ore than 5% of the packets are not received within the delay budget when monitored over a 10-second window. For LTE [11], an outage is counted if more than 2% of the packets are not received within the delay budget when monitored over the whole call. It is emphasized that even though the used outage criteria differ slightly between HSPA and LTE, the performance difference originating from different outage criteria is rather small, being inside the error margins of the simulations. In the UL direction there exists an alternative method to measure VoIP per- formance, which is used in the context of this chapter for HSPA. That is, UL VoIP capacity can be measured as the allowed number of users in a cell with an average noise rise of X dB measured at the base station. The optimal value for X may depend on the used system, and for HSPA the used value was 6 dB. Radio Interface Optimization for VolP The most important mechanisms to optimize air interface for VoIP traffic in (E-)UTRAN are presented in this section. 10.4.1 Robust Header Compression (ROHC) In streaming applications such as VoIP, the overhead from IP, UDP, and RTP head- ers is 46 bytes for IPv4 and 66 bytes for IPv6. For VoIP traffic, this corresponds to about 144-206% header of overhead, assuming that the most typical VoIP codec (AMR 12.2 kbps) is used with 31 bytes of payload. Such a large overhead is exces- sive in wireless systems due to the limited bandwidth and would lead to serious degradations in the spectrum efficiency. In order to overcome this, a robust header compression (ROHC) technique [13] was adopted as a nonmandatory feature to 3GPP specifications from Release 4 onwards-hence, the ROHC feature is supported in HSPA and LTE radio sys- tems as well. With ROHC, the IP/UDP/RTP header is compressed down to a few bytes, with the minimum size being 3 bytes, which is also the most commonly used header size. Hence, with ROHC the header overhead may be compressed down to 25%, implying that the utilization of ROHC leads to significant improvements in t