Do we need LTE any sooner?

Ok, I have been thinking real hard on this. When do we exactly need LTE? Next year? Two years from now or three years from now? I am trying to understand various article which try to justify the right time to launch LTE, but I could read none of the articles completely. Either they are too long or they go over my head. So I thought I should do it in a simple way.

Lets break things in to two here. First, what do we baldy need that is lacking now. Second, what do we fancy/love to have.

Note that first need is something that is lacking us from doing things while second one is more like wow to have or something like that.

Before we categorize the needs we need to look at the market segment. That is how many people need that badly and how many are fancying it. The things which few people badly need could be fancy to few.

If we replace "we" with "I" in above statements it would make more sense to various individuals as the next generation is all about personalization.

So if you ask me I dont need LTE. Honestly I dont see its need for me atleast in next 5 years. I work in office where we run pretty good internet connection. I get back home and have a nice wifi to use. I really dont have big fat mobile so I skip my emails if I am traveling and can live with it. I am not a facebook addict, infact I dont know why I am still using it. I prefer to call up people and have conversations rather than facebook messages or what ever. SMS is the best way for me to send a message. In case I need to be hooked on to email all the time I would go get a black berry and use exiting 2G or 3G network. The only thing I fancy is a data card. It would be good to carry a data card hooked on to my laptop so that I can be at a remote place and still be connected. This is the case with me and also with tons of my friends. What good is LTE is going to do to me? In fact the whole corporate breed in India can fall into my category.

I want to hear from you. Do you need LTE? I would really appreciate your comments on this. Cheers!!

P.S: Please dont ask me to go back to stone age. I understand that few people do need mission critical applications, that need high data rates, running on their mobile devices all the time. I am just trying to figure out what is the right time to deliver LTE considering the huge costs involved with it. I would get a heart ache if LTE fails because it was launched too soon or too late.

I will add three more constraints. Available spectrum, back haul and number of devices. How will the time lines look considering these too.

LTE Tidbits

Few more interesting things on LTE.

GTP & PMIPv6

So, looking at the market I came to know that PMIPv6 is the one that will be deployed over S5 interface, no matter wether the existing service provider network is GSM based or CDMA based. Even I have been PMIP supporter for a while as it avoids complexity over S5. Reduces complexity? I was thinking over bearers one day and realized something.

In LTE a UE can have 11 bearers altogether established per APN. Note that EPS bearers are GTP based bearers. Now on S5 interface there is no concept of default bearer or dedicated bearer. That is there is only one bearer (PMIP) per UE per APN. The question is how are multiple bearers on S1_U interface mapped to single bearer on S5 interface. Lets call each bearer as a pipe with different quality of service running different applications. Since each pipe is identified by a TEID we can enforce all the QoS on it over S1_U. But how will the same be mapped over S5 interface? Interesting? Any clues?

Hint: There is an entity called PCRF which is responsible for bearer establishments, QoS enforcements etc. So SGW contacts the PCRF or rather PCRF informs to SGW on how to handle each GTP pipe over S5 interface.

More later.

Handovers & Tracking Area Update

Have you noticed that there are no NAS messages sent for handovers? That means all the handover decisions are taken by eNB based on the power measurements etc and UE is informed to modify its RRC connections.

But if you take a look at 23.401, there is a call flow for Tracking Area Update which gives us an illusion of handover. This is special case I guess so it is separately dealt, not as handover, in the spec. The difference between handover and Tracking Area Update is the later is UE initiated. Each MME(or SGW?) has a list of tracking areas which it tracks. This list is sent to UE during default bearer establishment. If UE detects that it has entered a new tracking area that is not present in the list sent by MME, then it will trigger a Tracking Area Update procedure.

Radio Bearers and EPS Bearers

If you look at the default bearer and dedicated bearer establishment procedure there is an interesting fact hidden with respect to radio and eps bearers. For default bearer, EPS bearer is established first and then the corresponding radio bearer is established. But for dedicated bearer radio bearer is established first and then the EPS bearer. Before I prove it, this fact leads to another two interesting facts. They are MME is responsible for assigning EPS Bearer Identities and we need modify bearer request for default bearer establishment. Let prove all the three.

Looking at default bearer establishment, Create session request is sent from MME to SGW after it receives Initial UE message with PDN connectivity request+Attach request. Create session request includes the EBI and same is informed to SGW. SGW sends the response if the bearer is accepted along with its user plane information. With Create Session Response the EPS bearer is established (EBI is assigned & SGW user plane info is known). Later MME goes ahead with establishment of Radio bearers for the same using the EBI. Once the radio bearer is established (eNB user plane info is known) the same is indicated to SGW in modify bearer request. This proves the third fact.

Next, take a look at dedicated bearer establishment. Dedicated bearer is network initiated. That means Create Bearer request is coming from SGW and it contains LBI, SGW user plane info, TFT etc but no EBI (set to 0?). Once this message is received by MME, MME assigns an EBI and goes ahead with establishment of radio bearer. Once the radio bearer is established (eNB fteid is known), then MME informs the same to SGW in create bearer response along with the EBI. This proves another fact that MME has to assign EBI, but not SGW.

So the above two explanations prove the first fact.

Why EBI is of 4 bits?

EBI is 4 bits because NSAPI is of 4 bits. NSAPI is used to uniquely identify a PDP context in GSM/UMTS networks. To maintain the compatibility EBI is also set to 4 bits. This means there cannot be more than 16 values for EBI. Out of these 16 values 5 are reserved which explains why there cannot be more than 11 bearers. Dont ask me my NSAPI is of 4 bits :-)

Thats it folks, I have run out the ideas. More to follow. As usual any corrections or comments are are greatly welcome.

LTE Backhaul

Let me start by wishing you all a very happy diwali.

Long back I wrote about MPLS being considered as backhaul for LTE. I believe it is still the choice. More random thoughts on the same below.

Many have said that with LTE bandwidths will explode, infact I say even with HSPA the bandwidths will explode. Lets not look at this from GPRS or UMTS or LTE perspective. The simple logic is if there are many users even with GPRS, bandwidth will explode if the link between SGSN and GGSN is thin. What is happening now is there is only one IP link in wireless core network, so there is not much emphasis on it. The case is same with HSPA. With LTE there are several IP links, devices that run on IP have increased and more over relation between these devices is many to many. The wireless core architecture is moving close towards the wired architecture.

Many IP devices bring in a Central management unit along with various OAM tools. So if Verizon wants to deploy LTE, how is it going to choose the EPC vendors. I dont think using MME from from NSN and SGW from ALU will be a good idea. This also means that EPC solution providers should also come up with Central management units for their devices. One place to configure and monitor the devices.

Next challenge is to keep up the service level agreements and end user quality of experience. This is very very challenging. I will touch base on this pretty soon.

Another challenge is the actual backhaul it self. While the fiber will be widely used there are other backhaul techniques like microwave and copper. I am not sure how many microwave links we will see, but yes, there are some solutions based on these links deployed and are running fine.

Next point is infrastructure sharing. Atleast in India I have seen BSNL lending its network to private operators. The same can be done with LTE.

Quite a few things lined up, it would interesting to see how they will shape up.

Cisco enters LTE?

First, I am neither a market researcher nor an analyst. These are the thoughts of a newbie in wireless segment.

Today Cisco (CSCO) announced that it is going to acquire Starent Networks (STAR). Read here.

What does this acquisition mean to Cisco? Cisco is huge company with lot of network equipment. But since the start of LTE, Cisco hardly made any noise. As far as I know Cisco was not big in the wireless segment. I am still not aware how many are using SGSN/GGSN's from Cisco. While its counter parts Alcatel Lucent and NSN were aloud with there wireless solutions.

Starent on the other hand is evolving organization. It is small, but it proved its point in wireless segment. Verzion, Sprint Nextel and Vodafone has announce Satrent as one of their EPC solution provider. Starent net revenue was high compared to last year. So Cisco did what it is best at doing, went ahead and bought Starent.

Now Cisco will have a strong foot hold in wireless division. More over both Cisco and Starent supported the Wimax. I am happy to see this deal happening. Signs of market recovery?

LTE Whitepaper from Wired n Wireless

I have been working on this White paper for quite long time and finally could finish. Here it is, all my work in LTE in single document. The document talks of LTE interfaces, network elements, radio network, user plane and control plane and handover scenarios. I havent gone too deep into technology as things are definitely complicated.

I hope it will be useful. I shall appreciate if you could take a look at it and pass on your comments.

I have uploaded the same in my Google code section here. The document is also available in scribd.

Direct link to the paper here. (Courtesy 3g4g.co.uk)

LTE Whitepaper

LTE End to End Signalling

LTE Initial Attach/Default Bearer Establishment

LTE Dedicated Bearer Establishment

Thanks to Zahid of 3G4G blog for the RRC call flows. (here and here)

LTE: QoS and Bandwidth

I want to touch base on two concepts here. I am quite not clear on things at this moment, may be some one can help!

Concept 1:

LTE QoS: Default bearers, Dedicated bearers, TFT and Bearer QoS (QCI, ARP, MBR, GBR) are the QoS variables in LTE (any more?). I am very much aware how these behave in EPC, but I am unable to map the same with UE. Assume a default bearer has been established. Note that there is no TFT associated with default bearer, but there is a bearer level QoS present in default bearer creation. This QoS might limit the bit rate on the network side. And these QoS values are indicated to the UE through a NAS message (Activate Default Bearer Context Request?). So far so good. Now UE wants to have a dedicated bearer for particular application. UE requests for a dedicated bearer using Bearer Resource Allocation NAS message which contains the TFT, or to say traffic flow aggregates. Once this message reaches EPC, PGW consults PCRF and allocates a QoS value to this particular TFT. The QoS allocated for the TFT is signaled to UE in Activate Dedicated Bearer Context Request NAS message. Now UE has the QoS values. The big confusion is how are these QoS values limiting the data flow from UE?

My theory: Basically there is QoS concept in air interface (what?). All the QoS rules are imposed by eNB, which means the bandwidth is controlled on the eNB side. So even if the UE gets all the spectrum it will not be able to do much as the bandwidth on the network side is limited. I believe the QoS for TFT is indicated to UE so that it can also regulate the usage of spectrum. Right? Totally out of mind?

Concept 2:

Considering the above "my theory" to be correct lets try and understand how service provider can offer services. In India there is no 3G yet (except for BSNL) on mobile phones. How ever there are mobile operators whose networks are 3G ready and have started offering 3G speeds over USB sticks (TATA and Reliance). When I spoke to their customer care executives I was told the USB sticks come with a limited data plan. They also told me that this restriction was imposed by the government. The restriction can be explained using "my theory". If we start offering unlimited data service over USB sticks people might start consuming all the spectrum all the time leaving nothing for others to use. Base stations are always throttling. This can be avoided by placing data usage limit or time limit. Good move.

On the other hand, all the Blackberry's in my office run on EDGE. These handsets are provided with unlimited data usage plans. This means I can connect a Blackberry to laptop and start using it as a modem and get unlimited access to internet. Only problem as of now is these devices are running on EDGE which means low speeds.

The authorizing body is imposing data limit usage on USB sticks while allowing unlimited access on mobile phones. What will happen when a 3G phone arrives? If the network is 7.2 Mbps ready and so is the device then people can get unlimited access using mobile phone at high rates. Its just a matter of USB cable which can give high rates on laptop too. So what will the plans be when 3G arrives here? I ask my boss, "Hey with 3G you will get 3 Mbps speed but you will be able to do only 3 GB per month". This actually surprised him and he said I would rather stay on EDGE and get unlimited access. People using blackberry's dont want to fall into limited usage schemes as its frustrating to keep monitoring how much data they have used all the time. These things seem quite contradictory.

I have no idea how things are going to turn up. Can anybody enlighten me? Thanks!

New revision of 3GPP Specs

Folks, September revision of LTE specs are out. I could download latest revision of 3GPP TS 29.274 - v8.3.0 (GTPv2) spec but looks like rest of the specs are not on 3GPP website yet. I am guessing all the specs should be available in next two days as some final discussions are going on in email threads. The CR list for this spec is huge, will try and see what all has changed. If you are working on LTE then it is the time to start looking at these new specs.

NSN Finlad Folks!

Any one from NSN Finland reading the blog? Would really like to have a word. Thanks. My email ID can be found in About Me section.

LTE Initial Setup

Reference:

3GPP TS 36.213: EUTRAN Physical Layer Procedures

3GPP TS 36.331: EUTRAN RRC Protocol Specification

LTE Cell Search:

When the UE is powered up it needs a network to attach itself. The first towards it is Cell search. Cell Search is a procedure by which a terminal can find a potential cell to attach too.

As a part of cell search procedure the terminal obtains the identity of cell and estimates the frame timing of the identified cell. LTE supports 510 different cell identifiers divided into 170-cell identity group of 3 identities each.

LTE provides two signals in downlink;

-       Primary Synchronization Signal

-       Secondary Synchronization signal.

In first step of cell search, UE uses primary sync signal to find the timing on 5 ms basis. This signal is transmitted twice in each frame(as LTE frame is of 10 ms).

Terminal can use this signal to identify the frame timing with a 5 ms ambiguity. Here terminal locks it local oscillator frequency to the base station carrier frequency. The terminal also finds an identity within the cell. It also obtains partial knowledge about reference signal structure.

In the next step terminal detects the cell identity group and determines the frame timing using secondary synchronization signal.

Random Access Procedure

To transmit data terminal needs a connection setup with the network. So a terminal has to ask for one. Random access procedure is used to establish uplink and unique terminal ID.

-       First step consists of UE transmitting a Random Access Preamble allowing the eNB to estimate the transmission timing of the terminal.

-       In the next step network transmits a Random Access Response. This consists of timing advance command to adjust the terminal transmit timing, based on timing measurement received in the first step. In addition to establish uplink synchronization this step also assigns uplink resources to be used in next steps to the terminal. Temporary identity is also assigned to UE for further communication with the network. This response is sent on PDCCH.

-       Third step consists of transmission of mobile terminal identity to the network using UL-SCH. The exact content of this signal depends on the state so of terminal whether the network previously knows it or not. (RRC_IDLE)

-       4^th step consists of contention resolution message from network to terminal on DL-SCH.

RRC Procedures

There are two RRC states in LTE. RRC_Idle & RRC_Connected.

In RRC_Idle there is no signaling radio bearer established, that is there is no RRC connection.

In RRC_Connected there is a signaling radio bearer established

Signaling Radio Bearers(SRB) are defined as Radio bearers that are used only to transmit RRC and NAS messages. SRB’s are classified into

Signaling Radio Bearer 0: SRB0: RRC message using CCCH logical channel.

Signaling Radio Bearer 1: SRB1: is for transmitting NAS messages over DCCH logical channel.

Signaling Radio Bearer 2: SRB2: is for high priority RRC messages. Transmitted over DCCH logical channel.            

RRC Procedures:

-       Paging

o   To transmit paging info/system info to UE in RRC_IDLE state.

-       RRC Connection Establishment

o   The purpose is establishing SRB1.

o   This procedure is initiated by UE when upper layers requests of a signaling connection when UE is in RRC_IDLE mode.

-       RRC Connection Reconfiguration

o   The purpose is to establish/modify/release radio bearers.

o   Also to perform handovers

o   Network initiated procedure(?)

-       RRC Connection Re-Establishment

o   To re-establish RRC connection which involves SRB1 resumption and reactivation.

-       Initial Security Activation

o   Activate security upon RRC establishment.

o   eNB initiated procedure.

RRC release procedure.