Multi-link TGs are supported only between T4 nodes. Refer to Figure 37-1 for an illustration of the relationship of TGs with respect to other common SNA routing components within the context of a subarea routing environment. IBM SNA Explicit and Virtual Routes Routes between subareas assume either an explicit or virtual route form. Explicit routes are the physical connections between two subarea nodes and serve as the ordered sequences of subareas and connecting transmission groups. Explicit routes are unidirectional, and two explicit routes are required to create a full-duplex path.
Virtual routes are two-way logical connections formed between two subarea nodes.
A virtual route flows over both an explicit route and a reverse explicit route that follows the same physical path. Virtual routes include values defining transmission priority and global flow control, which is provided by pacing, where a receiver with sufficient buffer space grants pacing windows to the sender. Each pacing window enables the sender to transmit a certain amount of information before the sender must request the next pacing window. Refer to Figure 37-1 for an illustration of the relationship between explicit routes and virtual routes, and their relative position in the context of an SNA subarea routing environment. Depending on user requirements, different COSs can be specified in an SNA network.