13.8 Passive Optical Networks
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channels, and then reorganizes the channels so that they can be delivered efficiently
to their destinations. In conjunction with the OLT, an ONT allows dynamic bandwidth
allocation based on varying demands of users to enable smooth delivery of data traffic
that typically arrives in bursts from the users.
Optical Network Unit (ONU)
An optical network unit (ONU) is similar to an ONT but normally is housed in an
outdoor equipment shelter near the user premises. These installations would include
shelters located at a curb or in a centralized place within an office park. Thus the
ONU equipment must be environmentally rugged to withstand wide temperature
variations. The shelter for the outdoor ONU must be water-resistant, vandal-proof,
and able to withstand high winds. In addition, there has to be a local power source
to run the equipment, together with emergency battery power backup. The link from
the ONU to the customer premises can be a twisted-pair copper wire, a coaxial cable,
an independent optical fiber link, or a wireless connection.
13.8.3 Controlling PON Traffic Flows
Two key network functions of an OLT are to control user traffic and to assign bandwidth dynamically to the ONT modules. Up to 32 ONTs use the same wavelength and
share a common optical fiber transmission line, so some type of transmission synchronization must be used to avoid collisions between traffic coming from different ONTs.
The simplest method is to use time-division multiple access (TDMA) wherein each
user transmits information within a specific assigned time slot at a prearranged data
rate. However, this does not make efficient use of the available bandwidth because
many time slots will be empty if a number of network users do not have information
to be sent back to the central office.
A more efficient process is dynamic-bandwidth allocation (DBA), wherein time
slots of an idle or low-utilization user are assigned to a more active customer. The
exact DBA scheme implemented through an OLT in a particular network depends on
factors such as user priorities, the quality of service guaranteed to specific customers,
the desired response time for bandwidth allocation, and the amount of bandwidth
requested (and paid for) by a customer.
As shown in Fig. 13.42, the OLT uses time-division multiplexing (TDM) to
combine incoming voice and data streams that are destined for users on the PON. As
a simple example of this, if there are N independent information streams coming into
the OLT, each of which is running at a data rate of R b/s, then the TDM scheme interleaves them electrically into a single information stream operating at a higher rate of
N × R b/s. The resulting multiplexed downstream signal is broadcast to all the ONTs.
Each ONT discards or accepts the incoming information packets depending on the
packet header address. Encryption may be necessary to maintain privacy because the
downstream signal is broadcast, and every ONT receives all the information destined
for each end terminal.
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channels, and then reorganizes the channels so that they can be delivered efficiently
to their destinations. In conjunction with the OLT, an ONT allows dynamic bandwidth
allocation based on varying demands of users to enable smooth delivery of data traffic
that typically arrives in bursts from the users.
Optical Network Unit (ONU)
An optical network unit (ONU) is similar to an ONT but normally is housed in an
outdoor equipment shelter near the user premises. These installations would include
shelters located at a curb or in a centralized place within an office park. Thus the
ONU equipment must be environmentally rugged to withstand wide temperature
variations. The shelter for the outdoor ONU must be water-resistant, vandal-proof,
and able to withstand high winds. In addition, there has to be a local power source
to run the equipment, together with emergency battery power backup. The link from
the ONU to the customer premises can be a twisted-pair copper wire, a coaxial cable,
an independent optical fiber link, or a wireless connection.
13.8.3 Controlling PON Traffic Flows
Two key network functions of an OLT are to control user traffic and to assign bandwidth dynamically to the ONT modules. Up to 32 ONTs use the same wavelength and
share a common optical fiber transmission line, so some type of transmission synchronization must be used to avoid collisions between traffic coming from different ONTs.
The simplest method is to use time-division multiple access (TDMA) wherein each
user transmits information within a specific assigned time slot at a prearranged data
rate. However, this does not make efficient use of the available bandwidth because
many time slots will be empty if a number of network users do not have information
to be sent back to the central office.
A more efficient process is dynamic-bandwidth allocation (DBA), wherein time
slots of an idle or low-utilization user are assigned to a more active customer. The
exact DBA scheme implemented through an OLT in a particular network depends on
factors such as user priorities, the quality of service guaranteed to specific customers,
the desired response time for bandwidth allocation, and the amount of bandwidth
requested (and paid for) by a customer.
As shown in Fig. 13.42, the OLT uses time-division multiplexing (TDM) to
combine incoming voice and data streams that are destined for users on the PON. As
a simple example of this, if there are N independent information streams coming into
the OLT, each of which is running at a data rate of R b/s, then the TDM scheme interleaves them electrically into a single information stream operating at a higher rate of
N × R b/s. The resulting multiplexed downstream signal is broadcast to all the ONTs.
Each ONT discards or accepts the incoming information packets depending on the
packet header address. Encryption may be necessary to maintain privacy because the
downstream signal is broadcast, and every ONT receives all the information destined
for each end terminal.