 Dual connectivity brings a number of benefits for small cell deployments by combining reli- able mobility in the macro layer with the high capacity and enhanced data rates available from the small cells. Despite these benefits, LTE dual connectivity was not implemented in live networks. The number of small cell deployments was too low to justify the com- plexity of dual connectivity. However, the functionality was still an important starting point for the dual connectivity between LTE and 5G. The principle of dual connectivity is illustrated in Figure 16.16. 16.3.5 Millisecond Latency with Shorter Transmission Time Interval Reducing latency is an evergreen quest, similar to increasing bit rates. Low latency is not only driving Transmission Control Protocol (TCP) throughput performance, it is Figure 16.16 Dual connectivity between macrocells and small cells. Macrocell Small cells 16 LTE-Advanced Evolution Frame size Round trip time Figure 16.17 Shorter TTI for faster round trip time. 14-symbol TTI 10-20 ms 7-symbol TTI 5-10 ms 2-symbol TTI <2 ms 0.14 ms also important in reducing, for example, buffering requirements at high data rates in the radio equipment. Even more importantly, low latency determines user experience in interactive applications, and will drive new delay-sensitive use cases. LTE-Advanced Pro tackles the latency problem by reducing the TTI length and optimizing the physi- cal layer control of the air interface resources. Figure 16.17 illustrates the evolution of the LTE frame structure and TTI. A shorter TTI directly scales down the air interface delay. Shortening the TTI by reducing the number of symbols is the most promising approach when seeking to maintain backward compatibility and usability in existing LTE bands. The regular 1 ms TTI gives in practice a 10-20 ms round trip time, while an LTE-Advanced Pro solution should provide a round trip time of even less than 2 ms and a below 1 ms one-way delay. The evolution of the round trip time from High Speed Packet Access (HSPA) to LTE, LTE-Advanc