Service manual AlphaCom XL transceiver Rev. 1.4 10-08-2011 Blz. 70/74
and TCP divides the byte stream into appropriately sized segments. TCP then passes the resulting packets to
the Internet Protocol, for delivery through a network to the TCP module of the entity at the other end. TCP
checks to make sure that no packets are lost by giving each packet a sequence number, which is also used to
make sure that the data are delivered to the entity at the other end in the correct order. The TCP module at
the far end sends back an acknowledgement for packets which have been successfully received; a timer at
the sending TCP will cause a timeout if an acknowledgement is not received within a reasonable time, and
the (presumably lost) data will then be re-transmitted. The TCP checks that no bytes are damaged by using a
checksum; one is computed at the sender for each block of data before it is sent, and checked at the receiver.
The TCP protocol can be used to transmit alarm messages. However, the security mechanism used by TCP
is not reliable enough to be 100 percent sure that a message is received. Checking this must be done by the
application software. Since TCP requires an established connection, people might scan the receiver side for
active ports and take advantage of this knowledge.
UDP
UDP is the second of the core protocols used on the internet. The User Datagram Protocol functions at the
same level as TCP, but uses an other technique of transmitting. Unlike TCP, the UDP does not use
checksums. At UDP level, the transceiver side is not informed on whether or not the sent package is
received. The advantage this gives is that the transmission speed is much higher (up to 4 times). UDP is
therefore mostly used by applications which need a fast transmission and a short reaction time and which
does not require an active check on the reception of the messages. Applications which often make use of
UDP are video conferences, Domain Name Services or online games.
One would say that in security applications, TCP would be preferred over UDP. However, security
equipment should not rely on the underlying UDP and TCP layers and the reception check must be made by
the application software. So, the additional check with a checksum as used by TCP becomes unnecessary,
making UDP and TCP equal for security purposes. UDP now gives the advantage of a higher transmission
speed and not the need for an established connection, making it harder to trace the active port.
WAN
A wide area network or WAN is a computer network covering a wide geographical area, involving a vast
array of computers. This is different from a local area network (LANs) that is usually limited to a small area.
The best known example of a WAN is the Internet.
WEBSERVER PORT
For information over a PORT, see PORT. A webserver port is the port number for a web application. A web
application listens to the programmed port number to see if information is coming in, for example „ show me
the homepage‟. Most applications use webserver port 80. This is the reason why we advise to not use port
80, but to use port 81 or 82.
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