ATM users don't need to worry about data eavesdroppers.
The folks at MFS Datanet, who have offered ATM service domestically for nearly a year and who earlier this year began service between the U.S. and U.K., say they hear common questions about security issues.
One common concern contrasts traditional time division multiplexing (TDM), which keeps data physically separate from other users' data, with ATM, which in effect "mixes" data from various users.
What keeps one company's data from being read by someone else, in this era when ex-Cold War spies have turned to industrial espionage to fill their idle hours and keep food on the table?
Well, while TDM uses time slots to assign users to channels, ATM uses headers or labels to assign traffic to virtual circuits. The ATM protocol includes header verification and error checking to assure that data is delivered only on the appropriate circuit.
In TDM, a frame (or period of time) is divided into a fixed number of equal-duration time slots. One user is assigned one slot or channel, regardless of bandwidth needed. For instance, Fred's data will always be in the time slot right after the framing bit, and in every third time slot after that.
With ATM, however, there is no relationship between a particular user and time slot. Each ATM message begins with a header containing an address. The message is identified by this address rather than by its position in the data stream.
For as long as that address is used, the channel looks like a circuit provisioned through a digital cross-connect system. The bandwidth available to that user can be expanded or contracted, depending on his or her needs.
What TDM and ATM have in common is that both are methods of sharing a communications medium. The difference is essentially that TDM creates separate channels for each user while ATM's addresses create virtual channels.
The technical breakdown is like this: In the header of each ATM cell is the address. This cell header includes a flow control field, then the Virtual Channel Identifier and Virtual Path Identifier, making up the cell address. There are other fields for payload type and cell loss priority, then the final byte of the header, the Header Error Control field. This provides protection against incorrect delivery of cells due to address errors.
When data needs to be transmitted via ATM, the first layer encountered is the ATM Adaptation layer. This enables handling of different service types and divides user information into segments that can be packaged into cells and transmitted.
Once the adaptation layer's sublayers finish packaging data, a physical layer Transmission Convergence sublayer packages cells inside the transmission frame. This sublayer is responsible for generating and receiving transmission frames.
It calculates the header error control information and inserts it into the proper position before transmitting the cell. Also, before handing off a recovered cell to the ATM layer, the sublayer checks the header against the received Header Error Control information and discards the cell if the address bytes are not error-free.
The result of this Transmission Convergence sublayer error-checking assures that no cells with damaged addresses are misdelivered by the network.
Another consideration is that ATM is a "virtual circuit" connection-oriented service. In a TDM setting, a channel is established between two points consisting of physical wires, reserved time slots in a TDM mux, physical cross-point connections in a switch or other real connections. An individual channel can't be shared because there is no way to identify the traffic in the channel.
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|Title Annotation:||Broadband Forum; asynchronous transfer mode|
|Date:||Jul 1, 1994|
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