User's Manual

58 ALFOplus 24GHz (North America) - Release 01.05.0x - MN.00395.E - 004
7.12 MAB PROTOCOL (MICROWAVE ADAPTIVE BANDWIDTH)
Microwave radio transmission capacity depends on radio channel propagation conditions. In case of Ether-
net ring, capacity degradation can affect one side of the ring, while the other is still working at nominal
capacity. In case of congestion events, Microwave radio prioritizes the Ethernet traffic, by reducing or dis-
carding the lower priority frames. Similar situation can be found even in other network topologies, where
the microwave link bandwidth changes must be reported to an upstream Ethernet switch or router in order
to adjust the relevant traffic accordingly its traffic shaping and/or forwarding rules. By knowing the capac-
ity of the radio link, external equipment can optimize the transport on ring networks, by means of forward-
ing rules that choose the best direction for each traffic type, or select additional criteria for the quality
management (e.g. Router/Switch shapes the traffic allowing only selected data to be transmitted through
the microwave link).
The Microwave Adaptive Bandwidth (i.e. MAB) protocol has been developed by SIAE and CISCO in order
to improve the behaviour of the Ethernet ring protection when using the adaptive code and modulation
(ACM) on microwave links. Thus the MAB offers reliable QoS and optimized performances even in worst
radio propagation conditions because it allows forwarding traffic on the Ethernet ring according to the
bandwidth available for every ring branch.
For a more comprehensive scenario, the following Ethernet ring topology is assumed, where adaptive mod-
ulation microwave Radio and Ethernet switch or router are deployed.
Fig.24 -
The main ring topology advantages are:
• to offer a protection path in case of equipment failure or radio link unavailability due to deep fading
• to increase Ethernet throughput (up to double) from the core towards the border of the network by
distributing on the two ring branches the Ethernet traffic by means of control plane protocols (i.e.:
G.8032) in order to open the Ethernet loop in a suitable segment.
The standard ring protection mechanisms are usually triggered by failures, because the control protocols
stop receiving
messages due to loss of connectivity among sites.
However, the traditional Ethernet ring protection mechanisms do not take into account of the degradation
of radio links capacity due to ACM intervention, because the protection protocol packets (usually marked
as high priority) still continue to run on the ring, and this leads to an unbalanced operation of the ring with
different capacities on the two branches.
In fact, the traditional Ethernet ring protection are not sensitive to variations on the radio bandwidth oc-
curring in presence of modulation level reduction (i.e. ACM enabled) caused by atmospheric phenomena
(e.g. fading due to multipath and rain events). Therefore, in case of fading phenomena, the Ethernet ring
protocol does not activate the ring protection because of lack of both Link Loss Forwarding messages com-
SIAE
radio
Cisco
ASR
SIAE
radio
SIAE
radio
Cisco
ASR
SIAE
radio
SIAE
radio
SIAE
radio
SIAE
radio
SIAE
radio
SIAE
radio
SIAE
radio
Cell site A
Cell site C
Cell site B
Core network
Ge Interface Radio link