequency domain this can be shown to be related to coherence bandwith (Δf)c as

(Δf)c = 1/Tm

What this means is two frequencies separated by the coherence bandwith will fade differently through the channel. If a signal’s transmission bandwith is less than this amount, the channel is said to be frequency-nonselective or frequency flat. Otherwise, it is frequency-selective.

Multipath varies greatly in relation to the terrain. Table 1 prepared by the Institute of Technology Zurich is a nice summary of the range of values that multipath spread can take.

Table 1: Delay Spread for Different Terrains
Delay Spread Values

Apparent from this table are:

  • Urban areas have a smaller delay spread as compared to rural.
  • Indoor environments have small delay spreads.
  • Where there is a significant LOS path, delay spread is less.
  • There is some variability due to carrier frequencies and bandwith. In general, the terrain determines the delay spread.

Let us take 3 µs as the delay spread for an urban area. The coherence bandwidth would be 333 kHz. This is more than 200 kHz bandwith of GSM. Thus, this environment is frequency-flat for GSM. For WCDMA, the coherence bandwith is much smaller than 5 MHz. This is an important reason why WCDMA is called “wideband”. The channel is frequency-selective. Rake receivers are therefore quite important for WCDMA to reconstruct the signal without distortion.

For GSM, the fact that the channel is frequency-flat is used to good advantage. In other words, if the channel fades at one frequency it may not at another that’s at least one coherence bandwidth away. GSM employs frequency hopping which gives an improvement of 2-3 dB. On the other hand, if we consider a rural environment with delay spread of 0.03 µs, the coherence bandwith is 33 MHz which is quite high. GSM does not have frequencies spread over such a wide band. Thus frequency hopping is not likely to improve performance in such an environment, at least on this aspect. But frequency hopping does more than combat slow fading. It also mitigates interference which is why it is still useful when the delay spread is low.

Channel Coherence Time

For this we need to consider the time variations of the channel which is seen as a Doppler spread. Let us say that Sc(λ) is the power spectrum of the signal as a function of the Doppler frequency λ. The range over which this spectrum is non-zero is the Doppler spread Bd. Now we can define the coherence time (Δt)c as

(Δt)c = 1/Bd = co/fv where

co is the speed of light
f is the carrier frequency
v is the velocity of the receiver

A slowly changing channel has a large coherence time and a small Doppler spread. For example, in WCDMA if we consider a carrier frequency of 2 GHz, a vehicular speed of 250 kph, we get a coherence time of about 2.2 ms. This is smaller than UMTS 10 ms frame. Thus channel characteristics do change within a frame. This means that a UMTS frame experiences fast fading. How does the design combat fast fading? It does it simply by dividing the frame into 15 slots. Transmission power can be adjusted from one slot to the next. TPC bits sent in each frame enable this fast power control. Thus fast power control at the rate of 1.5 kHz help in combating fast fading. The net effect of this is that at the receiver, the required dBm is maintained to meet the target BLER set by outer loop power control.

In GSM, the same calculation yields a coherence time of 4.8 ms which is much larger than a GSM slot (0.577 ms). So GSM experiences slow fading. For GPRS on the other hand, multiple slots can be assigned to the MS. A frame is about 4.6 ms. So GPRS is on the border of slow to fast fading. This is tackled by transmitting an RLC/MAC block in four bursts over four frames.

Figure 2 is a representation of the time and frequency selective characteristics of a channel [4].

Figure 2: Time-and-Frequency Selective Channel
Time and Freq Selective Channel

References:

  1. John G. Proakis, Digital Communitations, Fourth Edition, McGraw-Hill, 2001.
  2. Data Capabilities: GPRS to HSDPA and Beyond, Rysavy Research, 3G Americas, Sept 2005.
  3. Prof. Dr. H. Bölcskei, Physical Layer Parameters of Common Wireless Systems, Fundamentals ofWireless Communications, May 3, 2006.
  4. Klaus Witrisal, Mobile Radio Systems – Small-Scale Channel Modeling, Graz University of Technology, Nov 20, 2007.

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Going by the number of mobile phone subscribers, India has become the world’s fastest growing region. Cellular mobile services were introduced in India in August 1995. The initial growth was lacklustre. Subscribers were added at the rate of 0.1 million per month at best for the first 5-6 years. In August 2007, 8.31 million subscribers were added. In May 2006, subscription crossed the 100 million mark. By September 2007, it had doubled. The accelerating pace of this industry in India is clear. This growth has outpaced all predictions. Some predicted 100 million by 2007 and 200 million by 2010. Now the estimate is 500 million by 2010.

Although cellular networks were introduced in India 7 years later than in China, India’s growth has overtaken China’s in the same time frame since inception. A chart (Figure 1) taken from India-cellular.com reflects the market share of the main players as of September 2007. Of the 204 million subscribers (TRAI quotes a figure of 209 million [1]), 75% were in GSM and the rest in CDMA. The four Metros accounted for 19% of the market.

Figure 1
Cellular Market Share Sept07

With an estimated population of 1.136 billion and about 72% living in rural areas, we can estimate that the urban population is 318 million. Given that only 2% of the rural population have access to mobile phones (16 million) [Gartner Research], the next growth segment is in the rural areas. This implies an urban penetration of 92%.

Table 1
Subscribers Sep07

What I found to my great surprise was wireline connections when compared against wireless. Table 1 released by TRAI on 22 Oct 07 tells us the impact of wireless [1] . Wireline compares feebly against it. We can infer many things from this table and associate some reasons for the growth of wireless.

  1. Since wireless has only marginal penetration, we can infer that wireline connections to rural areas are quite low.
  2. Capturing the rural market will never make business sense by laying down copper wires. Going wireless is the way.
  3. A typical urban household has one mobile phone per person (best case) whereas only one landline per household. This accounts for the much higher wireless subscriptions we see.
  4. Given the comparable pricing of wireline and wireless connections, and the added advantage of mobility that wireless brings, people are switching from wireline to wireless.
  5. Wireline has a fixed monthly rental charge. Wireless has this but more importantly the ability to offer prepaid service which works out cheaper for many income groups.

It’s worthwhile to look at some of these issues in some detail.

Urban areas are generally well connected by copper. Many people, especially among the retired and elderly, have been slow to change from their wired connections to wireless. This too is set to change as owning a mobile phone becomes cheaper than a landline. The competition among cellular operators and against landline providers has driven prices down across the industry. Phone calls from the mobile in India are among the lowest in the world. To take the example of ownership plans for Karnataka, I pay a monthly minimum of Rs. 375 for my Vodafone connection. Calls are as cheap as 30 paise per minute. For those who use their phones less often, plans as cheap as Rs. 199 are available from Vodafone. For prepaid connections, a person needs to spend only Rs. 99 to get connected.

While the market in urban areas appears to be saturating, there is yet room for growth. This growth is likely to come in the form of more and better value-added-services (VAS). Back in 2004 people were familiar with only a handful of VAS – roaming and voicemail [2]. For long, the best that users could get were ringtones, wallpapers, themes, icons and caller tunes. Today people are not only aware but starting to use MMS, mobile web browsing, call conferencing, call forwarding, call waiting, m-banking and access other data services. India now ranks as the highest in the world for mobile data services [Mobile Web Requests India]. It is hard to believe this when my own first-hand experience does not verify it. I know people who have high-end phones with GPRS but they haven’t used GPRS yet. On the other hand, I have heard of youths in Bangalore reading blogs while they are commuting (stuck in traffic, waiting for a bus). I have heard of skilled staff (IT, telecoms, …) browsing from their mobiles in their offices where employers restrict access to many sites. If it is happening in Bangalore, it is likely to be the case in the four Metros as well.

In order to provide the higher VAS, consumer needs and behaviour must evolve. There must be a clear demand before operators and content providers can supply. We are already in the midst of this evolution in which Indian mobile culture is changing. Operators are preparing for this. 3G licences are being discussed and we may be seeing the launch of 3G in India sometime next year. It is too early to comment on this. For the moment, operators are focusing on expanding their subscriber base by providing competitive plans with low call rates and monthly rentals. VAS is not yet their priority. Likewise, content providers so far have maintained a low profile in the Indian market mainly because operators take home most of the revenue. This is unlike the market in China. Content providers in India get only 15-25 % whereas their Chinese counterparts get 85% [3].

Despite the high penetration in urban areas, ARPU (Average Revenue Per User) is quite low, one of the lowest in the world [3]. Table 2 is a comparison of EBITDA (Earnings Before Interest, Tax, Depreciation and Amortization) and ARPU [4]. If anything, ARPU is falling. For operators, this is offset by increased subscription. However, profit margins are decreasing and to stay in good shape operators have to leverage on larger economies of scale. One trend in this aspect is the sharing of towers and base station location sites among operators.

Table 2

 

Q2 2007 EBITDA margin

Q2 2006 EBITDA margin

Q2 2007 ARPU (in rupees)

Q2 2006 ARPU (in rupees)

Reliance

42%

37%

375

375

Bharti

41.40%

38.90%

390

441

IDEA Cellular

34.70%

33.70%

320

362

Spice

28%

27%

333

400

Hutchison Essar

33%

433

Source: Company financials
Information is incomplete for Hutchison Essar, which is now part of Vodafone

There are many ways to interpret the low ARPU in India. This may have something to do with the willingness to spend from the Indian consumer. For many in the low income groups having a mobile phone is nothing more than a status symbol. They make calls only infrequently. As such, prepaid subscriptions are much more popular than postpaid. Cost for signing up is less. Incoming calls are free. BSNL has quoted that prepaid calls per day on its network is about 10 while postpaid is about 20 [5]. Data from 2004 shows that prepaid takes up two-thirds of all mobile subscriptions in the Indian market [2]. More recent data from IDC endorses the popularity of prepaid in emerging Asian markets (Bangladesh, Pakistan, Sri Lanka and Vietnam), where 95% of subscriptions are prepaid.

As an example, just two days ago I heard a first-hand account of an auto-rickshaw driver owning a prepaid mobile phone. He received an SMS in English which he didn’t understand. My friend, who was riding in his vehicle, read out the message. He needed to top-up his account. He wasn’t bothered because he could still receive calls. As another example, I had some relatives from the US who had come to Bangalore for a ten-day visit. For the duration of their stay, one of them took a prepaid subscription. Strictly speaking, she is no longer a subscriber now that she is back in the US. Likewise, it may be the case that many such prepaid connections are no longer in active use. It makes me wonder if the statistics that we see are entirely accurate. Do they reflect market penetration? Buying a subscription is one thing. Using it is another. May this be another reason for the falling ARPU?

Looking at growth potential in rural India, there is a clear case. Interest in WiMax is growing. The deployment of WiMax in India does not have a clear path yet. While ISPs are attempting to get into this space as a natural extension of their wireline broadband services, the government has left them out of initial discussions. Only cellular operators have been involved. We will need to wait and see.

While wireless to rural India will bring benefits, the nature of these benefits is not yet clear. The average rural Indian is financially a poor chap. Landowners and small-medium businesses may benefit from wireless connections but what about the landless labourer on the field? Will wireless on its own solve problems of poverty, education, water shortage and racial tensions? It is clear, in my opinion, that if wireless has to benefit rural India, a socialist model will be more appropriate than a capitalist one. A capitalist model will give wireless connectivity but will not help in attracting potential subscribers many of who are below the poverty line. Government must actively use wireless connectivity to provide remotely a host of services that would otherwise be available only in towns and cities – medical services, online education (for adults and children), e-governance (forms, birth/death certificates, marriage certificates, land registration & others), online banking and so on. An effective and viable partnership between the government and the private sector is the key to success.

The future is bright for India. Per capita income last year was about $800. Today it is around $1000. The Finance Minister has said that it will double every nine years. By 2025 it will be $4000 [6]. Urbanization of rural India is happening faster than ever before on the wave of steady economic growth. The Indian mobile market is growing and expanding in a competitive environment. It is now possible for subscribers to switch operators with the same number. The recent introduction of National Do Not Call Registry (NDNC) places consumers before intrusive marketing firms.

For long, India has been a great place for foreign companies to outsource work due to lower cost and skilled workforce. Even today, a lot of 3G and IMS specific work is done in India although their markets are elsewhere. The time has now come for the world to take notice, for the biggest market is India; and it’s growing.

References:

  1. Press Release No.91, TRAI, 2007.
  2. Vipul Chauhan, Mobile Value Chains in India, India Mobile Seminar, 2004.
  3. Mobile growth in India fastest, but realisations lowest, The Financial Express, Nov 21, 2005.
  4. Nicole Willing, Mobile Growth Blooms in India, Light Reading, Aug 03, 2007.
  5. Ravi Sharma, BSNL has an ambitious plan to expand customer base in State, The Hindu, Feb 17, 2007.
  6. India per capita income seen $1000 by ’07/08, Reuters India, Nov 5, 2007.

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Identities of a Dual-Mode Phone

This post is not meant to be comprehensive but I shall attempt to cover the most important identities used to identify the UE/MS. It has been a difficult task the reason being that information is spread across many specifications. If there are errors, do let me know. I also appreciate assistance on making this list complete.

Identities are summarized in Table 1. Shaded cells refer to group identities. Many of these identities can exist at the same time. Some identities are mutually exclusive. For example, TLLI cannot exist at the same time as C-RNTI. The former’s context is with the GPRS CN and the latter’s is within UTRAN.

An RNTI (Radio Network Temporary Identifier) exists only when the UE has an RRC connection. Depending upon the context and allocator, different RNTIs exist as listed in Table 1. RNTIs are deallocated when RRC connection is released.

Table 1: UE/MS Identities

Identity

Description

Length

(bits/

digits)

Context

C-RNTI

Cell Radio Network Temporary Identifier

  • Allocated by the CRNC.
  • Unique within the cell controlled by the CRNC.
  • Can be reallocated when UE changes cell with a Cell Update procedure.
  • Deallocated when RRC connection is released
  • Used by MAC to address a UE-UTRAN connection when DCCH/DTCH are mapped to common transport channels.
  • Can be used by MAC on UL and DL transport channels.

16

UE, UTRAN

DSCH-RNTI

Downlink Shared Channel Radio Network Temporary Identifier

  • Used only for TDD.
  • Used on the SHCCH.

16

UE, UTRAN

E-RNTI

E-DCH Radio Network Temporary Identifier

  • For E-DCH (HSUPA) only.
  • Two variants are possible: primary and secondary: both can be active at the same time.
  • RRC signalling configures the UE to use either primary or secondary.
  • Identity is used by PHY to address the UE on E-AGCH. Identity is used as a mask on the 16-bit CRC.

16

UE, UTRAN

G-RNTI

GERAN Radio Network Temporary Identifier

  • Applicable from Release 5 onwards for GERAN Iu Mode.
  • Allocated by RRC in the Serving BSC and unique within the BSC.
  • Reallocated when the SBSC changes
  • Used at RLC/MAC during contention resolution. If there is no allocation, a random G-RNTI is used; otherwise the allocated identity is used.
  • Deallocated when RRC connection is released.
  • Derived from TLLI codespace. Used on Iu interface and quite similar in purpose to TLLI used on A/Gb.

UE, GERAN, CN

H-RNTI

HS-DSCH Ratio Network Temporary Identifier

  • For HSDPA only.
  • Identity is used by PHY to address the UE on HS-SCCH. Identity is used as a mask on the 16-bit CRC.

16

UE, UTRAN

IMEI

International Mobile Station Equipment Identity

  • Unique number allocated to each Mobile Equipment (ME) in the PLMN.
  • Unconditionally implemented by the MS manufacturer.
  • Generally used to block stolen mobiles. IMEI can be changed but it is illegal in some countries (UK for example).
  • Composed of Type Allocation Code (TAC) + Serial Number (SNR) + spare digit = 8 + 6 + 1= 15 digits.
  • IMEI will be used for emergency call establishment and re-establishment only when SIM/USIM and IMSI are not available.

15

UE, CN

IMEISV

International Mobile Station Equipment Identity with Software Version Number

  • Similar to IMEI; composed of Type Allocation Code (TAC) + Serial Number (SNR) + Software Version Number (SVN) = 8 + 6 + 2= 16 digits.
  • Optionally included by the mobile in Authentication and Ciphering Response.

16

UE, CN

IMGI

International mobile group identity

??

??

??

IMPI

IP Multimedia Private Identity

  • Used in IMS.
  • Could be based on IMSI if not allocated explicitly.
  • Of the form “user@domain”.

Textual

UE, IMS

IMPU

IP Multimedia Public identity

  • Used in IMS.
  • Of the form “sip:user@domain”.

Textual

UE, IMS

IMSI

International Mobile Subscriber Identity

  • Variants exist based on the type of CN: IMSI-DS-41, IMSI-GSM-MAP, IMSI-and-ESN-DS-41. Here we are refer to IMSI-GSM-MAP when we say IMSI.
  • Allocated to each mobile subscriber in the GSM/UMTS system.
  • ITU-T refers to this as International Mobile Station Identity.
  • This is the common identity across both CN domains.
  • RAN uses it for coordination with UE. CN transfers identity to RAN using RANAP. RAN deletes it when RRC connection is released.
  • Consists of MCC (3 digits) + MNC (2 or 3 digits) + MSIN.
  • Used by RRC for CN originated paging.
  • Generally avoided on the air interface in lieu of TMSI or P-TMSI unless the latter is not recognized by the CN or not available at the mobile.
<= 15

UE, CN

IMUN

International Mobile User Number

  • A diallable number allocated to a 3GPP System user.
  • Not sure how this is used.

??

??

IUI

International USIM Identifier

??

??

??

LMSI

Local Mobile Station Identity

  • Allocated by the VLR and mapped to IMSI.
  • Used for communication with HLR although HLR makes no use of it. HLR merely sends it back in its responses.
  • Facilitates easy search of database at the VLR.
  • Advantage is that TMSI could be reallocated while LMSI is persistent.

32

VLR

MSID

Mobile Station Identifier

??

??

??

MSIN

Mobile Station Identification Number

  • Part of IMSI.
  • Identifies the mobile within the PLMN.
  • The leading digits may in some cases used by the PLMN to identify quickly the HLR.
  • If MNC has just 2 digits, this identity has a leading zero.

<= 10

UE, CN

MSISDN

Mobile Station/Subscriber ISDN Number

  • Number by which the mobile can be called.
  • Composed of Country Code (CC) + National Destination Code (NDC) + Subscriber Number (SN).
  • An NDC is required for each PLMN. Some PLMNs may require more than one NDC.
  • Parts of the identity are used by SCCP for routing messages to the HLR.

Variable??

UE, CN, PSTN/ISDN

MSRN

Mobile Station Roaming Number

  • Allocated by the VLR.
  • Used to route calls to the mobile.
  • Passed by the HLR to GMSC for routing purpose.
  • Identity addresses the MSC/VLR where the mobile currently has a context.
  • Has a structure similar to MSISDN.
  • A mobile may have more than one MSRN.

Variable??

UE, CN

MUI

Mobile User Identifier

??

??

??

NMSI

National Mobile Station Identifier

  • Part of IMSI.
  • Composed of MNC + MSIN.
  • Managed within relevant national bodies.

<= 12

UE, CN

NUI

National User / USIM Identifier

Network User Identification

??

??

??

P-TMSI

Packet Temporary Mobile Subscriber Identity

  • Variants exist based on the type of CN: P-TMSI-GSM-MAP, P-TMSI-and-RAI-GSM-MAP. Here we refer to P-TMSI-GSM-MAP when we say P-TMSI.
  • Used instead of IMSI to implement subscriber confidentiality in the PS domain.
  • Allocated by SGSN which maps it to the IMSI.
  • Extensively used by GMM.
  • Generally reallocated when Routing Area changes. Has local context within a RA, outside which it has to be combined with RAI.
  • Used by RRC for CN originated paging. If it exists, preferred over IMSI for packet paging.
  • A P-TMSI signature exists to enable GMM establish the context of a mobile.

32

UE, CN

S-RNTI

SRNC Radio Network Temporary Identifier

  • Allocated by the SRNC and unique within the SRNC.
  • Reallocated during SRNC Relocation.
  • Deallocated when RRC connection is released.
  • A shorted version (10 bits) exists for signalling purpose only during a handover to UTRAN.

20

UE, UTRAN

TLLI

Temporary Logical Link Identity

  • Used on the GPRS air interface (RLC/MAC) to address the mobile.
  • Allocated by the MS based on P-TMSI (local or foreign TLLI) or directly (random TLLI).
  • Auxiliary TLLI is allocated by the SGSN but this is no longer required from R99 onwards.
  • Part of the TLLI codespace is reserved for G-RNTI.
  • Used by LLC in SGSN to uniquely identify the MS.
  • TLLI along with NSAPI is used by SGSN to identify the PDP context to which a packet belongs.

32

UE, GERAN, CN

TMGI

Temporary Mobile Group Identity

  • Used to identify an MBMS Bearer Service.
  • Allocated by BM-SC.
  • Used for either broadcast or multicast service.
  • This is the radio resource equivalent of MBMS Bearer Service Identification consisting of IP multicast address and APN.
  • Contains 24 bits of MBMS Service Id and optionally MCC (12 bits) MNC (12 bits) if the session is from a non-local PLMN.
  • Identity is used by RR/GRR for packet paging procedures for MBMS (pre-)notification. An optional MBMS Session Identity is used along with TMGI if notification is done on A/Gb mode.
  • Identity used by Layer 3.

24, 48

UE, CN

TMSI

Temporary Mobile Subscriber Identity

  • Variants exist based on the type of CN: TMSI-DS-41, TMSI-GSM-MAP, TMSI-and-LAI-GSM-MAP. Here we are refer to TMSI-GSM-MAP when we say TMSI.
  • Used instead of IMSI to implement subscriber confidentiality in the CS domain.
  • Allocated by VLR which maps it to the IMSI.
  • Extensively used by MM.
  • Generally reallocated when Location Area changes. Has local context within an LA, outside which it has to be combined with LAI.
  • Used by RRC for CN originated paging.
  • Format of TMSI is not standardized.

32

UE, CN

U-RNTI

UTRAN Radio Network Temporary Identifier

  • Identifies the UE uniquely within UTRAN.
  • Made of two parts: SRNC identifier (12 bits) + S-RNTI (20 bits).
  • Used to identify UE in many RRC procedures in CELL_FACH (Cell Update, URA Update, UTRAN Originated Paging).
  • Used by MAC to address a UE-UTRAN connection when DCCH/DTCH are mapped to common transport channels.
  • Not used by MAC in the UL but can be used by RRC. Used by MAC only in the DL when DCCH is mapped to FACH for SRB1 (alternative is the C-RNTI).
  • During a handover to UTRAN, RRC signalling uses U-RNTI-Short in which the S-RNTI component has only 10 bits. The shortened S-RNTI is expanded to 20 bits to (MSBs set to zeros) and the final U-RNTI is generated.
  • Rel5 RRC signalling uses this in a modified form to address a group of UEs for RRC connection release.

32

UE, UTRAN

Some identities need explanation. Since C-RNTI and U-RNTI are usually meant for use in CELL_FACH states, it may be puzzling to note that these can be reallocated in CELL_DCH state. Such a reallocation will generally happen in the case of handovers when the CRNC or SRNC change. The UE will merely store the identities until a later time when they may be required. For example, after the handover the channel conditions may worsen triggering a Cell Update procedure on CCCH. This procedure will then make use of the U-RNTI within RRC Cell Update message. The Cell Update Confirm may be on CCCH or on DCCH. In the former case, the U-RNTI will be returned within the message. In the latter case, U-RNTI is used at MAC. Either way, Cell Update Confirm may in its turn allocate new identities.

Specifications [TS 24.008] define the conditions under which the mobile shall use IMEI, IMSI, TMSI or P-TMSI. Some of these have been mentioned in Table 1. Specification titled “Numbering, addressing and identification” [TS 23.003] is a good source of information. The use of some of these identities by RR/GRR/RRC can be found in relevant specifications [TS 44.018, TS 44.060, TS 44.160, TS 25.331]. The use of E-RNTI and H-RNTI is mentioned in a PHY layer specification [TS 25.212]. Some details on TMGI is in an MBMS specification [TS 23.246].

Other important identities for the mobile are DLCI, XID, NSAPI and TI. Because these pertain more to connections at a specific layer rather than the MS/UE as a whole, they have been left out of this discussion.

 

 

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Those of us initiated into the design and workings of cellular systems will know the truth of the matter. Those of us who have tried browsing on a GPRS mobile will know that we rarely get the promised maximum data rate of 171 kbps. Engineers are not to be blamed for this. They know for sure what’s possible and under what conditions. Misrepresentation, as we may call it, comes from the marketing guys.

These days, with the growing affluence of the Indian middle class and their spending power, I find more and more TV adverts for cars – sleek, stylish and spacious. These cars are fitted with ultra-modern features and latest gadgets. In these adverts, the cars zip across a lush countryside. The ride is smooth and noiseless. The roads are wide, free of traffic and spotlessly clean. Overall, the cars promise a great driving experience. Exactly the same cars on Bangalore’s congested, pot-holed, water-logged and narrow roads struggle to live up to expectations.

Just as these adverts have failed to point out that enjoyment of cars is only as good as the roads, phones are only as good as the network and its capacity. Operators have failed to inform subscribers that there is a great deal of difference between average data rates and peak data rates. Most of the time users will not get the data rates that they have come to expect, be it for GPRS or for HSDPA. Let’s take the example of GPRS.

Cellular systems were first designed for GSM. GSM networks were deployed such that there is enough C/N (Channel to Noise ratio) for the required BER at the cell edge. This meant that users closer to the BTS often had better quality. However, GSM did not utilize this to provide higher capacity, higher data rates and better QoS for subscribers. This didn’t matter much because GSM offered CS voice calls and CS data at only 9.6 kbps. Then GSM 14.4 kbps data service was introduced. The difference with the 9.6 kbps was that this had less error protection by puncturing more bits [TS 45.003]. The end result was that GSM 14.4 kbps did not offer the same cell coverage as 9.6 kbps although it did deliver the promised bit rate because of the circuit switched nature of the connection.

Enter GPRS. Four coding schemes were introduced, the difference being the level of error protection: CS-1, CS-2, CS-3 and CS-4. Cell coverage decreases as we move from CS-1 to CS-4 and the coverage is worse without frequency hopping. So only users close to the base station will be able to use CS-4 at bit rates as high as 171 kbps. Even this is an ideal situation which can happen only if neighbouring cells are lightly loaded (lower interference), current cell has sufficient spare capacity so that all the slots in a frame can be allocated to the user requesting the high data rate.

The situation is similar with HSDPA. When there are multiple users in the same cell, it is unlikely that a single user will be allocated all the 15 codes. Even when there is only one user, he has to be close to the Node-B for ideal channel conditions. At the cell edge, it is has been shown that HSDPA can offer only 250 kbps with 15 codes and HSDPA using 80% power [1, Chapter 11]. In fact, at cell edge DCH/DSCH can achieve 384 kbps mainly because of soft handover gain. Soft handover is not possible with HSDPA.

It is certain that GPRS, E-GPRS, HSDPA and HSUPA all increase network capacity. This improvement is advertised to the user as higher data rates. This is true in terms of average bit rates which are significantly lower than peak rates. Peak data rates are possible only under good channel conditions that enable higher modulation and lower FEC coding. So although spectral efficiency has improved with newer access technologies, the difference between average and peak efficiency has also increased. The exceptions are AMPS and GSM for which cells were planned for what they were meant to deliver. Figure 1 summarizes the efficiency gap [2].

Figure 1: Average vs Peak Spectral Efficiency over Time

Average vs Peak Spectral Efficiency

This may be one way seeing why the cost of mobile/wireless data has always been not as competitive as fixed data. If it were cheap, lots of subscribers would try it for a start. This would increase the load, which would reduce the effective data rate per user. Reduction in data rate per user occurs because available capacity has to be shared and more users means more interference. The user would be disillusioned with the service when he had been promised a much higher rate. Thus, it is only natural that data is for premium users and not for the mass market. If all mobile users were to start using data at the moment and expect speeds possible with their ADSL modems, today’s cellular networks are not ready to deliver. We have the technology but not the delivery mechanism. Operators have to perform cell splitting and provide HSDPA at the level of microcells rather than at the macrocell level. This would result in more handovers. They would have to upgrade their backhauls (Iub, Iur and Iu). Naturally, link budgeting has to be revised along with other factors associated with cell planning.

Will it make business sense to make these upgrades? To answer this we need to look at the predicted growth of data when compared to voice so that a proper cost-benefit analysis can be made. Hopefully, this will be another post. But then, there are also competing technologies such as WiMAX/WiFi (microcell) and femtocells (picocell).

References:

  1. Harri Holma and Antti Toskala, WCDMA for UMTS, Second Edition, John Wiley & Sons, 2002.
  2. “What Next for Mobile Telephony?”, Agilent Measurement Journal, Issue 3, 2007.

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Following the article on GPRS cell reselection and NACC, I thought it was appropriate to mention neighbouring cells that the MS monitors. The list of neighbour cells is formed by a complex process in which information is gathered from different sources. As an easy reference, this post attempts to summarize this process. This post will look at:

  • GSM Neighbour Cell List for GSM
  • GSM Neighbour Cell List for GPRS

The term “GSM Neighbour Cell List” is so named to distinguish it from 3G Neighbour Cell List. It is applicable to GPRS as well. It is simply a list of neighbours that the MS maintains. It makes measurements on these cells and reports them to the network if configured to do so. Cell reselection depends on these measurements.

The final list that the MS maintains is the Neighbour Cell List which is a concatenation of GSM Neighbour Cell List and 3G Neighbour Cell List. For the purpose of reporting measurements, the lists are treated separately.

Every cell maintains a BCCH Allocation List or BA List which is a list of frequencies of neighbouring cells. This information is useful for the MS for performing and reporting measurements, and eventually in cell reselection and handover. The BA List is formed from information in the SI. For GPRS, BA(GPRS) exists which is formed from information in PSI. If the cell does not have a PBCCH, BA(GPRS) equates to BA(list) obtained from SI. The key difference between BA(list) and BA(GPRS) is that the former is only a frequency list and the latter is a list of frequencies paired with Base Station Identification Code (BSIC). BSIC allows for cell changes across cells that are not necessarily neighbours in a geographic sense. So an MS could do a cell reselection across cells with the same frequency but different BSIC. Such a possibility arises if the immediate neighbours are hidden (no line of sight) whereas a far away cell can provide better service due to clear LOS radio path.

Figure 1: GSM NC List (GSM)
GSM NC List - GSM

Now let us look at the formation of GSM Neighbour Cell List for GSM (Figure 1). In idle mode, BA(list) is formed from SI2/2bis/2ter while in dedicated mode SI5/5bis/5ter are used. If the latter set is missing completely in dedicated mode, the former will be used. SI2quater enhances the BA(list) by providing BSICs. The combination of the two makes the GSM NC List. SI2quater was introduced for enhanced measurements, a feature I will describe in a separate post. In dedicated mode, Measurement Information (MI) is used instead of SI2quater but the end result is the same as in idle mode. If SI5 and MI are both not sent to the MS in dedicated mode, the GSM NC List in dedicated mode is same as in idle mode.

Figure 2: GSM NC List (GPRS)
GSM NC List - GPRS

Figure 2 illustrates the formation of GSM NC List for GPRS. In packet idle mode without PBCCH on the cell, the BA(GPRS) is only a frequency list. Combined with BSIC information in SI2quater this becomes the GSM NC List. If PBCCH is present, PSI3/3bis is used to make the BA(GPRS) which contains pairs of frequency and BSIC. This is also the GSM NC List. In packet transfer mode, the GSM NC List formed in packet idle mode is reused. Elements of this list can be deleted or new elements can be added using Packet Measurement Order (PMO) or Packet Cell Change Order (PCCO). Addition happens on the GSM NC List and is based on pairing of frequency and BSIC. Deletion happens on the BA(GPRS) and is based on frequency which implies that all cells with that frequency are removed from the GSM NC List regardless of the BSIC. Such a definition is useful if the BA(GPRS) is only a frequency list as in a cell without PBCCH.

Among the cells in the GSM NC List for GPRS, some may be cells with both Iu and A/Gb modes. There may be cells with only Iu mode. In the final count, the list can have a maximum of 96 cells and 32 unique frequencies.

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