Download IP Link Presentation

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
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An example network with connection to Internet
2
An example network with ”repeater”
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• 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
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• 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.
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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.
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• 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”.
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• 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
TX1
TX2
TX3
1
RX1
RX2
RX3
3
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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
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• TCP is a reliable connection
oriented protocol.
APPLICATION
LAYER
APPLICATION
APPLICATION
APPLICATION
APPLICATION
TCP
UDP
• 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.
TRANSPORT
LAYER
NETWORK
LAYER
ICMP
IP
LINK
LAYER
ARP
NETWORK
INTERFACE
PHYSICAL
LAYER
PHYSICAL
INTERFACE
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•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 data is 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 data is 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".
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Typical TCp transfer with one client
downloading a webpage.
red = download
yellow = upload
green = 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.
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