Level 1 of the OSI model

Formats of data block in physical layer1 (see Figure 3) ISDN differ depending on a block of data sent outside the Terminal (from terminal to network) or within the limits of the Terminal (from network to Terminal). The length of the data block is 48 bits, of which 36 bits are user information. Bits of "F" provide synchronization. Bits "L" regulate the average value of bits. E-bits are used to resolve conflicts when several terminals in a passive bus compete for one channel. Bit "And" activates the device. Bits "S" have not yet been appointed. Bits "B1", "B2" and "D" are intended for user data and management accordingly.

Figure 3-Format of the frame of the ISDN physical layer

Physically to the one circuit many ISDN user devices can be connected. For such configuration the collisions may be result of the simultaneous transfer of the two terminals. Therefore, ISDN provides means to identify conflicts in the communication channel. When a device NT receives bit D from TE it reflects this bit as echo signal back to the neighboring position of E-bits. TE expects that the neighboring E-bits must be the same as the bit D from the last transmission.

Terminals cannot transmit in the D-channel as long as they do not recognize a specific number of units (indicating "no signal"), with predetermined priority. If TE device detects a bit in the channel with echo signal (E) other than its bits (D), it should immediately stop transmission

This simple technique ensures that only one terminal may transfer its D-message. After a successful transfer of D-messages priority of this Terminal becomes lower, it is provided through presentation of the claim before transmissin to detect more units. Priority of the terminal may not increase until all other devices on this line will not be able to send D messages. Telephone connections have the biggest priority than other services, and signaling information takes precedence over the nonsignaled.

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