Subject: 2.5g3g wg comments

From: "Cobus de Beer" <[email-masked]>
Date: Wed, 1 Aug 2001 09:59:31 +0200

From the 3GPP specifications:

The size of the RLC PDUs can vary and is the same size as the MAC SDU.
The maximum value is 4992 bits as specified in the Transport Format Set
(TFS) from the Radio Resource Controller (RRC) (3GPP 25.331). A
different TFS may yield a different RLC PDU size.

The number of times the RLC will try retransmission before quitting is
specified by the value MaxDAT. MaxDAT is given in the ASN.1 messages
(Chapter 11) of 25.331 and is specified as maximum of 40, 
MaxDAT ::=	ENUMERATED {dat1, dat2, dat3, dat4, dat5, dat6,
				dat7, dat8, dat9, dat10, dat15, dat20,
				dat25, dat30, dat35, dat40 }

This information conflicts with the following paragraph:

2. 2.5G and 3G Link Characteristics

   Link layer characteristics of 2.5G/3G networks
   have significant effects on TCP performance.  ARQ and FEC are
   discussed in [1]. Justification for link layer ARQ is discussed in
   [10], [12].

   This section provides further details on a specific 3G technology,
   namely, Wideband CDMA (W-CDMA).

   W-CDMA (Wideband CDMA) uses RLC (Radio Link Control)
   [3], a Selective Repeat and sliding window
   ARQ. RLC uses protocol data units (PDUs) 336 bits long (including a
   16 bit RLC header). This is the unit for link layer retransmission.
   The SDU (IP packet) is fragmented into PDUs for transmission by RLC.

   In W-CDMA, one to twelve PDUs (RLC frames) constitute one FEC frame.
   The actual size of the FEC
   frame depends on the link conditions and bandwidth allocation. The
   FEC frame is the unit of interleaving.

   RLC uses "status report" type acknowledgments: the poll bit in the
header
   explicitly solicits the peer for a status report containing the
   sequence number that the peer acknowledged. The use of the poll bit
   is controlled by timers and by the size of available buffer space in
   RLC. Also, when the peer detects a gap between sequence numbers in
   received frames, it can issue a status report to invoke
retransmission.
   RLC preserves the order of packet delivery

   The maximum number of retransmissions is a configurable RLC
   parameter, but it cannot exceed a value of 10. Therefore, RLC can be
   described as an ARQ that can be configured for either
   HIGH-PERSISTENCE or LOW-PERSISTENCE, not PERFECT-PERSISTENCE,
   according to the terminology in [10].

   In general, the link layer ARQ and FEC can provide a packet service
with a
   negligibly small probability of undetected error (failure of the
   link CRC), and a low level of loss (non-delivery) for the upper
   layer traffic, i.e. IP. The SDU (IP packet) is fragmented into PDUs
   for transmission. Retransmission of these by ARQ introduces latency
   and jitter to the SDU flow. This results in relatively large BDP
   (Bandwidth-Delay Product). Typical values may range around
   50KB, but may exceed 64KB. [These BDP figures to be completed
   according to the DoCoMo measurements.]

   The above explains why the transport layer sees the underlying W-CDMA
   network as a network with a relatively large BDP and
   jitter. The loss rate of IP packets is low due to the ARQ, but the
   recovery in layer two appears as jitter to the higher layers.