Subject: RE: [mobile-ip] Handover Smoothing - What to Do?

From: James Kempf <[email-masked]>
Date: Tue, 24 Jul 2001 13:39:17 -0700 (PDT)

Hongyi,

I agree  with your analysis. Basically, what you are saying is that
there can be jitter between the old link and the new that could
result in duplicated packets going to the MN on the new link, correct? 

One way to mitigate this problem is for the FA to get better
handover sequencing information from L2. For example, if nFA
gets an L2 trigger when L2 handover begins, it can start buffering
until the new L2 is fully stabilized. This reduces the window
during which the nFA is buffering. Although it won't synchronize
packet delivery, it will reduce the amount of buffered packets
and therefore the potential for duplication. 

Another way to mitigate it is to make sure the wired network is 
properly provisioned for the expected amount of traffic, to
reduce the possibility of congestion and thus jitter. This will
not, of course, solve the problem of the radio link on the 
oFA slowing down. That can be mitigated with reducing cell
sizes, etc. to avoid overloading, but, in general, if the
radio propagation conditions become poor enough that retransmits
are necessary, as you point out, oFA could lag behind the
bicast stream. 

In general, I don't know any way to synchronize packet delivery
between oFA and nFA. I believe CDMA does this by having a 
very short window (something like 20 ms) during which soft handover
occurs, and using GPS (in cdma2000) to synchronize soft handover
between BTSs. This is not possible in IP, if for no other reason than
the packet sizes can vary, and not all radio protocols have access
to GPS synchronization. There are a variety of heurstics that
could be used, however, and I think it makes sense to write
them down.

		jak

>From: "Hongyi Li" <[email-masked]>
>To: "[email-masked]'" <[email-masked]>
>Subject: RE: [mobile-ip] Handover Smoothing - What to Do?
>Date: Tue, 24 Jul 2001 15:37:27 -0400
>
>See my comments inline
>
>-Hongyi
>
>> -----Original Message-----
>> From: Karim El-Malki (ERA) [mailto:[email-masked]]
>> Sent: Tuesday, July 24, 2001 12:44 PM
>> To: [email-masked]'
>> Subject: RE: [mobile-ip] Handover Smoothing - What to Do?
>> 
>> 
>> Comments inline.
>> 
>> -----Original Message-----
>> From: Hongyi Li [mailto:[email-masked]]
>> Sent: den 24 juli 2001 18:27
>> To: [email-masked]'
>> Subject: RE: [mobile-ip] Handover Smoothing - What to Do?
>> 
>> 
>> Hi Karim, 
>>   I think George was right in this point. 
>>   For a single interface MN, if there is no synchronization 
>> between the 
>> two FAs, the MN will experience either packet loss or packet 
>> duplication 
>> depending on the direction of the handoff. 
>>   Here is an example, at time t, assume FA1 is transmitting 
>> packet with 
>> seq no. x; FA2 is transmitting packet with seq no. x+2.
>> 
>> >> Why is it transmitting x+2? It would be x-2 if FA2 (nFA) 
>> is buffering.
>> >> If so my suggestion was to set the buffer to the L2 
>> handoff duration.
>
>[hyli] this is just to show the different transmission speeds on
>the two bicasting legs. The transmission speed depends on the traffic
>conditions on different legs that include radio links as well. I don't
>think we can over provisioning radio links in near future.
>Buffering at the new FA still doesn't solve the problem. This is
>because the transmission speed at old FA still matters. Let's assume
>for example bicasting started with packet of seq no. x. If the old FA
>is transmitting x-2 at handoff (due to poor radio link), packet x-1
>will be lost; if the old AR is transmitting x+2 at handoff (good radio
>link), the MN has packet duplication. 
>
>
>> 
>>  If the MN handoff 
>> from FA1 to FA2, packet x+1 will be lost. If the MN handoff 
>> from FA2 to FA1 
>> the MN will receive duplicated packet x+1. 
>>   Buffering without synchronization won't help here. 
>> 
>> >> does it take zero time to perform a L2 handoff in this example?
>> >> If it does then I would expect FA2 to transmit x, while 
>> FA1 transmits x.
>> >> Then you don't have packet loss or duplication.
>
>[hyli] Whether handoff takes zero time or dt is not important here. If a
>system wants both FAs transmit x at the same time, good synchronization
>mechanism is needed. 2G CDMA systems implement this synchronization
>in soft handoff. However, this synchronization reduces the overall
>throughput of the CDMA systems. You may find some HDR papers that explain
>this problem.
>  
>> 
>> 
>> > -----Original Message----- 
>> > From: Karim El-Malki (ERA) [mailto:[email-masked]] 
>> > Sent: Tuesday, July 24, 2001 12:03 PM 
>> > To: [email-masked]' 
>> > Subject: RE: [mobile-ip] Handover Smoothing - What to Do? 
>> > 
>> > 
>> > > GT> The nFA has no way of know at which point the 
>> > > transmission was severed 
>> > > at the oldFA...i.e.: if the bicasted packets are buffered 
>> > at the newFA 
>> > > (which I assume they will so this is useful) the TCP 
>> issues remain! 
>> > 
>> > Just a quick answer to this point. You're assuming that you 
>> > will buffer 
>> > and that may not be needed. It's a MAY in low-latency. If you 
>> > do buffer, 
>> > and have a fixed buffer/delay at nFA of the size of the 
>> > duration of the 
>> > L2 handoff (when the MN is disconnected at L2 while moving 
>> between oFA 
>> > and nFA) then you shouldn't get duplicates. That's a 
>> possible solution 
>> > since the L2 handover period is normally known in wireless systems. 
>> > 
>> > Rgds 
>> > /Karim 
>> > 
>>

---

From: Sridhar Gurivireddy <[email-masked]>
Date: Tue, 24 Jul 2001 16:16:33 -0500

James Kempf wrote

> result in duplicated packets going to the MN on the new link, correct

> and therefore the potential for duplication.

If during handoff,  the MN (or) the old FA  get  information(trigger) about
handoff before the actual handoff takes place, passing this information to new
FA through some upper layer protocol  may itself induce some delay. In that
time, actual, L2 handoff may have already have started....Of course, if this
trigger information is passed fast enough, it may help in
Handover-smoothing...

This delay is even more when the Acess router and access point are at
different points in the network..What I feel is, some other Layer-3 protocol
should be written to reduce this delay...instead of relying on Layer-2
triggers...

--
Sridhar Guriviredddy,
Alcatel- Network Strategic Group,
Richardson,
TX-75080

Ph:  972.996.2048

---

From: Arthur Ross <[email-masked]>
Date: Tue, 24 Jul 2001 14:47:54 -0700

>could be used, however, and I think it makes sense to write
James et al:

The issue here is the nature of the diversity that is available in 
any given RAN technology. The simplest is unsynchronized multicasting 
- packets are sent from more than one source, without any attempt at 
synchronization other than that imposed by sequence numbering.

The various flavors of CDMA technology, on the other hand, employ a 
very sophisticated symbol-by-symbol optimal ratio (in the downlink) 
combining. This substantially improves error performance under 
adverse propagation conditions. That combining is, in essence, a 
vector sum of received symbol amplitude estimates from whatever 
sources are available. The signal components add amplitude-wise, 
while the noise components add power-wise (i.e. rss), hence the 
improvement in overall SNR of the resultant. This technique relies on 
the same sequence of symbols be radiated the various BTSs 
participating in this "soft" handoff.

The CDMA return link diversity combining is done in a different way, 
but also makes use of multiple symbol streams from the various BTS 
receivers involved in the handoff.

As you say, this is done with the aid of GPS-derived precise timing. 
Synchronization is not fundamentally necessary, but it is extremely 
convenient in implementing the combining, and helps keep the latency 
low. Other less precisely timed forms are possible in principle, but 
have not, as far as I know, ever been proposed or implemented. To do 
so would require substantially buffering and de-skewing of the symbol 
streams. That buffering and deskewing would contribute objectionable 
latency and variance of latency, both very objectionable in a speech 
service.

The 20 ms of which you speak is the CDMA frame size. That size has 
its origins in the size of the vocoder frames. The FEC is blocked in 
those 20 ms frames - each frame can be decoded and CRC checked 
independently. Again, not fundamental, but because of the dominance 
of the speech service, it is probably here to stay, or at least will 
be around for a long time.

Re "handoff smoothing" - It is, I think, desirable to recognize the 
variety of diversity techniques that may be used in the radio link, 
depending on the particular technology, and to provide the 
tools/mechanisms the can accommodate that variety somehow. This is, 
obviously, intimately intertwined with the QoS issues. What is ok for 
a dumb datagram service may be manifestly awful for a speech service.

Just my 2 cents worth ...

    -- Regards, AHMR

---

From: Sridhar Gurivireddy <[email-masked]>
Date: Tue, 24 Jul 2001 16:16:33 -0500

James Kempf wrote

Ph:  972.996.2048