Making a Case for MMDS
telecoms-mag.com
Focus On
Siddhartha Shankar Menon
MMDS (multichannel multipoint distribution service) provides several
advantages over competitive terrestrial Internet access methods
because it effectively utilizes full duplex service provisioning. Despite
its flexibility, however, the technology, also known as wireless cable,
has some important technical drawbacks compared to DSL and cable
modems.
MMDS comprises eight channels in the E and F Group at 2596 MHz
to 2644 MHz. There is also a bank of 125-KHz channels assigned
for upstream voice and data applications as well as 28 downstream
channels in the A through G Group. The downstream spectrum also
includes 2686.0 MHz to 2689.875 MHz.
The technology's upstream and downstream specifications reveal
some technical advantages over competitive technologies. MMDS
systems broadcast downstream data in a TDM format to subscriber
modems, a plus given the asymmetric nature of Internet traffic.
Multiplexing techniques therefore play an indispensable role in
handling Internet traffic given its bursty nature; the average
downstream and upstream rates can vary as much as 5:1 to 20:1.
MMDS's ability to broadcast this traffic directly to subscribers via RF
(radio frequency) waves instead of passing it through the switched
network is particularly advantageous. While cable modem systems
are also able to dedicate transmission to Internet data, many have to
be built from scratch to provide full duplex usage and are more costly
to implement.
MMDS systems have distinctive data rate and bandwidth
specifications. These systems provide a downstream raw data rate of
30 Mbps and an attainable speed of 27 Mbps, which factors in FEC
(forward error correction). MMDS's downstream bandwidth
capability utilizes 6-MHz channels that allow 540 users to access the
system simultaneously at 50 kbps (see Figure 1). If the Internet traffic
is particularly bursty and there is a sudden upsurge in demand, the
system can handle up to 2700 subscribers, a loading factor of five
times the typical capacity. The ratio of upstream to downstream
subscribers in typical 6-MHz channels is approximately 3:1.
Upstream, MMDS consists of 48 channels that are each 125 MHz
wide. These transmissions have a data rate of 200 kbps with no FEC.
The capacity is at least at 36 percent of the MAC (media access
control) efficiency of 72 kbps, which translates into 38 active
simultaneous users uploading data at an average rate of 38.4 kbps.
Upstream transmissions can accommodate up to 187.5 subscribers at
a five times loading factor in heavy traffic (see Figure 1).
Upstream
48 channels each 125 KHz
wide
Raw data rate 200 kbps
Capacity 36% MAC efficiency
37.5 active users, 38.4 kbps
averate rate with burstiness of
20x
187.5 subscribers @ 5x
loading factor
For 48 channels
accommodates 9,000
subscribers per 6 MHz TV
channel
Downstream
64-QAM 5 MHz channel
Raw digital rate 30 Mbps
540 simultaneous users @ 50
kbps throughput rate
27000 subscribers @ 5x load
factor
The ratio of U/S to D/S in a 6
MHz channel is approximately
3:1
Fig.1 Two-way MMDS service specifications.
MMDS systems utilize innovative technologies to compensate for
their shared bandwidth infrastructure. MMDS data is sent as packets
of payload and address data that requires each subscriber's modem
to monitor the downstream flow and screen for information
specifically intended for that particular user. The shared nature of the
downstream flow requires an algorithm--most commonly MAC--to
separate the upstream and downstream bandwidth resources among
subscribers, especially when Internet traffic is heavy. The upstream
traffic is sent to the POP, which typically constitutes the software and
hardware used by the local ISP (servers, modems, gateways) and
connects to the headend and consumer via RF wireless broadcast
signals (see Figure 2).
This RF upstream and downstream MMDS system is preferable to
competing technologies, especially SDSL (symmetric DSL), in that its
asymmetric broadcast capabilities allow it to be more responsive to
two-way technical requirements and usage patterns. Perhaps the most
compelling reason is that two-way RF systems specifically deployed
to handle Internet traffic are the most flexible in meeting the distinct
patterns of Internet users because they do not have to accommodate
telephony-related traffic. In addition, these systems can operate
entirely from the headend without feedback from the subscribers to
prevent heavy data users from monopolizing the downstream channel
capacity.
MMDS is taking advantage of spectral efficiencies of digital video
compression to increase the number of RF channels per system and is
closing the capacity and speed gap with DSL and cable. For
example, cellularization techniques utilize multiple hub sites to offer
signals to geographically dispersed groups of subscribers. This
augments capacity by sending different information from different cell
sites using the same RF channels. Because the frequency is reused,
users can send more bandwidth-intensive graphical and audio files
while simultaneously conserving bandwidth.
While the limitations of DSL and cable modems have been well
documented, MMDS has its own technological disadvantages. One is
a line-of-sight constraint, which can pose difficulties for MMDS users
in some geographic terrain. Once the signal comes into contact with a
physical barrier it rapidly diffuses or attenuates, and the data is lost. A
second disadvantage is that xDSL generally offers faster data rates.
While MMDS might have a comparatively impressive downstream
rate of 27 Mbps, the upstream is relatively inadequate at 200 kbps.
Coaxial cable systems offer other advantages over MMDS, including
greater user capacity and available bandwidth as well as the absence
of the line-of-sight constraint. Cable provides more bandwidth,
especially in terms of downstream spectrum, which is in the 50-MHz
to 550-MHz range. MMDS, on the other hand, only offers 6-MHz
channels in comparison. Even in terms of upstream capability some
cable modem systems can far exceed MMDS. For example, while
optimal coaxial systems offer upstream transmissions in the 5-MHz to
35-MHz range, wireless cable is limited to channels 125 KHz wide.
Cable systems were better designed to accommodate video capacity
requirements, which is particularly useful for innovative high-end
Internet use such as transmitting high-definition pictures and video
files. However, cable systems were not designed to carry full duplex
traffic, and much of the existing cable plant must be upgraded or
replaced to offer this functionality.
MMDS that uses two-way RF broadcasting to route Internet data
proves to be more flexible than DSL or cable because the systems
are dedicated to the specifications unique to Internet use. The
technology is more effective and efficient than xDSL, which often
funnels its traffic, at least part of the way, through the PSTN. MMDS
does not burden networked corporate users with high infrastructure
costs associated with updating cable systems. While the technological
specifications and factors involved with MMDS are highly dynamic
and complex, MMDS is the most appropriate emerging technology
for two-way Internet applications. *
Siddhartha Menon is a new media consultant and marketing
executive at IMI Inc., a media consultancy firm. He holds a
graduate degree in communication, culture and technology from
Georgetown University and is a member of the International
Communication Association |
