Contributed by Christian Wietfeld, Siemens and R.W.T.H. Aachen.
During the development of the DSRC MAC protocol a number of different
approaches have been considered and analysed. Here we initially present
the DSRC MAC protocol as proposed by the European
standardization body CEN TC 278 WG 9 [8].
The standardized DSRC MAC protocol was primarily designed to support an
asynchronous communication mode, but is also able to support the a synchronous mode.
An second approach is the standard proposal in North America
[1], which was also considered during the
development of the DSRC MAC protocol,
Figure 5: DSRC MAC protocol
The DSRC MAC protocol [8] is based on half-duplex TDMA.
The beacon (as primary station)
offers two different types of up-link windows
to the vehicles: public and private windows.
-
A public window consists of time slots (public slots),
which may be accessed by every vehicle within the communication zone.
Together with the Beacon Service Table, which holds information about
the valid application and protocol parameters at a specific beacon
site (such as length of public and private windows),
public windows are offered periodically by the beacon to
newly arriving vehicles (address acquisition or connection phase).
- A private window allocation reserves a time period
for one specific vehicle and therefore protects it against data
collisions after the address of a vehicle is known (transaction phase).
To avoid unnecessary delays during the address acquisition phase, it is
essential to avoid and resolve collision situations effectively.
By randomly distributing the transmission
of newly arriving vehicles over several public slots the probability
of data collisions can be reduced.
This improves the DSRC MAC performance. (See also pages on ALOHA, stability, collision resolution, and stack algorithms for a generic treatment.)
Figure 6: Synchronous protocol
In contrast to the asynchronous DSRC MAC protocol,
the TDMA protocol proposed for the ITS communication architecture
[3] is based on a fixed frame structure, the so-called
Open-Road Frame, which consists of the following elements:
- a Reader Control Message (RCM),
- several Data Slots (DS), and
- a number of Activation Slots (AS).
The activation
slots are used for the transmission of the ID of newly arriving
vehicles (similar to public slots in the DSRC approach).
The data slots may be used for down-link transmissions or
are reserved by the beacon for up-link transmissions (similar to
private windows). Reading the RCM, the vehicles get to know the
assignment of down-link and up-link slots. As the data slots have a
fixed length and the frame structure is fixed, data slots, which do not need
be used completely or not at all, produce a certain overhead
especially for single application scenarios.
From a performance point of
view, the DSRC MAC solution offers more flexibility for optimization
especially for applications with a high priority as
Automatic Fee Collection.
This is because the assignment of public and private
windows can be adapted to the actual traffic situation easily.
Nevertheless, the DSRC MAC protocol can
also be used in a synchronous mode by setting the BST retransmission
intervals to a fixed length.
