||Voice over Internet Protocol (VoIP) is a rapidly developing technology. Most cable, broadband and phone service providers are planning to start adding Internet telephony service to their standard packages. Despite hardware constraints, the operators have to tackle the problem of providing voice quality over the current Internet. VoIP is a time-sensitive application and requires real-time support for its quality of service (QoS) requirements. The traditional Internet, which uses a best-effort mechanism, fails to support the QoS requirement of most multimedia application like VoIP. Differentiated Service (Diffserv) is a scheme designed to support multimedia QoS requirements in a scalable manner. Two per-hop-behaviors (PHB): Expedited forwarding (EF) and Assured Forwarding (AF) have been defined for Diffserv. They are designed to provide low loss, low latency end-to-end service and assured bandwidth service respectively. In addition, AF is capable of being configured as a low latency service. In this thesis, simulations of VoIP on both PHBs are carried out and the resulting performance improvements over best-effort service are investigated. The performance in delay, jitter and effect to background traffic are examined. EF PHB is capable of providing guaranteed delay transmission; however, it is not able to utilize the reserved resources. Alternatively, AF is a good means to provide acceptable latency end-to-end service and with good utilization in bandwidth reservation. This thesis is aimed to compare their performance in terms of latency and fairness.