Subject: RE: [seamoby] cart-before-seamoby

From: "Gary Kenward" <[email-masked]>
Date: Wed, 3 Jan 2001 19:46:13 -0500

-----Original Message-----
From: Dave Martin [mailto:[email-masked]]
Sent: Wednesday, January 03, 2001 7:37 PM
To: Kulwinder Atwal; 'Michael A. Ramalho'; Kenward, Gary [WDLN2:AN10:EXCH];
seamoby
Subject: Re: [seamoby] cart-before-seamoby

<Technical diatribe on RSSI>
I think we need to be careful when talking about RSSI.  My interpretation of
RSSI is the received RF power as measured by the mobile of one or more of
the base station carriers it is monitoring.  The mobile does not know that
the RF power is generated by a particular base station, only that RF power
exists in a band that those base stations operate in.  In the In some
technologies (e.g., CDMA), the RF power received in a mobile's carrier
frequency does not necessarily correlate to the performance (e.g., bit error
rate).  RSSI is only a good indication of RF in situations where there is
low or no interference.  In high interference situations (dense urban
environments or having several ISM band CDMA products co-located) *raw* RSSI
may not be a good indication of the best serving base station.  A better
metric than RSSI would be some Signal to Noise (actually Signal to
Noise+Interference) metric, however this requires that the signal be
demodulated first and then compared to the ambient noise+interference.  A
fairly costly (from the power perspective) thing to be doing constantly.
</Technical diatribe on RSSI>
 
Now a question ... I am having trouble understanding why layer 3 need to be
invoved in physical layer metrics?  I  
 
Layer 3 is not supposed to be involved with a particular physical layer
metric. That's how
this discussion began. I would prefer us to agree upon a "handoff trigger"
event delivered to layer 3 from layer 2, as an abstraction of the possible
layer 1-2 phenomena that actually might trigger a real handoff.
 
 would not expect a wireline router to route on some bit error rate metric,
but would rather have the layer 2 protocols deal in their own way with bit
errors, resulting in a throughput and latency metric, which I do think is
valid for L3 decisions. For the wireless environment, I am not convinced
that enhancing L3 to overcome shortcomings of poor (from the wirless
mobility perspective) L2 protocols is the right approach.  
Actually, good point. I recall vaguely from a previous life, reading about
failover in routers. Some interfaces keep track of physical layer quality
(sometimes BER, sometimes simply dc signals), and report a link failure
event, but not the quality measurement used to indicate link failure. I
think there is an analogy here.
 
Gary