Broadband Wireless Access Technology Based on DOCSIS Protocol

Broadband wireless access technology is worth learning a lot. Here we mainly introduce the DOCSIS-based broadband wireless access technology, from the DOCSIS Protocol release to the current development trend, many related equipment manufacturers use DOCSIS Protocol in their 3.5 GHz MMDS products because DOCSIS uses many mature technologies to support IP services well, it has adapted to the current technological development trend, and has adopted a large number of Integrated Circuits Used in Cable MODEM. Due to the large demand of such devices worldwide, large-scale production has made them cheaper.

Although the DOCSIS specification was originally intended for CATV applications, due to the above characteristics, a number of companies have joined forces to establish a wireless digital user line Association, in order to establish a DOCSIS enhanced version for broadband wireless access technology applications, the trend in the field of broadband wireless access technology. Specifically, the new DOCSIS + standard is based on the original specifications, but some specific expansion specifications are added for the LMDS/MMDS market. These extensions will be included in a new standard or an extension of the existing DOCSIS standard), which is currently being studied by the IEEE 16th Committee. At present, the market has launched a DOCSIS-based wireless access system. However, how to ensure real-time business and reliable application in the wireless environment requires further lab verification, which is also a further improvement of this standard.

Since the DOCSIS specification was initially used for data applications of CATV, the QOS and real-time service support of DOCSIS1.0 are not yet well guaranteed. How to Implement connection-oriented real-time services in DOCSIS is further upgraded and extended in DOCSIS 1.1. The expansion of 1.0 mainly includes the addition of all QoS functions of CBR, VBR, and ABR. To support voice services, it provides clear request policies for active authorization services and real-time round-robin services; standardize IP multicast, increase the division of CMTS on upstream code streams, and compress PFS by using the Net Load header) to make more effective use of upstream and downstream code streams. The core of these extensions and support capabilities for real-time services lies in the MAC layer and the control units designed by the system. The DOCSIS Protocol can transmit four types of MAC frames on the air interface:

MAC frame of the packet data PDU. It supports packet data PDU of the Ethernet type with a variable length. MAC frame of an ATM cell indicates that an atm pdu exists in the frame. It is a MAC frame reserved for the future PDU type; MAC frame used for specific MAC control. The first three types of MAC frames are mainly used for data transmission, and specific MAC frames are used for special functions, including scheduled messages, management messages, request messages, cascading messages, and segmented messages. These functions support downlink timing and uplink ranging/Power modulation, the bandwidth requested by the terminal to the base station, the bandwidth allocated by the base station to the terminal, and the cascade of multiple MAC frames. For IP data services, the MAC frame of the packet data PDU is transmitted. Through configuration, the system specifies the minimum guaranteed bandwidth and maximum bandwidth for each terminal, and provides QoS Assurance for data services through statistics multiplexing and Dynamic Allocation of air bandwidth.

Due to the high real-time requirements of circuit services, the outstanding characteristic is constant traffic. DOCSIS uses 14-bit SID business type identification to set bandwidth for a single user and set different levels of business, and uses an active authorization mechanism to expand and adapt the package type, encapsulate the circuit business into the PDU type and retain the MAC ?, The special packet transfer mechanism is used to provide a TDMA-like environment to ensure the delay and jitter of circuit services. At the same time, the circuit clock is transmitted to ensure the clock synchronization between the upstream and downstream networks. Therefore, according to the specified configuration, the E1-Based Circuit service can be assigned to a specific terminal, which occupies Mbps of traffic in the air. Relatively speaking, because the uplink channel supports segmentation, the non-Voice authorization service can be segmented and moved to the authorization boundary of the voice Authorization Service to minimize the impact of the conversation audio service, however, the downlink communication and audio service is guaranteed based on the priority of different queues. Therefore, the downlink channel guarantees real-time circuit services less than the uplink channel.

Therefore, the implementation of DOCSIS-Based Circuit services is basically achieved through the data packet service. The basic principle of QOS is to classify data packets transmitted in the RF band and classify them into a specific business flow. The so-called business flow refers to a single data packet flow that provides a specific service quality, the base station and the remote station provide QOS with different priorities through business shaping, negotiation or business flow. the high-speed processing control of the maximum level of active authorization and control unit is implemented when the MAC layer is set to a frame to support circuit services. Therefore, the possible problem is that when both circuit and VoIP services are supported, especially downstream channels, when short frame services such as VoIP suddenly increase, it may cause the impact of VOIP on the E1 circuit service, and may affect the delay, jitter, and error rate of the circuit service. At the same time, the DOCSIS standard also mentioned that some unexpected events, such as the loss of MAP uplink bandwidth allocation messages in the following line, or the uplink frequency hopping may cause Jitter to exceed the jitter window. Therefore, the implementation of real-time services based on DOCSIS is quite different from that of traditional TDM transparent real-time services. Traditional TDM is oriented to connection services and does not require QOS Assurance.

Another noteworthy problem is that once a traditional TDM circuit Service establishes a connection, the allocated time slot will be retained for the service until the connection is removed, therefore, the latency and jitter are fixed and guaranteed. The delivery of each packet in the DOCSIS circuit service may be different in different segments, so the delay is not fixed. In addition, because the wireless network has different characteristics, DOCSIS is used for broadband wireless access technology. The existing DOCSIS standard limits large-scale Broadband Wireless Configuration, media access control MAC) layer and physical PHY) the design of the layer must be improved.

Multi-path fading, frequency offset, and Interference Problems in wireless systems are all negative factors for wireless systems. Therefore, the robustness of the physical layer design should be sufficient to support the reliable operation of wireless networks, in the design, the problem of SNR and multi-path fading must be solved. The selection of Error Correction Codes and the performance of RF units will directly affect the system performance, especially the impact on real-time circuit business. DOCSIS standards require the use of R-S code, if the use of more effective Error Correction Code and balancing will be greatly improved DOCSIS standards, the complexity of its implementation is greatly increased, this is one of the reasons that the current DOCSIS-based equipment system indicators are low, such as the receiving threshold and the same/adjacent frequency load ratio ). Despite this, the DOCSIS standard still has a certain application environment with its simple technical structure, mature technology adoption, low price, and support for IP services. However, from the perspective of development, DOCSIS standards still need to be further improved: new versions are continuously released to better support circuit businesses, apply to wireless environments, and improve policies to improve QOS Assurance.