2.5g/3g ID additions (RE: pilc minutes (corrected))

["Farid Khafizov" <[e-mail]>]

Enclosed please find revised version of earlier submitted text
on VJC and Bandwidth Oscillation for 2.5g/3g ID.
Thanks,
--Farid





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1. Disabling Van Jacobson TCP/IP Header Compression

	Van Jacobson TCP/IP header compression (VJC) algorithm [35] is
negotiated between peer PPP layers. In CDMA2000 networks it could be
implemented between the Mobile Terminal Equipment, such as laptop computer,
and the Packet Data Serving Node. The algorithm was designed to increase
application layer throughput by reducing packetization overhead [11]. For
TCP segment size of 1000 Bytes, enabling VJC increases throughput by about
4%, if there is no packet loss. 

	However, experiments have shown that in the presence of wireless
link errors, VJC is not desirable [n4]. If a wireless link error is not
recovered, it will cause TCP segment loss between peer PPP layers, and then
VJC does not allow TCP to take advantage of Fast Retransmit Fast Recovery
mechanism. VJC algorithm transmits not the TCP/IP headers but only the
changes in the headers of consecutive segments. Therefore, loss of a TCP
segment on the VJC link causes the transmitting and receiving TCP sequence
numbers to go out of synch. When a TCP segment is lost, none of the
following segment will be forwarded by the link until RTO expires [11]. 

	It is recommended to disable VJC algorithm unless packet loss
between peer PPP layers is very low. Robust Header Compression [34] was
designed to address deficiencies of VJC. At the time of writing of this
document, IETF was working on a proposal for negotiating Robust Header
Compression over PPP [n5].


2. Bandwidth Oscillation

	Limited RF spectrum along with high data rate requirement for
2.5G/3G wireless systems necessitate dynamic resource sharing among
concurrent data users. Various scheduling mechanisms can be deployed in
order to maximize resource utilization. Some of the limited resources in
CDMA based systems (e.g., UMTS, CDMA2000) are orthogonal codes and RF
transmit power. Shared channels in UMTS [N1] and supplemental channels in
CDMA2000 [N2], designed for high speed traffic, utilize relatively high RF
power and require higher portion of orthogonal code resources. Time division
sharing of these resources may result in TCP throughput degradation. Usually
these resources are allocated on per needed bases (bandwidth on demand) and
released when there is no data to send. There could, however, be situations
when resources are de-allocated while significant amount of data is still
waiting in the queue. If a number of users require large data file transfer
at the same time, the system (e.g., the scheduler) may have to repeatedly to
allocate and de-allocate resources from each user.

	In this section we refer to periodic allocation and de-allocation of
high-speed channel as Bandwidth Oscillation. Bandwidth Oscillation effects
such as spurious retransmission were identified elsewhere (e.g., [17]) as
throughput degradation factors. However, it is important to note that in
case of some 3G wireless network configurations Bandwidth Oscillation can be
the single most important factor in reducing throughput by as much as
30%-50%. In the next paragraph we give an example of Bandwidth Oscillation
and define notation needed for further discussion. Although, the example is
based on CDMA2000 system, the same considerations are applicable to many
(wireless) systems with time scheduling of high-speed data traffic. 

	CDMA2000 1x standard, IS-2000.2 [n2], provides means of transmitting
data over two type of traffic channels: Fundamental (FCH) and Supplemental
(SCH). Fundamental channel has a fixed low bandwidth (e.g., 9.6 kbps).
Bandwidth of SCH is a multiple of that and could be as high as 32 times of
FCH bandwidth. To simplify notation, we assume that FCH rate is fixed at 9.6
kbps, we denote (SCH+FCH)/FCH bandwidth ratio by O. Hence, O is proportional
to the SCH rate. FCH is always assigned before data transmission begins. SCH
is assigned on per needed basis. When SCH is being used we say that the call
is in burst. There are two types of SCH assignments: finite and infinite
[n3], which will be referred to as finite burst and infinite burst,
respectively. Infinite burst means that SCH can be used for transmitting
data until a release command is issued. Finite burst mode of operation
limits the SCH usage to one of fourteen finite time intervals [n3] before it
must be released. We denote the duration of SCH allocation by B. After SCH
is released, it can be acquired again after certain delay (D). 

	One of the ways of detecting congestion in TCP is RTO expiration.
RTO computation algorithm [32] was designed to follow closely round trip
time (RTT), but is known to work poorly when delay variance is high [11].
During high bandwidth (FCH+SCH) RTT is low and, if B is relatively long
(e.g., 5.12 seconds), RTO converges to RTT. When SCH is released, suddenly
RTT increases (proportionally to O) and low RTO expires forcing TCP into the
Slow Start state, while actually none of the TCP segments were lost. 

	            B            
	   |<--------------->|      |-----------------|      |
	   |                 |      |                 |      |
	   |   SCH+FCH       |   D  |                 |      |
	---|                 |<---->|                 |------|   
	                       FCH
	------------------------------------------------------
	Figure 1. Bandwidth oscillation. Full cycle time is B+D. SCH and FCH
are used for transmitting data for time B, then SCH is released and only FCH
carries data for time D.

	The best approach to avoiding adverse effects of Bandwidth
Oscillation, is, perhaps, proper wireless sub-network design [11].
Simulation results as well as lab measurements suggest [N4] that when TCP
parameters (and FCH rate) are fixed the level of throughput degradation (and
achievable throughput) is a function of <O, B, D>. For some combinations
degradation of throughput could reach 55%. When B and/or D are low, the
throughput degradation is less severe. However, deploying some 2.5/3G
wireless systems with low B and/or D values could be impractical. Higher
throughput is achieved when B is high, while signaling delays impose limits
on reducing D. Avoiding finite burst mode of operation is also not practical
because limited RF resources require time-sharing of SCH resources (e.g.,
scheduling users). 

	Therefore, one has to consider other techniques that could reduce
spurious retransmissions due to bandwidth oscillation. One obvious method
was to adjust computed RTO value (or configure appropriately the minimum RTO
value) at sending TCP. This technique, however, can not be recommended as a
practical solution. 

	Experiments have shown that RTO algorithm implementation compliant
with RFC2988 [32] (e.g., minimum RTO=1 sec and initial RTO=3 sec) reduce
number of spurious re-transmissions. Although RTO timer management specified
in RFC2988 is not mandatory, implementation of retransmission timer restart
when an ACK is received (section 5.3 of RFC2988) will further reduce (or
even eliminate) spurious retransmissions. Secondary effects, such as TCP
segment loss, in combination with Bandwidth Oscillation may not allow
avoiding all spurious re-transmissions.

	Analysis of RTO algorithm along with an alternative (Eifel)
algorithm are presented in [17]. Eifel algorithm requires timestamp option
and at least one RTO expiration before TCP "learns" that retransmission was
not necessary. Enabling timestamp option enables increased RTT sampling
which can reduce spurious re-transmissions due to Bandwidth Oscillation.
Other options that could reduce spurious re-transmissions due to Bandwidth
Oscillation are increase CWND and reduce delay ACK timer at Receiving TCP to
< 100 ms (however, this technique may have side effects in case bandwidth is
limited in the opposite direction).




	[n1] "WCDMA for UMTS", edited by Harri Holma and Antti Toskala, John
Wiley & Sons, Ltd., 2000
	[n2] TIA/EIA/IS-2000.2-A, March, 2000, "Physical Layer Standard for
cdma2000 Spread Spectrum Systems", 
	[n3] TIA/EIA/IS-2000.5-A, "Upper Layer (Layer 3) Signaling Standard
for cdma2000 Spread Spectrum Systems", March, 2000
	[n4] F.Khafizov, M.Yavuz, "Running TCP over IS-2000", to appear in
Proc. of IEEE ICC 2002
	[n5] C.Borman, "ROHC over PPP", Internet Draft, November 2001,
http://www.normos.org/ietf/draft/draft-ietf-rohc-over-ppp-04.txt