WIMAN Systems WIMAN2A24 FHSS RF Modem User Manual Introduction to WIMAN technology

WIMAN Systems Inc FHSS RF Modem Introduction to WIMAN technology

Manual6

51 / 90l4 Hardware Installation - All data cables (upper and lower end) - All small external housings (if used) - All antennas on large external housings (if used) - the radio modem designated as synchronization mas-ters, with the additional designation "Sync master", 4.3 Installation at the Radio Tower 4.3.1 Installing the tower standoff at the radio mast 4.3.2 Installing the WIMAN hardware at the tower standoff Attach the WIMAN hardware (external housing and antennas) to the tower standoff at the suitable positions and align. If a WIMAN is to take over the function of the synchronization-master and is not clocked from a remote location, always use the unit aligned to the north (0º) as the synchronization master. 4.3.3 Installation Install all data cables. Connect the data cables with the Hybrid cables coming from the WIMAN radio modems. Attention: For correct function and error-free installation, be sure that the sync cable is installed before testing. 4.3.4 Start tests Now test all installed components on correct function and instal-lation. You find a specification of the tests in chapter 6 on page 58. 4.3.5 Checking the antenna adjustment • Check to see that all antennas are facing the correct direc-tion. • Note the adjustments of all antennas. 4.3.6 Test all devices • Make sure all devices on the radio tower are switched on and are connected to the synchronization cable. • Test each WIMAN radio modem again (see chapter 6, page 58.) to make sure that there is no error caused by the synchronization cable.
52 / 90 4 Hardware Installation 4.3.7 Save all configuration data of the WIMAN radio mo-dems at the radio tower • Use the same name conventions used for the cables and WIMAN radio modems. • Record the following information with a terminal program: - Parameter („show“-command) - Statistics („stat“-command) 4.4 Grounding It is extremely important to ground all installed devices on the radio tower. This will reduce the amount of damage should lightning strike. The following steps will also help to reduce possible damage caused by lightning: • Do not mount the WIMAN at the highest point of the radio tower. This is the point most likely to be struck by light-ning. • Check that the outdoor housing and the tower standoffs form a well-grounded metal-on-metal connection with the tower frame. • Avoid using rubber washers or seals. • Install lightning protection devices between the data cable and the hybrid-sets on both the top and bottom of the tower. • Ground the data cable to the tower at (a minimum of) three different places. (1) to the center of the tower, (2) to the base of the tower where the cable bends (before the bridge from the tower to the shed) and (3) before the cable runs into the equipment shed. The best way to do this is to strip away the outer casing of the cable and affix a grounding clamp to the cable shielding, then connect this clamp to a second one which is fixed to the tower. • Make sure that all equipment (Switches, Routers, etc.) at the base of the tower is properly grounded to the rack in which it is mounted. Also make sure that the rack itself is properly grounded.
4 Hardware Installation 53 / 90l4.5 Burst-Synchronisation Burst-synchronization is the coordination process of frequency hopping tables, receipt, and points of transmitting time for sev-eral WIMAN networks within the same geographical area. Burst-synchronization is achieved by both hardware and soft-ware items. The hardware item is a synchronization cable, which is only a wire, which connects the X.21-interfaces among them-selves. For the X.21 Interface, the synchronization cable is enclosed in the hybrid cable type 3. This is connected to further radio mo-dems with additional cables and special T-connectors. The software section for synchronization consists of the parame-ter SyncMode, which is to be entered in the basic configuration of a master or a Slave. One master radio modem (STAR or LINE) is determined as synchronization master for all radio modems at that location. The synchronization master is adjusted as follows: • SyncMode = Master All further master radio modems should be adjusted as follows: • SyncMode = Slave 4.6 Extended Point-to-Point Connections An estendet Point-to-Point connection can be structured by ar-ranging two WIMAN LINE “back-to-back”. For this applica-tion, additional hardware is necessary. Please contact your WIMAN supplier. For an extended point-to-point connection the parameter SyncMode has to be set to the base WIMAN LINE configura-tion. The example configurations specified below refer to an ex-tended X.21 Point-to-Point-connection. In this structure, the WIMAN LINE Slave 1 is coupled to the WIMAN LINE Master 2.
4 Hardware Installation 54 / 90 Line Master 1 Line Slave 1 NetId = 1 NetId = 1 NodeId = 0 NodeId = 1 Destination = 1 Destination = 0 SyncMode = (according to local Network) SyncMode = Master Table 1 parameters of an extended point-to-point connection (connection 1) Line Master 2 Line Slave 2 NetId = 2 NetId = 2 NodeId = 0 NodeId = 1 Destination = 1 Destination = 0 SyncMode = Slave SyncMode = (according to local Network) Table 2 parameter of an extended point-to-point connection (connection 2)
5 Reception quality and transmission speeds 55 / 90l5 Reception quality and transmission speeds For the examination of the receipt quality as well as to error de-tection, test loops can be generated. The type of test loop can be influenced by the configuration of the parameters LoopData, LoopMode and LoopTest. The parameter LoopData enables the setting of the Byte-values that are to be generated (see page 18). This parameter can be produced on a WIMAN ACCESS only. The parameter LoopTest enables a switching to a test loop, with which the data, which can be transmitted, is produced independ-ently by the WIMAN radio modem (see page 19). This test loop can already be activated in the lowest authoriza-tion level and is, in combination with the statistics analysis on the wireless interface, an outstanding inspection procedure for radio communication. The parameter LoopMode determines, which bit pattern will be transferred with the back loop in the loop test operation from the WIMAN radio modem (see page 18) Attention: If the back loop test is execute in an operating radio net, avoid all values except normal. Use of any other value may result in loss of performance. 5.1 Configuration of a TestLoop with Independently Generated Data Communication 1. Access the command line level of authorization level one (see chapter 2 on page 13) and change into the configura-tion mode. You will see an output similar to: WIMAN-II (config) > 2. Type in the command looptest true. 3. Check with show looptest the value for the parameter Looptest. This should now be switched to true. The follow-ing output appears: WIMAN-II (config) > show looptest LoopTest false ( true) The present and the new configuration (in parentheses) of the parameter are displayed.
5 Reception quality and transmission speeds 56 / 90 4. To take over the modifications into the current configura-tion leave the configuration mode by entering the command exit (see page 27). The following output appears: Configuration changed, do you want to save (y)es / (n)o / (c)ancel ? You now have the choice: - To transfer the new configuration into the current con-figuration and to leave the configuration mode by pressing the key „y“, - To discard the modification but to leave the configura-tion mode anyway by pressing the key „n“ or - To not take over the modification into the current con-figuration but to stay in configuration mode by press-ing the key „c“. Press the „y“ key to activate the looptest. The modification of the parameter becomes part of the current configuration and the WIMAN starts transmitting bit samples. 5.2 Test after a Radio Tower Installation with Syn-chronisation After all devices are correctly installed, a final test must be exe-cuted. This final test checks if all devices are installed correctly and whether a trouble free transmitting and receiving mode is possible. • Switch on the first WIMAN radio modem. Always begin with the WIMAN determined as synchronization master. • Radio test - If the synchronization Master is a WIMAN STAR or a WIMAN LINE Master, conduct a loop back test from a properly configured WIMAN ACCESS or LINE Slave. - It the synchronization Master is a LINE Slave, conduct a loop back test from its LINE Master. • Check the X.21-interface by connecting the X.21-plug of the Hybrid cable type-2 to the Router.
5 Reception quality and transmission speeds 57 / 90l- For the WIMAN STAR enter the commands „stat wl“ and „stat wl<nodeID>“ to ensure proper functioning of the data exchange. - For the WIMAN LINE enter the command „stat sync“ to check the setting of both signals (both signals have to be set to „On“). 5.2.1 Continue the Tests • Switch on the WIMAN radio modem next to the synchroni-zation master. • Switch off the synchronization master. • Perform a reception test. • Check the X.21-interface (see chapter X.21-Test above). • Switch the synchronization master back on. • Perform another reception test to make sure the synchroni-zation cable does not produce any errors (the radio statistics should not differ substantially from the preceding ones). 5.2.2 Test the Remaining Modules • Switch on the next WIMAN. • Switch off all WIMAN radio modems that were tested be-fore. • Perform a reception test. • Check the X.21-interface (see chapter X.21-Test above). • Switch on all WIMAN units that were tested before. • Connect the synchronization cable to the last tested WIMAN radio modem. • Perform another reception test to make sure the synchroni-zation cable does not produce any errors (the radio statistics should not differ substantially from the preceding ones). 5.3 Transmission Speeds 5.3.1 FTP-Download from an FTP-Server The maximum transmission speed of the WIMAN radio modem at optimum conditions is about 25 … 30 Kbytes/s at 2FSK and about 55 … 62 Kbytes/s at 4FSK (depending on the extend of utilization of the network).
58 / 90 6 Frame Relay 6 Frame Relay 6.1 Technical Description of the Frame of Relay Fea-tures The WIMAN STAR supports the multiplexing of Frame Relay packages. In multiplexing procedure, the packages received from the Frame Relay Switches are transferred to the WIMAN ACCESS, which is connected to a Frame Relay Router at the user’s site. Figure 10 Frame Relay connections with the WIMAN For addressing the WIMAN ACCESS the DLCI number (Data Link Connection Identifier) of the Frame of Relay protocol is used. The following restrictions apply to the Frame Relay support: • Only static connections are supported (PVC = Permanent Virtual Connection) • DLCI numbers must be configured statically on the Frame Relay Switch and the Frame Relay Router • 2-, 3- or 4- Byte-Frame Relay-address-arrays are supported, • Since the WIMAN node address is embedded in the DLCI number (10-bit DLCI with implemented WIMAN node identifier), the DLCI allocation of numbers is reduced • Up to four virtual connections are supported for each Frame Relay user • Up to 250 Frame Relay users are supported at a WIMAN STAR (currently 9 users possible, Software Version T0.7). • The following Frame of Relay features are not supported:
59 / 90l6 Frame Relay - LMI (Local Management Interface of ITU-T Q.933 or ANSI T1.617), since this procedure uses DLCI 1023 or DLCI 0, - Establishing of connections for SVCs (uses DLCI 0), - Multiple transmissions (uses DLCI 1019 ... 1022). 6.1.1 Frame Relay-Address array Table 3 shows the structure of the 2 Byte long Address array: 8 7 6 5 4 3 2 1 DLCI 10 DLCI 9 DLC I 8 DLCI 7 DLCI 6 DLCI 5 C/R EA DLCI 4 DLCI 3 DLCI 2 DLCI 1 FECN BECN DE EA Table 3 Structure of the 2 Byte long address array Explanation: • DLCI Data Link Connection Identifier • C/R Command Response Bit • EA Address Array Extension Bit • FECN Forward Explicit Congestion Notification • BECN Backward Explicit Congestion Notification • DE Discard Eligibility Indicator The node address of the WIMAN is determined by the high or-der-bits (DLCI 03...DLCI 10) of the DLCI number. The low or-der bits (DLCI 1...DLCI 2) are used for virtual connections. 2-Byte-Adressarray
60 / 90 6 Frame Relay The LCI value for the Frame Relay Router of the user is calcu-lated as follows: DLCIm = 512 + NodeId * 4 + m m = [0 ... 3] Table 4 lists the valid DLCI numbers for appropriate node iden-tifiers (NodeId) on use of the 2-Byte-Frame of Relay address ar-ray. WIMAN NodeId DLCI array Note 0 512 – 515 reserved (WIMAN STAR) 1 516 – 519 2 520 – 523 3 524 – 527 4 528 – 531 5 532 – 535 6 536 – 539 7 540 – 543 8 544 – 547 9 548 – 551 10 552 – 555 11 556 – 559 12 560 – 563 13 564 – 567 14 568 – 571 15 572 – 575 Table 4 NodeId with 2-Byte-Frame Relay address array In the following, the implementation of the Frame of Relay Pro-tocol within the WIMAN software is listed briefly. Exclusively the static software-Version of the WIMAN STAR supports the Frame Relay Protocol with the following characteristics: • The maximum size of the Frame Relay information field amounts to 4096 byte. • The WIMAN star rejects Frame Relay framework with in-valid DLCI number (transmitter and receiver). DLCI value calculation Frame Relay-support of the WIMAN Software
61 / 90l6 Frame Relay 6.1.2 DLCI-areas when the 2-Byte-Address array is used (ITU Q.922) Table 5 lists the allocation of the DLCI numbers on use of the 2-Byte-address array. DLCI-area Meaning 0 Signalizing in the transmission channel, if necessary 1 – 15 Reserved 16 – 511 Network option: on not-D channels, usable for the support of user information 512 - 991 logical connecting identifier for the support of user information (the use of semi permanent connections can reduce the DLCI numbers available within this area) 992 - 1007 Layer 2-Management of Frame-transport services 1008 reserved 1023 Layer 2-Management in the transmission channel if necessary (only usable without d-channel) Table 5 DLCI allocation in connection with 2-Byte-address array DLCI Range 10 9 8 7 6 5 4 3 2 1 0 0 0 0 0 0 0 0 0 0 0 1 - 0 0 0 0 0 0 0 0 0 1 15 0 0 0 0 0 0 1 1 1 1 16 - 0 0 0 0 1 0 0 0 0 0 511 0 1 1 1 1 1 1 1 1 1 512 - 1 0 0 0 0 0 0 0 0 0 911 1 1 1 1 0 1 1 1 1 1 992 - 1 1 1 1 1 0 0 0 0 0 1007 1 1 1 1 1 0 1 1 1 1 1008 - 1 1 1 1 1 1 0 0 0 0 1022 1 1 1 1 1 1 1 1 1 0 1023 1 1 1 1 1 1 1 1 1 1 Table 6 bit sequence for different DLCI identifiers
62 / 90 6 Frame Relay 6.2 Frame Relay-configuration samples 6.2.1 Sample configuration with CISCO-Routers StarAccess 1Cisco 1005Cisco 1005 Service-NotebookGW192.168.33.0 /24.3.20 .1 .2 (515).3 (519)192.168.40.128 /29.129.130.34(515).33(515)Access 2 Service-Notebook.4 (523)192.168.40.136 /29.137.138.38(515).37(515)Access 9 Tiny Router Service-Notebook.11 (519)192.168.40.240 /29.195.194.66(515).65(515)Cisco 1005D 516D 520 Figure 11 Simple WIMAN Network with CISCO-Router CISCO1, connected to STAR unit: ! Configuration Cisco Star ! no service password-encryption no service udp-small-servers no service tcp-small-servers ! hostname Cisco_Master ! enable password wiman ! ip subnet-zero ! interface Ethernet0 ip address 192.168.33.20 255.255.255.0 ! interface Serial0 no ip address encapsulation frame-relay IETF no keepalive no fair-queue ! interface Serial0.1 multipoint ip address 192.168.40.1 255.255.255.224 no arp frame-relay frame-relay map ip 192.168.40.2 515 frame-relay map ip 192.168.40.3 519 frame-relay map ip 192.168.40.4 523 frame-relay map ip 192.168.40.5 527
63 / 90l6 Frame Relay frame-relay map ip 192.168.40.6 531 frame-relay map ip 192.168.40.7 535 frame-relay map ip 192.168.40.8 539 frame-relay map ip 192.168.40.9 543 frame-relay map ip 192.168.40.10 547 frame-relay map ip 192.168.40.11 551 ! interface Serial0.2 point-to-point ip unnumbered Ethernet0 no arp frame-relay no cdp enable frame-relay interface-dlci 516 ! interface Serial0.3 point-to-point ip unnumbered Ethernet0 no arp frame-relay no cdp enable frame-relay interface-dlci 520 ! interface Serial0.10 point-to-point ip unnumbered Ethernet0 no arp frame-relay no cdp enable frame-relay interface-dlci 548 ! ip classless ip route 0.0.0.0 0.0.0.0 192.168.33.3 ip route 192.168.40.32 255.255.255.252 Serial0.2 ip route 192.168.40.128 255.255.255.248 Serial0.2 ip route 192.168.40.36 255.255.255.252 Serial0.3 ip route 192.168.40.136 255.255.255.248 Serial0.3 ip route 192.168.40.64 255.255.255.252 Serial0.10 ip route 192.168.40.192 255.255.255.248 Serial0.10 ! line con 0 exec-timeout 0 0 line vty 0 4 exec-timeout 0 0 password wiman login ! end CISCO2, connected to ACCESS01: ! Configuration Cisco Access 01 ! no service password-encryption no service udp-small-servers no service tcp-small-servers ! hostname Cisco_Access_01 ! enable secret 5 $1$9xE0$1jVP/hVttHmwhWi/b1Dzv0 ! ip subnet-zero ! interface Ethernet0
64 / 90 6 Frame Relay ip address 192.168.40.129 255.255.255.248 ! interface Serial0 no ip address encapsulation frame-relay IETF no keepalive ! interface Serial0.1 point-to-point ip address 192.168.40.33 255.255.255.252 no arp frame-relay no cdp enable frame-relay interface-dlci 515 ! interface Serial0.2 point-to-point ip unnumbered Ethernet0 no arp frame-relay no cdp enable frame-relay interface-dlci 516 ! ip classless ip route 0.0.0.0 0.0.0.0 Serial0.2 no cdp run ! line con 0 exec-timeout 0 0 line vty 0 4 exec-timeout 0 0 password wiman login ! end Configuration STAR: ## WIMAN II configuration file # # Air Antenna 8mn360 MaxNodeId 1 MaxRetry 9 NetId 250 RadioPower Normal # Serial ConBaudrate 9600 ConDataBit 8 ConHandShake Soft ConPageSize 24 ConParity None ConStopBit 1 PS1 WIMAN_Star SerBaudrate 2048000 SerCRC 16 SerEncode NRZ # Network IPDefaultGW 192.168.40.1 IPEthAddress 0.0.0.0 IPEthMask 255.255.255.0
65 / 90l6 Frame Relay IPSerAddress 192.168.40.2 IPSerMask 255.255.255.224 IPTFTPServer 192.168.33.178 Location Area_01 # Sync SyncMode Off passwd enable crypt Av/WbhGC.i1HA3E Configuration ACCESS01: ## WIMAN II configuration file # # Air Antenna 8mn360 LoopData FF LoopMode Long LoopTest False MaxRetry 9 NetId 250 NodeId 1 RadioPower Normal # Serial ConBaudrate 9600 ConDataBit 8 ConHandShake Soft ConPageSize 24 ConParity None ConStopBit 1 SerBaudrate 2048000 SerCRC 16 SerEncode NRZ # Network IPDefaultGW 192.168.40.1 IPEthAddress 0.0.0.0 IPEthMask 255.255.255.0 IPSerAddress 192.168.40.34 IPSerMask 255.255.255.252 IPTFTPServer 192.168.33.178 IPWLAddress 192.168.40.3 IPWLMask 255.255.255.224 Location Area_1 # Sync SyncMode Master passwd enable crypt Av/WbhGC.i1HA3E
7 Troubleshooting 66 / 90 7 Troubleshooting 7.1 Techniques and Methodologies Used for Trou-bleshooting 7.1.1 General Problems This section lists some common problems that may occur and cause a malfunction in the WIMAN system: Bad RF-Link between STAR (Master) and ACCESS (Slave): • STAR units are not synchronized ! see stat sync • Bad hardware on the STAR or the ACCESS - Check RF statistics from the STAR to other ACCESS de-vices ! see stat wl ext If all other connections are functioning properly, STAR is not defective. • If the star is working correctly consider the following ques-tions: - Is the ACCESS device configured correctly (correct STAR, correct sector)? - Are there any obstacles between the STAR and the ACCESS? - Is the antenna cable attached correctly? - Is the antenna adjustment correctly? - Is the ACCESS device itself defective (defective transmit-ting or receiving part)? ! If so, exchange the device. - Are two ACCESS devices within a network configured with the same NodeID? Check the ACCESS configuration, the network configuration and the documentation of the other ACCESS radio modems within in the same network. No data communication from the STAR to the ACCESS: • Check the radio connection between the ACCESS and the STAR. • Check the wiring of the STAR and ACCESS - Check the other radio modems attached to this STAR If data can be transmitted to the other ACCESS de-vices then the wiring at the STAR is OK.
7 Troubleshooting 67 / 90lIf you are still uncertain whether there is a problem with the wiring of the STAR, proceed as follows: Check the statistics with the commands stat serial ext and stat wl<NodeID>. If you transmit a Ping, the Rx and Tx-counter should be increased. Check whether the data cable is wired according to the specifications shown in chapter 10, page 85. Check all modules for correct wiring. Check the hybrid cables. Check the interface converters. Check the cross over cables. - Check the wiring on the ACCESS: Check the statistics with the commands stat serial ext (see page 49) at the ACCESS-side. If you transmit a Ping, the Rx and Tx-counters should increase. Check whether the data cable is wired according to the specifications shown in chapter 10 on page 85. Check the wiring of all modules. Check the hybrid cables. Check the routing tables. • With the instruction stat hw compare the serial number en-tered in the device table (peer-table) with the actual serial number of the device. If the serial number does not match, all data packages will be discarded. Enter the following to delete an existing entry in the device table: PEER <NodeID> <ENTER> Afterwards reset the device. • Check whether the looptest at the ACCESS radio modem is still active (show looptest). The Parameter „LoopTest“ must be set to „false“. • Defective Router at the customer side: - Check the configuration - Check the Hardware • PVC - PVC was built on the wrong port - PVC was built with wrong DLCI (according to the appro-priate NodeID of the ACCESS) • Routing tables
7 Troubleshooting 68 / 90 • Bad port on the switch - Check other customers who are attached to the same STAR - In case no further customers are attached to the same STAR, try attaching the device to another port. WIMAN Baud rate parameters are not adjusted correctly: If the WIMAN radio modem does not interface with the termi-nal program, the Baud rate may be set incorrectly on the WIMAN and/or the terminal program. Frequency table adjusted incorrectly If the WIMAN Slave cannot construct synchronized connections and you are using generated frequency tables, check that the pa-rameter "FtabMode" is adjusted to "user". Make sure that all parameters are configured correctly. Parameter destination not adjusted correctly (LINE only) This situation cannot occur after a loop test. If the Socket pro-gram cannot structure a connection, check the network and en-sure that all parameters "destination" are set to the correct value. Baud rate in the Socket program not set correctly If the Socket program cannot construct a connection, it could be that the Baud rate is set incorrectly on the Socket program and/or the WIMAN. Parameter NodeID not set correctly If an ACCESS radio modem receives synchronization impulses, but no data can be transmitted, it could be that the parameter NodeID is adjusted incorrectly. The double assignment of a node number in the same network leads to malfunctioning. Serial number does not correspond with the device table (Peertable) If the WIMAN STAR is adjusted to a serial number that differs from the one used in the Peertable, malfunction may occur. IP-Parameter in the Socket-program not set properly If the Socket program over the ACCESS radio modem cannot construct a connection, check whether all IP parameters are ad-justed correctly. Damaged or defective antenna cable Damaged or defective synchronization cable
7 Troubleshooting 69 / 90lAny of these problems may lead to poor or no radio communica-tion. Check the antenna cables for damages. If there are no dam-ages, check the synchronization connection. If the problem per-sists, the WIMAN may need to be replaced. 7.1.2 Troubleshooting with Radio Tower Installations • If the WIMAN radio modem can not be accessed over the RS-232-interface, the problem may be caused by: - A non-corresponding Baud rate of the terminal program and the WIMAN (usually the Baud rate is adjusted to 9600 Baud) - Incorrectly attached cables - Faulty Hybrid-2 or Hybrid-3-cables. Exchange the Hybrid-2-cable first and then the Hybrid-3-cable (if necessary). - Faulty contacts inside the data cable plug. Check the con-figuration and transmission with an extra 25pin data ca-ble. • In case the ACCESS can get no RF-synchronization signal (indicated by the Status-LED at the front side of the unit) or if the synchronization signal reception is periodically inter-rupted, the problem may be caused by: - Incorrect configuration of the ACCESS or STAR. Check whether all parameters are correct. - The operating voltage at the star radio modem is too low. If the operating voltage at the WIMAN radio modem drops below the given threshold value, a restart is performed automatically. It is advisable to constantly apply a volt-age at the radio modem by at least 12V. - Defective or unattached antenna cable, - A Faulty Hybrid-2 or Hybrid-3-cable. Replace the Hybrid-2-cable first and then the Hybrid-3-cable (if necessary). - Defective RF filters. - Defective WIMAN STAR or ACCESS. - Faulty contacts inside the data cable plug. Check for perfect configuration and transmission with an additional 25pin data cable. - Defective synchronization cable (short-circuit in the plug) of and to the testing device. - Defective T-connector (short-circuit)
70 / 90 Appendix A: WIMAN Hardware 8 Appendix A: WIMAN Hardware !!!!!!!!!!Still being revised!!!!!!!!!!!!. 8.1 Technical description Below you will find pictures of the WIMAN units: Figure 12 front side of the WIMAN radio modem Figure 13 rear side of the WIMAN radio modem

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