Computer Guide

Attaching images into the body of mail
by George Skarbek - 21 February 2006

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Q. Can you tell me how to prevent the graphics in some emails arriving as attachments instead of in the body of an email?

A: This is not set by you, but by the sender. They decide how they wish to attach the images. It can be that they only know one method and always use the same one, so you should ask them to send them in a manner that you prefer. It also depends on what e-mail reader program that you are using.

Q. I recently emailed three high-resolution JPEG pictures to a friend. With my slow dial-up connection, these took almost 30 minutes to send. The three pictures amounted to 4.73 MB, and the email message amounted to 6.487 MB, but 7.5 MB had been sent and 250,000 Kbps had been received. How can this be?

A: Files such as photos and programs are referred to as binary files as they contain special characters, and all binary files must be converted to a text stream for transmission and then decoded at the other end. This encoding adds to the overhead. This can also explain why occasionally you may have received a file that was composed of readable but funny characters and letters. An additional part of the extra bytes is due to the properties of the Internet Protocol (IP), which sends the information in packets. Each packet must have the destination address as well as your address so that acknowledgments can be send back plus a sequence number of the packet and check-sum information to guard against corruption of the packet. As for the received figure, this may be due to re-transmitting some packets if an acknowledgement has not been received in the required time frame, plus the incoming acknowledgments for each packet sent.

Q. At times when there is no obvious activity on my machine, the screen icon indicates communication is taking place between my machine and my ISP. Can you tell me what this might be?

A: I have been able to capture and analyse every byte coming from and going to my "idle" computer. The results show nothing sinister; no constant probing of my computer, nor was any spyware sending out data. More than 96 per cent of the traffic consists of Address Resolution Protocol requests from a router and the dutiful replies from my computer. An Ethernet network uses two hardware addresses that identify the source and destination of each frame sent by the Ethernet. ARP is used by the internet protocol to map IP network addresses to the hardware addresses used by a data link protocol. This is the constant background traffic. I have searched through thousands of captured packets and found no data packets, therefore you should not be charged for this background traffic. The other 4 per cent was data being sent by my server polling for a network computer that was recently connected, but I have since shut it down in order to eliminate any local traffic.

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