Broadband access system. At that time, ATM was recognized as the best link layer protocol, and PON was recognized as the best physical layer protocol. The combination of the two naturally produced APON technology. Both FSAN and ITU have published APON technical specifications (the technical specifications used for APON have been accepted as ITU-T Rec. G.983). Currently, in countries such as Japan, the United Kingdom, and Germany. A mature APON system has been put into use.

 

1. How APON works

 

APON combines ATM technology with passive optical network technology to provide existing services ranging from narrowband to broadband. The APON is composed of an OLT (Optical Line Terminal), an ONT (Optical Network Terminal), and a POS (Passive Optical Splitter). Its structure is shown in Figure 1. In the figure, the POS is a passive optical splitter/coupler that splits and distributes the incoming optical signal onto multiple optical fibers or combines them onto one optical fiber depending on the direction in which the light is transmitted. The ONT mainly performs service collection, interface adaptation, multiplexing, and transmission functions; the OLT mainly performs interface adaptation, multiplexing, and transmission functions. In addition, the OLT provides a network management interface to the network element management system.

 

APON is a point-to-multipoint transmission system using two transmission wavelengths, one for the downlink transmission wavelength and the other for the uplink transmission wavelength, the two transmission wavelengths at the central office optical line termination (OLT) and each final Information is transmitted between the optical network terminals (ONTs) of the subscriber stations. The OLT allocates one time slot and a certain amount of bandwidth to each ONT (downstream and uplink). The APON network topology is a star, and the bidirectional transmission mode generally adopts a dual fiber space division multiplexing (SDM) method, and is also based on a single fiber wavelength division multiplexing (WDM) method. Point-to-multipoint propagation uses downlink time division multiplexing (TDM) and uplink time division multiple access (TDMA). Typical line rates for existing products are 622 Mb/s downstream and 155 Mb/s upstream.

 

In the downstream direction, the OLT transmits a wavelength through a fiber and broadcasts information to the ONT using ATM cells. This fiber is separated on the passive splitter and can connect up to 32 ONTs.

 

While sending each cell packet in the downstream direction, there is also an ID field to identify the destination address. Technically, all information is transmitted to the ONT, but the ONT only accepts information with the correct address.

 

From the physical point of view, the OLT always broadcasts downlink information, but only one ONT can decode this information according to the password. Structurally, it is similar to the print server settings for various printers, and the printer only prints information that is tagged with its own address.

 

In the upstream direction, the ONT waits for the designated time slot and uses the second wavelength transmitted through the same fiber to send back control information to the OLT.

 

The uplink and downlink data rates of APON can reach 155 Mbps, the data rate of the asymmetric application in the downlink direction can reach 622 Mbps, and the access rate of users can be flexibly allocated from 64 Kbps to 155 Mbps. The most obvious benefit of APON to end users is that it greatly increases the amount of bandwidth used by the home. This advantage will open up new avenues for applications such as video on demand and eliminate any time waiting for surfing the web. For example, on-demand video applications, many users can subscribe to 6Mbps movies. When the movie starts, its required bandwidth can be increased, and when the movie is over, the amount of bandwidth required for the user's service can be restored to a normal level. Although the bandwidth required for APON to provide services is high, it does not increase the user's cost. In some cases, such as the Small Home Office (SOHO) for residential areas, APON can also effectively reduce the cost per bit.

 

Like ADSL, APON provides end users with a more secure environment than cable modem services. APON uses a so-called "churning" security mechanism to encrypt data. In this environment, the ONT generates and sends an uplink key to the OLT to control the data and send back the downlink data. These keys change dynamically, making it difficult to decipher.

 

2. Key technologies of APON

 

The key technologies of APON mainly include: control of time division multiple access, fast bit synchronization and reception of burst signals.

 

Time division multiple access control

 

The APON system adopts a time division multiplexed broadcast mode in the downlink direction, and the frame is composed of consecutive time slots, each of which is filled with a 53-byte ATM cell. Since the downlink direction is the broadcast mode, each ONU will receive all the frames and autonomously take out its own cells from the corresponding time slots, so no OLT is required for control in the downlink direction.

 

In the uplink direction, since the ODN of the PON is actually a shared transmission medium, proper access control is required to ensure that the uplink signals of the respective ONUs completely reach the OLT. G.983 recommends the use of TDMA uplink access control. In this way, since the access multiplexing of the ATM-PON is implemented in the time domain, there is no conflict in the distance from the OLT to the ONU, and effective uplink access is an important consideration. Different distances cause different delays. Therefore, in order not to cause the uplink signals to collide, the OLT must measure the distance to each ONU and inform the ONU of the specified delay. Each ONU coordinates with the specified delay when transmitting the uplink signal. , multiplex the respective ATM cells into the upstream frame.

 

The ranging process is implemented by using in-band signaling data carried by uplink/downlink cells (PLOAM cells), that is, ranging permission signals carried by PLOAM cells, PLOAN permission, data permission, ONU sequence number, PON? ID, and measurement. It is implemented from the time message. When multiple ONUs are connected to the line at the same time, the OLT first uses one of the binary tree exclusion mechanisms according to the ONU serial number to perform ranging. The approximate process of ranging is: the OLT opens the ranging window to measure the uplink cell delay from the ONU; the OLT informs the ONU of the equivalent delay; the ONU adjusts the transmission delay.

 

Fast bit synchronization

 

After the adopted ranging mechanism controls the uplink transmission of the ONU, the uplink signal still has a certain phase drift. There is a 3-byte overhead in each time slot of the upstream frame, the guard time is used to prevent small phase drift from damaging the signal, and the pre-bit pattern is used for synchronous acquisition. When receiving the uplink frame, the OLT searches for the synchronization pattern, and quickly acquires the phase information of the code stream to achieve bit synchronization; then determines the boundary of the ATM cell according to the delimiting pattern to complete the byte synchronization. The OLT must implement bit synchronization within the first few bits of the ONU uplink burst to recover the ONU signal. Synchronous acquisition can be achieved by correlating the received code stream with a particular bit pattern. However, the general sliding search method has too long delay and is not suitable for fast bit synchronization. Therefore, a parallel sliding correlation search method can be used to sample the received signals with clocks of different phases, and the results are simultaneously (parallel) and synchronized patterns. Perform correlation operation, compare the operation result, take the sampling signal corresponding to the maximum value as the output when the correlation coefficient is greater than a certain threshold, and use the clock of the phase as the optimal clock source; if several correlation values ​​are equal, the phase can be taken Signal and clock in. This is actually at the expense of the complexity of the circuit in exchange for time gains.

 

Transmitting and transmitting of burst signals

 

In the uplink access using TDMA, each ONU must complete the transmission of the optical signal within a specified time interval to avoid collision with other signals. In order to implement the burst mode, special techniques must be adopted at the transceiver end. The optical burst transmission circuit is required to be turned on and off very quickly, and the signal is quickly established. Therefore, the automatic feedback control used in the conventional electro-optical conversion module is not applicable, and a laser with a fast response speed is required. At the receiving end, since the signal optical power from each user is different and varies, the burst receiving circuit must adjust the receiving level (threshold) each time a new signal is received. The adjustment is implemented by the pre-bit of the time slot in the ATM PON system. The threshold adjustment circuit of the burst mode preamplifier can quickly establish a threshold within a few bits, and the receiving circuit correctly recovers the data according to the threshold.

 

Second, Ethernet passive optical network (EPON)

 

EPON is a point-to-multipoint optical access network technology that emerged after APON, which provides a low cost for configuring optical access lines between the Network Central Office (CO) and the user site. High-performance approach. It builds on the ITU's standard G.983 for APON and seeks to build a full-service access network capable of transmitting aggregated data, video and voice information over a single optical access system.

 

Source: IT World Network