Cellular Network Security 309 the Internet makes no assumptions on the content of traffic and simply passes it on to the next node, the cellular network identifies the traffic content and provides a highly tailored service involving multiple service nodes for each type of traffic [26]. Until this gap is bridged, such attacks will continue, but bridging the gap itself is a major process because either the design of the cellular network must be changed to match the Internet design, or vice versa, which is unlikely to happen soon. Hence a temporary fix would be to secure the gateways connecting the Internet and core network. As a last note, Internet connectivity filters attacks not only into the core network, but also into the PSTN network. Hence PSTN gateways must also be guarded. Security Implications of PSTN Connectivity PSTN connectivity to the cellular network allows calls between the fixed and cellular networks. Though the PSTN was a closed network, the security-free SS7 protocol stack on which it is based was of no consequence. However, by connecting the PSTN to the core network that is in turn connected to the Internet, the largest open public network, the SS7- based PSTN network has "no security left" [27]. Because SS7 protocols are plaintext and have no authentication features, it is possible to