identi- fies the ABR control layer, B2B3 = 00 identifies the adjacent node maintenance layer. The B2 bit ("congestion state") and the B3 bit ("cell type") in cells to be switched (B1 = 0) has no significance for the switching actor, which only switches cells according to their inVCI, inVPI and the corresponding outVCI and outVPI, that are set in its control store (similar to how the X.25 VC actor works). The meaning of the B2 and B3 bits is first revealed when a cell is terminated in an AAL. The B2 bit tells the receiving AAL if the channel is congested (B2 = 0) or not 6.4 ATM Cell Switching (B2 = 1). An originating ATM agent sets B2 = 1 when it sends a cell. During transport of the cell through the switching network, any node may change this bit to 0 (to warn the destination ATM agent) if the node experiences a conges- tion situation for the actual VC. 4. The B3 bit is the encoding of a cell type identifier, CTi7 (B3 = 0 means cell type 0; B3 = 1 means cell type 1). This bit is end-to-end transparent, meaning that when an AAL element delivers the payload for a cell, it also delivers a CTi value. The CTi will join the cell through the network and finally be delivered to the terminating ATM element at the other end of the VC as a kind of connection identifier. Thus the switching layer encodes the CTi in the B3 bit, but has no knowledge of what the AAL uses it for. The CTi implies that in every virtual circuit there can exist two permanent sub- streams of cells, one cell-type 0 stream and one cell-type 1 stream. The AAL agents may not use this separation, but it can also be used by a discrimination function in a terminating ATM actor. Figure 6.12 shows how this function works in ATM. B-ISDN media- services stratum element CTI=0,1,0&1 adaptation strata CTI=0,1,08 {data} {VPI,VCI,CTi,data} cell-switching terminating stratum element ATM terminating actor agent Fig. 6.12 CTi discrimination in an AAL. CTi=cell type identifier The discrimination function in the terminating actor is dynamically controlled from the B-ISDN 