S.Kuznetsov, B.Ognev, S.Polyakov

  4.5 KILOMETERS FSO CONNECTION WITH CARRIER CLASS RELIABILITY
  Practical results

  Introduction

  Currently widespread as a means of delivering traffic to the "last mile", and as a backup, emergency, temporary and operational communications were wireless atmospheric optical communication lines (Free Space Optics, FSO). Along with the main FSO advantages (high speed and secrecy of information, speed of deployment, lack of licensing of the optical range, high noise immunity, etc.) are widely known and its main drawback - the dependence of the availability of connections to the weather conditions.

  For example, in the middle of Russia the availability of wireless connections on the criterion of 99.7% to the FSO-equipment "Artolink" M1 FE-2A (Fast Ethernet) is provided at distances only up to 1.3 km . However, there is a need to constantly use FSO on over long distances, but on such conditions the FSO channel availability decreases.

  To maintain high reliability connections offered the use of contingency-based radio systems, broadband Wi-Fi, which, unfortunately, did not give good positive results [1, 3].

  Only the use of specially calibrated solution (based on 5 Hgz Wi-Fi equipment) as backup channel has produced acceptable results in practice.

  As for this article shows results of the six-month field test of the hybrid radio-optical equipment "Artolink" model M1 FE-2A-R (manufacturer - Ryazan State Instrument-Making Enterprise), delivery of which began in early 2008.

  The measurement scheme and method

  Testing line was deployed in 4.5 km length distance.

  On the way the beam is water meadows with the two bodies of water (in the off-season where there are frequent fogs, and in summer - ascending heat flow) and the big road area.

  The equipment M1 FE-2A-R was composed of two serial transmit-receiver modules (TRM) with 100Base-TX interface and supporting dual channel, and calibrated backup channel equipment.

  One TRM was fixed to a stationary base on the roof of the 6-floor industrial building, and the second TRM - by a temporary scheme on the balcony of the house 4-th floor (on a tripod ) .

  Supply was standard - by the external interface devices (EID) with cable length 50 m .

  Measurement scheme is shown in Fig. 1.

Fig . 1. Hybrid channel measuring scheme

  Both TRM's through their first ports were connected to the Fast Ethernet switches, through the second ports - to radio modules, forming a backup channel on the 5.8 GHz frequency.

  On the each channel side tester Ethernet ETest [4, 5] and computer was connected.

  One of the computers (in the Fig.1 is located on the left) to manage the testing process and channel monitoring, and was connected to another Web-camera to monitor the link route and storing images.

  In the continuous equipment operation the following data was recorded:

• Parameters of both FSO-equipment TRM's (working channel type, the temperature inside the TRM, the TRM's target and tracking systems parameters);
• Link trace route images from the Web-camera;
• Packet loss values, the classification of time intervals, equivalent bit errors (BER) values, derived from Ethernet testers ETest [5].

  From the obtained data the primary parameters database was formed.

  Database records period was 3 minutes, and the overall recording time - six months (May-October 2008).

  During all this time, two Ethernet testers ETest permanently produced test traffic generation and analysis of its passage through the communication channel.

  Test traffic was a three minutes sessions of the continuous sending Ethernet packet with 1518 bytes length and the minimum interpacket interval (interframe gap, IFG) according to IEEE 802.3u standard.

  Results

  As an example of primary data Fig. 2 shows the dependence of the equivalent BER in the channel of the time for October 2008

FIG 2. BER level and dynamics of the backup channel switching (October 2008)

  In the Fig.2 in blue identifies the equivalent BER, ETest measured by the method given in [5]. Vertical red lines mark the transition to the backup channel, and vice versa, i.e. the backup channel working time.

  The channels averaged characteristics calculation results for the entire period of observation are listed in the table:

TABLE 1. Average channel characteristics

*«cold» backup means that the backup equipment supply is switch on at moment of the switching on it.

  The table shows that the real results on the availability of only the optical channel of communication were better calculated values: 98.67% instead of the expected 95.5% (calculated coming from FSO line budget and weather statistic ). Perhaps this is due to the fact that the observation period did not come the winter months.

  Using the backup channels on the calibrated solutions basis significantly increase the availability of channels - almost 99.99%.

  This was achievable due to the minimum switching time - no more than 2 seconds.

  The criterions of switch to backup was more than 1E-4 BER value, return to the optical channel - 10 seconds of optical channel faultless.

 In Table. 1 also showed the calculated values to allow for accessibility to the equipment reserve channel is located in the "cold" backup. In this case the switching time increases to 40 s, and the availability of the channel as a result of only 99.7%.

 It should be noted that values the hybrid channel availability directly depend on the optical channel quality, as the integral availability of hybrid channel essentially determined by loss of switch time.

 Using as the main optical channel Artolink FSO equipment allowed us to obtain such high rates for such a long distance, and without taking special measures to secure the equipment, but only realized through technical solutions such as:

• autotracking system, which is in the process of exploitation of the wireless communication channel automatically leads TRM's at each other with maximum accuracy (0.08 mrad), regardless of support mobility;
• the use of three in-phase transmitters with a narrow directivity radiation pattern (0.55 mrad), which is necessary for work at distances over 1 km to combat atmospheric turbulence;
• integrated protection against solar radiation, powerful optical selection of optical interference in the channel reception, as well as small angle field of view of an optical receiver (3 mrad);
• the use of encryption technologies consistent with multiple overlapping characters based on the HQN turbo codes specially adapted for data transmission through the atmosphere mainly in the reduced visibility due to weather (fog, snow). In addition, the receiving path is implemented proprietary technology equipment asynchronous data transfer excluding spurious effects of the phase noise characteristic of systems with a classical PLL;
• built-in errors control mechanism in optical channel with a firmware algorithm implemented in the switching between channels. All these measures helped to minimize the number of switches and the on-backup working time and result in a availability gain of channel .