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Broadband Powerline Communications Networks
In the second case, network stations using the packet voice service transmit data that
contains speech information only during so-called active periods of the talk (talkspurts).
A network station using packet voice transmits a request at the beginning of a connection
for its setup, such as in the case of the classical telephony service. After the connection
is established, the network station has to send a request at the beginning of each talk-
spurt. If we assume that transmission channels for voice can be dynamically allocated
(see Sec. 5.4.3 and Sec. 6.1.3), the voice station can start the transmission immediately
or very shortly after the acknowledgment from the base station is received. In this way,
the access delay can be reduced to its minimum, and the transmission delay of the voice
packets consists mainly of the signaling delay. The delay limits for the voice service in
access networks are set to relatively small values; for example, in wireless networks of
20 to 24 ms ([AlonAg00], [KoutPa01]), or of 25 ms to avoid the usage of echo cancel-
ers [DaviBe96]. The maximum signaling delay in the investigated network model is below
20 ms (Fig. 6.51). So, in this case, the two-step protocol can fulfill the delay requirements.
6.6 Summary
To specify a reservation MAC protocol the following four functions have to be defined:
reservation domain, signaling procedure, access control and signaling MAC protocol. An
optimal reservation domain has to be chosen in accordance with transmitted telecommu-
nications service. To avoid the transmission gaps occurring when the per-burst reservation
is applied, the per-packet reservation domain is proposed for the realization of data trans-
mission to improve network utilization. The signaling procedure and the access control
have to be simple with a limited number of signaling messages, ensuring a low probability
that the signaling exchange is affected by the disturbances. Among numerous proposals
for signaling MAC protocols in different communications technologies, it is possible to
identify two main protocol groups protocols with random and with dedicated access.
The generic simulation model, used for the investigation of various signaling MAC
protocols, implements the OFDMA/TDMA scheme, allowing implementation of multiple
disturbance and traffic models. Two types of traffic models are considered simple traffic
models, representing the data traffic causing rare and frequent transmission requests, and
multimodal traffic models, representing a nearly realistic behavior of Internet users. Two
disturbance models are applied to allow investigations of lightly and heavily disturbed
PLC networks.
Signaling delay, evaluated in the network using ALOHA protocol, is significantly
shorter than in the network with polling in the case of rare transmission requests. In
the case of frequent transmission requests, ALOHA protocol collapses and polling has
significantly better performance. The protocol performance can be improved by the appli-
cation of various protocol extensions. So, application of extended random access, using
free data channels for signaling, improves network performance significantly in the low
network load area as well as the piggybacking access method in the high loaded net-
works. On the other hand, with application of dynamic backoff mechanism, protocols
with random access can be stabilized. Generally, it can be concluded that polling pro-
tocols, implemented in their advanced variants, have some advantages, and as opposed
to advanced ALOHA protocols, they always achieve the theoretical maximum network
utilization. Furthermore, the polling-based reservation protocols can be improved by the