R&S M3SR 4100 Series, Руководство по эксплуатации, Страница: 586 / 706

M3SR Series 4100 IP over Air (IPoA)
5.40 Operating Manual 6175.4760.02 – 01
The available send buffer is clustered in data chunks of different sizes from 64 bytes
to 1524 bytes. When sending mainly large (e.g. 1500 bytes) or mainly small (e.g.
64 bytes) data chunks, not all of the above mentioned send buffer may be available. If,
for example, SECOM-V (only R&S M3TR) is the waveform used and only 1500 byte
packets are sent, source quench will not become active at a use level of 70 % of
315 kB (220.5 Kbyte), but already when the 1500 byte clusters are used up to 70 %,
which will be already at a use level of 155.6 Kbyte.
Automatic Repeat Request (ARQ)
ALE-3G / xDL and IPoA protocol for SECOM-V (only R&S M3TR), SECOM-H and OFDM
(only R&S M3TR) uses different data link layer protocols for channel access. This different
mechanism will be described in the following:
IPoA protocol ARQ
The data link layer (layer 2) of the IPoA protocol stack contains an ARQ (Automatic Repeat
Request) algorithm. The type of the ARQ algorithm is "Selective Repeat". Per default all
TCP/IP data, which is not mapped to any of the configured services are transmitted with ARQ
protection. Other protocols like UDP and ICMP are not transmitted with ARQ protection.
ARQ can be enabled or disabled for each service by means of R&S RNMS3000.
ARQ Functionality
Each IP data packet, which is sent to air by the transmitter, is acknowledged by the receiver
(ACK packet without payload).
The transmitter retransmits packets which are not acknowledged in time. Due to this, packets
that
have not been acknowledged yet
, remain in the transmission queue until an acknowl-
edgment was received, the maximum retransmission count is reached or the maximum
packet life time (TTL) is exceeded. In the case of maximum retransmission count or TTL, the
packet is not retransmitted anymore and is dropped.
ARQ contains a flow control mechanism called "Sliding Window". This prevents a packet
flooding of the receiver (if many packet losses occur).
The benefit of using ARQ with "Selective Repeat" in the data link layer is:
• Minimizing of data packet losses
• Selective Repeat: Only lost packets are retransmitted (in contrast to the ARQ algorithm
"Go Back N").
• The handling of ARQ in the data link layer reduces the retransmissions of data packets
in higher protocol layers (for example TCP) that have an own implementation of ARQ
and increases the data transmission performance.