Implementation of a wireless ATM Switch

1 Preface
Asynchronous Conversion Mode (ATM) is a communication technology that includes exchange, transmission, networking and other content. It is the core of Broadband Integrated Business Digital Network (B-ISDN. Because of its comprehensive service exchange capability, excellent service quality (QoS), and large capacity, ATM has been widely used in backbone networks. Whether or not ATM can be introduced into the wireless communication field has been or is being studied in China or abroad. This paper introduces a wireless ATM switch implementation method, and describes the hardware and software design based on the instance. It proves that ATM can also be used in the wireless field.
2. vswitch solution design
First, we need to ensure the feasibility of the technical system. Some solutions can refer to the technologies already achieved or researched in depth in the industry as much as possible, and pay attention to the advanced nature of the equipment on this basis. General standards should be adopted in software as much as possible; mature large-scale integrated circuits or even standardized modules may be used in hardware. This not only shortens the development cycle, saves manpower and money, but also facilitates interconnection with other devices, reducing the complexity of interconnection.
Vswitches should have the following features:
(1) diversified access services: wired and extremely wireless (mobile) data, voice and image (Multimedia) services;
(2) A vswitch not only has the access function but also the node function, that is, it can establish a network on its own.
3. Implementation of a wireless ATM Switch
A wireless ATM switch consists of various wired and wireless user network interfaces (UNI), network node interfaces (NNI), and switching units. The specific structure is shown in Figure 1.

3.1 Hardware Implementation
(1) Exchange Unit Design
Considering the actual situation of wireless users, we chose a bus-type exchange system, because: a) the mechanism is relatively simple and easy to implement; B) because each module is connected to the bus through the bus transceiver controller, therefore, it is highly versatile, which is very convenient in practical application. c) the technology is relatively mature and the chip was relatively easy to buy at that time.
At present, there are two Gbit/s bus-based switching chips available on the market, but it is difficult to reach the limit capacity in actual design. Therefore, to implement a larger switching system, it is a good choice to adopt a shared cache switching mechanism, and the versatility between modules is slightly lower.
(2) Central Processing Unit Design
The processor system of the vswitch is divided into two poles. Business Adaptation unit and central processing unit. The central processing unit and each user board transmit control information through the cell bus. The custom request answering machine is used to ensure the correctness of information transmission. The interfaces between the central processing unit and the cell bus are universal ATM test and physical interfaces (UTOPIA level 1 ).
(3) Class 1 Adaptation (AAL1) Business implementation
The ALL1 processing module mainly handles real-time services, such as voice and multimedia. Due to the special nature of the wireless environment, the frequency resources are very valuable. To save the channel bandwidth, the mobile user's voice uses 8 KB/s of G.729. The phone terminal has the signaling function (Q.2931 ). Mobile Multimedia is 16 kb/s low-speed multimedia; wired multimedia is 16-64 KB/s changeable low-speed multimedia. In addition, the switch also provides a 16 kb/s Delta M Digital Voice interface for radio station network users. Because the adaptation standards for low-speed real-time businesses were not perfect at the time of design, and no ready-made chips were available, we chose AAL1 as the adaptation standard for low-speed real-time voice, finally, the software method is used for implementation.
Real-Time and Fixed-bit-rate CBR (voice and multimedia) Service Transmission and Exchange require strict synchronization between the target and the source to completely restore real-time information, we have adopted a simple method, as shown in figure 2.

4) Category 5 adaptation to AAL5) business or signaling disassembly and assembly) Implementation
This unit has two functions: one is to adapt five types of data services; the other is to adapt five types of signals to complete segmentation and assembly (SAR) that is, the AAL5 and the public part converge on the layer CPCS) function. Data services mainly include general data terminals and high-speed sub-networking users.
5) NNI interface or base station E1 access) Implementation
This circuit unit is used to map the standard ATM cells to E1 transmission frames. On the contrary, it extracts the ATM cells in E1 and sends them to the switching unit. Its physical interface is a 75Ω coaxial cable connection. Its electrical properties comply with G.703 standard, HDB3/AMI encoding, the frame format complies with G.704 standard, and the ing between ATM cells and E1 transmission frames complies with G.804 standard. The 1-15 and 17-31 time slot of the E1 Channel carried by the ATM cell. The cell rate is adjusted by inserting an empty cell between the ATM cell. The Byte synchronization is used between the ATM Cell and the E1 transmission frame, instead of frame synchronization.
It is worth mentioning that mobile users connect to the base station through wireless channels, and the interfaces between the vswitch and the station are also 2 Mbps E1 interfaces, which are the same as NNI interfaces, it only works in UNI mode. The channel uses wireless ATM transmission. The channel is TDMA and CDMA.
6) Work Station access implementation
In a specific wireless environment, radio stations have a very important position. It is of great significance to connect radio station users to wireless ATM switches. The difference between a single radio station user and a general communication user duplex is that only one party can send a speech at the same time, while the other party can only receive the speech ). Therefore, when a single radio station user communicates with other users, in order to ensure the correctness and continuity of the communication, the data stream must be checked. The main function of the voice detection board is to calculate the input signal and determine whether it is a voice signal, so as to control the work mode of the radio station. Obviously, the voice detection effect directly affects whether the radio station's work mode is controlled correctly and quickly. Figure 3 shows a voice signal interface. We can see that each channel has two signals, uplink and downlink, and each channel has its own clock. Each signal has a delay in the processing process. To ensure that the processed signal does not cause any loss, various signals need to be stored and forwarded.

Because the voice detection board must perform real-time detection for each of the four channels with eight signals) and the delay cannot exceed 20 ms, the speed requirement on the processor is high, so the digital signal processor DSP is used.
3.2 Software Implementation
Vswitch software mainly includes maintenance software and command.
1) maintenance software
Maintenance software includes initialization of each Board and hardware drive degree; communication program between the main processor and each Access Board, for example, the Access Board reports the working status of the board, and the main processor queries information from each Access Board. Each Board uses the standard cell format to control the cell format when reporting information to the main processor), and encapsulates custom communication information in a certain format in 48 bytes.
2) signaling part
The signaling system of a wireless ATM switch includes two types of signaling: NNI and UNI. NNI signaling supports interconnection and communication between switches, while UNI enables normal transmission of information between the switch and the user. The signaling system references the ITU-T standard.
AAL5, CPCS, dedicated service-oriented connection protocol (SSCOP) and dedicated service coordination function (SSCF) constitute the SAAL layer, that is, the signaling adaptation layer. The implementation of the SAAL protocol is completed by the combination of software and hardware. The hardware implementation of SAR and CPCS is described above. In terms of software:
SSCOP provides Error Control and Error Correction Using retransmission mechanism when transmitting signaling messages. SSCF sets up, releases, and synchronizes Connection Control connections for SSCOP to meet special requirements of various users. When necessary, it sends connection establishment requests multiple times.
The UNI high-level protocol is designed based on the Q.2931 recommendation in the ITU-T. It establishes, maintains and releases the call connection at the user and network interface, and confirms the user parameters to ensure the service consistency; supports point-to-point and point-to-point connections and network resources such as bandwidth allocation through negotiation.
The NNI high-level signaling protocol is divided into two parts: message transmission part MTP-3b) and broadband integrated business user unit B-ISUP. The function configuration of the MTP-3b is basically the same as that of the narrowband MTP-3. On the one hand, the functional entities and the MTP-3b and its upper layer is the B-ISUP), The MTP-3b and its lower layer is the SAAL transferred between the primitive is different, at the same time, some messages are formatted, encoded, and programmed differently from MTP-3. B-ISUP is mainly composed of basic call, single connection control function, additional service function module, network interface and application processing, among which application processing is the most complex. At present, the wireless ATM switch has achieved G.729 telephone wired and wired, wired and wireless, wireless and wireless dial-up communication, signaling in line with the above B-ISDN signaling protocol stack standards.
4 Conclusion
This article briefly introduces an ATM switch used in a special wireless environment and its service implementation. Due to the low channel rate, in fact, the business in the wireless field can only be narrowband services, far from meeting the needs, from a technical point of view, it cannot give full play to the advantages of ATM. However, after all, a communication platform based on a wireless ATM switch is different from that of a new wireless communication network. This will certainly attract various businesses. Moreover, once Trunk Radio is successfully developed, it will in turn promote the further application of ATM in the wireless field. On the other hand, the exchange technology and wireless broadband access technology are developing rapidly today. We should pay close attention to the Mobile IP technology and other new exchange technologies, such as Multi-Protocol Label Switching MPLS.