Payload Header Suppression for WiMAX

Payload Header Suppression (PHS) is an optional feature in 802.16-2004. Since the bandwidth on the air interface is limited and shared by multiple users within a sector, it makes sense to optimize the use of such bandwidth. The idea of PHS is to remove redundant information in packet headers using known rules. These suppression rules help in reconstructing the header correctly at the receiving end. The rules are agreed in advance between SS and BS. In general, these rules are designed in such a way that fields of the header that do not change for the entire duration of the service flow are suppressed. Only changing fields are transmitted. Since the standard allows for multiple rules for every service flow,

The following important points are made about PHS:

1. PHS rules are specific to service flows. Thus, a rule agreed for a service flow cannot be applied to another service flow.
2. Every service flow can have multiple rules with one rule per classifier. Each rule is referenced using an index named PHSI. A classifier is associated with a rule using the PHSI. While the rule for a service flow can be changed, the new rule must be added to the service flow only after deleting the old rule. During this transition when no rule is defined, PHSI=0 will not be used.
3. PHSI will be omitted if PHS is not enabled. “Not enabled” is taken to mean that PHS rule is not defined.
4. The use of PHSI=0 preceding a higher layer PDU implies that suppression has not been applied for this PDU though PHS is enabled for this service flow.
5. Some service flows can have PHS enabled while others may wish to disable it.
6. When PHS is enabled, option exists to enable verification procedures at the transmitting end. Essentially, this CS layer verifies that higher layers have not changed the values of fields that have been identified for suppression.
7. While rules can be defined at either end (BS or SS), only the BS can allocated a PHSI.
8. In general, the responsibility to generate a rule lies with the higher layer (or manually configured using NMS). It is not part of the Convergence Sublayer (CS). This makes good sense for layered design. Nonetheless, intelligence can be built into CS for reasons described later in this document.
9. Rules can be created in a single message flow or in separate message flows. Typically, one could create a rule at provisioning based on certain known header fields. This could be altered later at service activation in order to achieve greater suppression.
10. Rule creation can happen as part of the creation of service flow and its classifier. It can also happen in a separate message flow.
11. Rule creation can use DSA or DSC messages.
12. Rule deletion can use DSC or DSD messages.
13. It is possible to delete a PHS rule, delete all rules, add a rule or set a rule. When deleting all rules, PHSI will be ignored. Add a rule that already exists is an error.
14. PHS rule consists of the following set – {PHSI, PHSF, PHSM, PHSS, PHSV, Vendor-specific PHS parameters}.
15. If there is any error in rule definition as part of DSA or DSC, the receiver should ignore the message. There is no mechanism in the standard to inform sender the source of error.
16. Since PHS happens before encryption, the latter has no effect on PHS. In other words, PHSF is constructed based on unencrypted data.

At a high level, enabling PHS is signaled as part of REG-REQ. This indicates capability of SS. PHS will be possible only if both BS and SS are able to support it and agree to use it after negotiation.

A brief summary of all parameters of a PHS rule follows.

PHSI

This is unique per service flow. This is used to identity the PHS rule. It precedes the higher layer PDU when PHS is enabled. It does not exist when PHS is disabled. If PHS is enabled and suppression is not done, PHSI=0 is used. It has the range 1-255.

PHSF

This is a specified number of bytes containing header information to be suppressed. It is stored and used on both sending and receiving sides. Number of bytes is same as the value of PHSS.

PHSM

This is a mask that determines which parts of the PHSF need to be suppressed. A value of 1 indicates a byte to be suppressed. Otherwise, the byte is included in the transmission. This has a maximum length of 8 bytes to cover the range of PHSS. Bit 0 related to first byte of PHSF. Bit n related to (n+1)th byte of PHSF. If PHSM is omitted, the default is to suppress all bytes of the PHSF.

PHSS

This indicates the size of the PHSF. Since this is just one byte, only a maximum of 255 bytes can be suppressed. During rule negotiation, if this is omitted or is of value zero, PHS is disabled.

PHSV

This controls verification which can be enabled or disabled as part of the rule definition. In general, it is desirable to have this enabled. If omitted, verification is enabled by default.

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MPLS on a WiMAX Base Station

Recently I was asked if WiMAX base stations come with MPLS support. This might have sounded an innocuous question for someone from an IP background. If you are from a wireless background like I am, it might sound a little strange. A WiMAX base station provides wireless connectivity at the physical layer. In particular, WiMAX provides last mile connectivity to the end user. It can also be used to provide point-to-point (PTP) backhaul links.

From this perspective, anything above the physical layer can run transparently on WiMAX. While this is so, WiMAX has defined Convergence Sublayers (CS) at its interface which can then be mapped correctly to 802.16 MAC layer before the packets are sent on the wireless channel. The supported CS Specifications include ATM and Packet (IPv4, IPv6, Ethernet, 802.1Q-VLAN) CS. So where does MPLS fit in, if at all?

MPLS is a technology that sits between Layer 2 and Layer 3. It can be seen to be outside the scope of a WiMAX base station and certainly outside the scope of the WiMAX standards. This is where we, as engineers, have to look at the whole thing from a deployment and operational angle.

Firstly, operators want MPLS because of the many advantages it offers. It leverages on both IP and Ethernet, technologies which are cheap and ubiquitous. It offers QoS. It provides multipoint connectivity but in a simpler way than IP. Its faster to switch at Layer 2 using labels than perform routing decisions at Layer 3. The attractiveness about MPLS in the coming years is that it is set to enable the move towards all-IP transport networks. When IP replaces TDM and ATM architectures, MPLS is set to play a major role.

So operators are interested in MPLS. Before they install new devices into their network, they want to know if it supports MPLS. The problem is that there is a clear distinction between core networks and access networks. MPLS is usually limited to the core. However, there has been significant push towards bringing MPLS to the access networks. This enables end-to-end traffic engineering, right up to the WiMAX base station. 3ROAM offers such a base station with MPLS built-in. Likewise, New Edge Networks is another company that is taking MPLS beyond the core to edge networks.

What if the WiMAX base station did not support MPLS? In this case, an MPLS-enabled network would terminate at an MPLS edge router (ingress or egress). This router would then be co-located and connected to the WiMAX base station. The problem for the operator in this situation would be to have a router for every base station. This is simply not cost-effective.

In general, WiMAX base stations operate in bridge mode (Layer 2) or routing mode (Layer 3). If a base station has to be MPLS-enabled, it has to work in Layer 3 mode. In other words, the base station doubles as an ingress/egress router. It does more than simply provide wireless connectivity.

Sprint has in its long-term roadmap this architecture in mind for backhauling of its WiMAX network. Sprint’s WiMAX base stations would be  MPLS-enabled and the backhaul between such a base station and its ASN Gateway would be IP over MPLS. One of Sprint’s providers for its WiMAX backhaul is Ciena which uses PBB-TE. This may very well carry IP over MPLS right up to the base station. The backhaul itself is wireless with equipment supplied by DragonWave.

To answer the question with which we started, WiMAX base stations may very well support MPLS for cost-effectiveness from operator’s point of view. Practically, it makes a difficult case since the MPLS-enabled network is likely to be from a different provider than the base station itself. Integration becomes an issue. It is well-known that managing an MPLS network is a challenge and requires a steep learning curve. Nonetheless, MPLS may be that small factor to choose one base station over another when an operator is unable to decide otherwise.

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Agilent Wireless Test World 2008, Bangalore, India

Earlier today I attended the Wireless Test World 2008 at the Le Meridien, Bangalore, a day-long seminar presented by Agilent Technologies. My particular interest was in learning about the test solutions they had in place for WiMAX and LTE standards. The former is fairly mature in both its flavours – Fixed and Mobile. The latter is not only new but also incomplete.

Lots of items in LTE standardization are still for further study (FFS). As can be expected, Agilent’s solution for LTE is an initial offering to test early PHY layer implementations. A full-fledged solution that incorporates a validated Conformance Test suite for full stack testing is still sometime away. Core standards are getting ready right now. We can expect the first test specifications later this year. Something concrete can be expected on this front from Agilent at about the same time since they are involved closely in the standardization process. Building upon their existing partnership with Anite for the conformance of GSM/UMTS implementations, LTE conformance solution from Agilent will take a similar route.

The interest on the day was greater for WiMAX, arguably because more companies in India are working on it than on LTE. The immediate future may be more promising for WiMAX, but LTE stands an equal chance from 2011 and beyond.

The seminar was primarily presentations about established and emerging technologies, and the test capabilities Agilent offers for these. There was sufficient time in between to look at Agilent’s test solutions and see live demonstrations of their equipment. The keynote address by Mr Bhaktha Keshavachar was about the challenges faced in integrating wireless connectivity into mobile devices. In particular, the future is about bringing different radio standards into the laptop. WiFi and Bluetooth are already standard on almost all laptops. We expect the same with GPS, 3G, WiMAX and LTE in the coming years. Well, some of these models are already out in the market. We are not talking about external plugin modules but about MiniCards fully integrated into the laptop. Apparently, MiniCard are standardized and they have an extremely small form factor. Key challenges are the co-existence of multiple radio standards on the same device (interference), platform noise, high peak currents, overheating, inter-working, system performance, battery life… let’s be assured that there may challenges that we may not yet be aware of.

When it comes to WiMAX, Agilent has an impressive line-up of equipment to meet all needs of a WiMAX programme – R&D, Design Verification, Pre-Conformance, Conformance, Manufacturing, Network Deployment, Service Assurance. An OFDM signal analyzer was available as early as 2004 and a signal generator in 2005. A one-box solution was available in 2007 which today has Wave 2 and MIMO functionalities.

Agilent WiMAX Test Equipment

There are many hardware models with accompanying software packages which are sold as licensed options. These support standard WiMAX test requirements – signal generation, signal analysis, modulation analysis, constellation diagrams, power measurements, peak-to-average metrics, spectral flatness, ACLR measurement, CCDF measurement, SEM measurement. This includes support for OFDM, OFDMA and MIMO.

Small companies with limited budget would have to make choices. The availability of similar equipment under different model numbers can make it difficult to choose the right one. The best option is to talk to the sales team and get as much information as possible. It’s about knowing if a particular choice meets one’s requirement. It’s also about knowing if we are buying more than what we really need.

Based on my understanding, I have put together a subset of WiMAX test equipment from Agilent. This covers only equipment specific to WiMAX. Of course, there are lots of complementary equipment that can be used for any radio standard – power supplies, logic analyzers, oscilloscopes, battery drain analysis equipment and others.

Model Number	Name	Remarks
N5181A MXG	Vector signal generator	Upto 6GHz.
N5182A MXG	Vector signal generator	Upto 6GHz. Has capability to be used with N7615B.
E4438C ESG	Vector signal generator	Has capability to be used with N7615B.
E8267D PSG	Vector signal generator	Has capability to be used with N7615B.
N7615B	Signal Studio for 802.16 WiMAX	Software that can be used with vector signal generators. Enables application specific signal generation without having to spend time in signal creation.
N9010A EXA	Signal Analyzer	Option 507 enables operation upto 7 GHz. Higher options are available. Better value for money than MXA series. Sophisticated user interface with colour coding of channels. Provides enhanced spectrum analysis. Provides support for measurement applications as optional extra – N9075A is for WiMAX. Generally comes with 89600 series vector signal analysis software. Examples are 89601A and 89601X.
N9020A MXA	Signal Analyzer	Higher specs of N9010A. For example, has WiFi testing capability which its EXA counterpart doesn’t have.
89601M	Vector signal analysis measurement application	Can be used with N9010A EXA.
89601X	VXA signal analyzer measurement application	Can be used with N9010A EXA.
N9075A	WiMAX measurement application	Can be used with signal analyzers N9010A and N9020A. Enables WiMAX specific signal analysis.
N8300A	Wireless Networking Test Set	One box solution with signal generator and analyzer. Only for Mobile WiMAX. Generally preferred over E6651A for manufacturing. Used with N6301A.
N6301A	WiMAX measurement application	Used for WiMAX transmitter testing. Used with N8300A.
E6651A	Mobile WiMAX Test Set	One box solution with signal generator and analyzer. Only for Mobile WiMAX. For R&D, pre-conformance and conformance testing. For conformance testing, used with N6430A. Used for Radiated Performance Test (RPT) by ETS-Lindgren.
N6430A	WiMAX IEEE 802.16-2004/Cor2 D3 PCT	For Protocol Conformance Test (PCT). Based on E6651A. In partnership with Anite supplied software. TTCN-3 runtime environment and interfaces from Testing Technologies.
N6422C	WiMAX Wireless Test Manager	Software with ready-to-use tests. Simplifies automated testing but not as formal as TTCN-3 based testing. Can be used for pre-conformance testing. Can be used with E6651A. Can be used with E4438C ESG or N5182A MXG with N7615A/B license.

Note: It’s easy to find information about WiMAX on Agilent’s website. Go to URL http://www.agilent.com/find/wimax

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A Review of Indian Mobile Market

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

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

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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