SATEL IP-LINK

1. SATEL IP-LINK • Makes wireless static TCP/IP routing possible • Is to be used with SATELLINE-3AS radio modem • Has the stable Linux operating system and specific software for the radio modem • The hardware does not have any moving parts and is therefore very reliable even in extreme conditions. • Has a 66 MHz Motorola Coldfire processor with 8 MB FLASH and 32 MB RAM • The radio modems use state-of-the-art narrow band modulation which makes distances up to 50 km possible

2. An example network with connection to Internet

3. An example network with ”repeater”

4. IP-LINK has two IP addresses; one for the Ethernet and one for the radio modem serial connection. • The IP addresses and routes make it possible to have separate sub networks in different address spaces. • The Ethernet broadcasts are blocked between different IP networks to avoid over loading the radio network • IP-LINKs can be pinged to find out network functionality • The IP-LINK has an random collision avoidance protocol to minimize collisions in the radio network

5. Data in the radio modem network is compressed at transmission and uncompressed in the receiving end. Compression works best if the application uses mostly text data, for example webpages with text. Compressed pictures ( JPG, GIF, PNG) can not be compressed efficiently. • The 64-bit shared key encryption can be used in the radio network to prevent eavesdropping. • The IP-LINK can be configured locally via RS-232 console port or remotely with Telnet connection. • The firewall rules can be configured, the implementation is based on IP Tables for Linux. • NAT makes it possible to hide radio network and the devices behind it.

6. TCP/IP and Radio Modems • The radio modems are often used in polling systems (e.g. Profibus, Modbus, IEC 60870-5-101) • No collisions occur and repeaters are possible. • In Profibus there can be multiple masters but they take turns and transmit one at a time so it is a polling system. • Ethernet is not a polling system. It is a random multiple access system.

7. In Ethernet every station can transmit at any time and the collisions are easy to notice in the cable • With radio modems it is impossible to notice collisions in the air. • To decrease the amount of collisions random transmission delay can be used. This has been done in IP-LINK. This will not totally remove collisions but there are a lot less of them. • Radio transfer has an additional property compared to Ethernet (and cable). All the radios may not hear every other radio, ”the hidden terminal problem”.

8. Radio modems 2 and 1 AND 1 and 3 can hear each others. • If 1 transmits, 2 and 3 will hear it and will not transmit (RX priority in radio modems and CD-line to IP-LINK) -> no collisions happen. • If the station 2 transmits, the station 3 cannot hear it and may transmit at the same time. This results in collision in the radio modem 1 and it cannot receive anything. • The collision depends on the signal strengths. If the signals have difference more than co-channel rejection (usually 20 dBm) the stronger signal will be received. 2 1 3 TX1 TX2 TX3 RX1 RX2 RX3

9. Random transmission and collision avoidance protocol • Before a packet is sent to the radio modem the IP-LINK checks the CTS (hardware handshake) signal from the radio modem. If the CTS allows transmitting a random value is waited. • After the waiting the CD signal is checked before transmitting. If the radio modem CD-signal is ON (RSSI above threshold or data on channel), the IP-LINK waits another random value. This will be tried as many times as is set in maximum retry limit. • The base delay is settable in 10 ms steps, the coefficient can be between 1-255 and the retry limit can be between 1-65000. • The radio modem RSSI threshold must be set with care to avoid noise preventing communication. Especially when CD signal follows RSSI in radio modem. Watch out for the orange CD led in radio modem. 6) Min Time Between Packets 10 7) Max Random Factor for Time 4 8) Packet Retry Limit 500 A) Use CD Signal for TX Control ON

10. TCP is a reliable connection oriented protocol. • Connection must be opened before any data can be sent. • The TCP opens connection to a IP address and port number (e.g 193.94.124.252:80) . • The hardware (MAC) addresses must be resolved in the Ethernet level (Link Layer). • MAC addresses are solved with ARP (Address Resolution Protocol) queries.

11. The TCP reliability means that every received data is acknowledged (ACK’d). • Good -> the lost packets in the radio are retransmitted by TCP • Bad -> The half duplex radio channel is used in both ways which with communication delays leads to reduced transfer speeds. • UDP is connectionless protocol without ACKs and mostly used for streaming audio and video where dropped packets do not matter. • TCP tries to send more and more data after the connection has been established. At first only small amount of datais sent before the are ACK’d but the amount is grown until a collision happens in the radio network and no ACK is received. • The unACK’d datais retransmitted after a timeout. • Timeout depends on transfer delays. The timeout length grows exponentially and can be up to minutes with radio modems. • Timeout growth makes a problem with multiple stations and narrow bandwidth called "TCP Unfairness".

12. Typical TCp transfer with one client downloading a webpage.red = downloadyellow = uploadgreen = download and upload almost simulataneously 25 kHz radio modem has maximum transfer speed of 19200bps (2,4 kBbs) Upload (ACKs) take about 10% of effective transfer speed. Effective transfer speed is usually 70% of maximum transfer speed.