Specifications

Windows provide an application protocol above TCP/IP called Server Message Block

(SMB) or common internet file sharing protocol (CIFS), as it is called now.

5.3.1.1 The server message block (SMB)

Server message block is a protocol that allows other operating systems to interact with

the Windows file system. It incorporates the same multi-user read-and-write operations,

locking, and file-sharing semantics that are used as if you were operating with the files

from the local machine.

This seemed like a very good solution. It meant that it was possible for the MP3 player

to directly talk to Windows. The main problem was that the Ethernut operating system

did not have this support built in. Consequently, it would have to be integrated into the

operating system as an application layer above TCP. Two options were available: either

develop the layer or find a suitable implementation and port the code to operate with the

Ethernut. A freely available implementation of an embedded version of SMB was found

from CodeFx [21]. It is free to develop with but if it was used in a product that was to

be sold licensing fees would apply. At a glance, it did appear that it was possible to use

this solution but there were two problems. Is the Ethernut fast enough and does it have

enough memory to operate this extra load. At this design stage it was estimated that

there would be about 25Kbytes for MP3 buffering and 40Kbytes per second network

speed might be possible once the MP3 decoder is integrated as well. It was decided at

this stage that a large amount of time could be spent developing this layer but there was

good chance that the resources required were not available on the hardware. An

alternative had to be found.

5.3.1.2 The Server program

After investigating how other embedded networking systems interacted with Windows,

it was found that they most commonly ran a server program on one of the computers on

the network to handle file operations. If the player could not handle these functions

themselves then the processing would have to be done on something with which they

can communicate. A server program was required to convert communication with the

player into the required Windows file operations.