peed request to the G.711 codec for fax pass- through mode can be signaled using a SIP REINVITE request or using NSE. The NSE option is supported by Cisco voice gateways and the CUBE. Modem Traffic Modem calls are less and less prevalent as many data communications systems move to using pure IP. However, modems are still used in many retail locations for credit card vali- dation systems (point of sale) and across enterprises for monitoring and triggering securi- ty and alarm systems. The main disadvantage of carrying a modem call over an IP 156 SIP Trunking network is the absence of a TDM clock, and therefore high-speed 56 Kbps connections cannot be achieved. Connection speeds of 28.8 Kbps are achievable when the IP network has no jitter or packet loss. Modem traffic can, like fax, be transported over IP networks as either pass-through or relay traffic. SIP trunk providers do not offer modem relay mechanisms, SO the only cur- rent method to carry modem over SIP trunks is as in-band G.711 traffic. For better call success rates, it is recommended to keep modem traffic on TDM PSTN connections until industry interworking for modem over IP improves or until modem applications become native IP applications and no longer need traditional TDM modem technology. Encryption Interworking Border element security features were covered in Chapter 7, including signaling (with Transport Layer Security [TLS]) and media encryption (with Secure RTP [SRTP]). The remaining interworking issue in the security category to discuss is how to interoperate between a network segment that is capable of TLS or SRTP to a segment that is not. Figure 8-5 illustrates three scenarios where such interworking might be desirable: Enterprise VolP SIP-TLS SIP-TLS d I b SP SIP SIP-TLS SIP-TLS d I b Internet Business A Business B Figure 8-5 Secure to Nonsecure Zone Interworking Intra-enterprise application interworking: Connecting SRTP-capable CUCM clusters with other non-SRTP-capable clusters or other applications. SIP trunk interworking: Connecting an SR