ach other, as illustrated in Figure 2.23, case 'A'. This was and is the typical approach in data-enabled mobile handsets. In these type of device the interface between MT and ME can be slightly 'looser' and utilize standardized channels and ATtention (AT) command sets - very much like traditional dial-up modems did [56]. The interface can also be 'tighter' and fully closed and/or using proprietary methods. An important property of case 'A' is that the TE is typically very aware of the MT's characteristics. Some UEs have evolved a bit from the original model and have switched to using a more distinct interface between TE and MT, for example, presenting TE to MT as an Ethernet interface (see more about Ethernet in Section 3.6.2). This approach is illustrated in Figure 2.23 case 'B' with interface I (these interfaces have been given illustrative names that are not used elsewhere). What is noteworthy in case 'B' is that it may be easier to support the MT in the operating system running on the TE. However, this approach causes TE to lose some visibility into MT, unless quite a bit of additional control software is used to bind the MT and the TE together. In this approach, differences in link models used between the TE and the MT and between the MT and the 3GPP network can cause issues, as we will further discuss in Section 4.9.1. However, the model of case 'B' can be helpful if the same type of interface is also used for interfacing external TEs. 2.6.2 Split-UE With the emergence of fast data connections on 3GPP accesses, it has become popular to share the cellular data connection with other devices via some local connectivity technol- ogy. We call this activity tethering throughout this book. The local connectivity method is typically WLAN or Universal Serial Bus (USB), but it can also be infrared, Blue- tooth, low-power radio technology, or anything human imagination can produce. With Figure 2.23's case 'C' interfaces Iel and Ie2 we illustrate the tethering scenario with a UE, case A UE, case B UE, case C Figure 2.2