tailieunhanh - Satellite networking principles and protocols phần 10

Mô tả chấp nhận được đối với hầu hết các dịch vụ và các ứng dụng của người sử dụng có thể chấp nhận cung cấp các quản trị viên nhận thức được thời gian truyền dẫn và tác động của nó trên truyền chất lượng của các ứng dụng của người sử dụng không được chấp nhận cho các mục đích quy hoạch mạng lưới chung, tuy nhiên, chỉ có một số trường hợp đặc biệt vượt quá giới hạn này. | 308 Satellite Networking Principles and Protocols Table Network delay specifications for voice applications ITU-T G114 Range ms Description 0-150 Acceptable for most services and applications by users 150-400 Acceptable provided that administrators are aware of the transmission time and its impact on the transmission quality of user applications 400 Unacceptable for general network planning purposes however only some exceptional cases exceed this limit the communications channel in this case the Internet . Excessive delays will mean that this ability is severely restricted. Variations in this delay jitter can possibly insert pauses or even break up words making the voice communication unintelligible. This is why most packetised voice applications use UDP to avoid recovering any packet loss or error. The ITU-T considers network delay for voice applications in Recommendation . This recommendation defines three bands of one-way delay as shown in Table . On-off model for voice traffic It has been widely accepted that modelling packet voice can be conveniently based on mimicking the characteristics of conversation - the alternating active and silent periods. A two-phase on-off process can represent a single packetised voice source. Measurements indicate that the average active interval is s in length while the average silent interval is s. An important characteristic of a voice source to capture is the distribution of these intervals. A reasonable good approximation for the distribution of the active interval is an exponential distribution however this distribution does not represent the silent interval well. Nevertheless it often assumes that both these intervals are exponentially distributed when modelling voice sources. The duration of voice calls call holding time and inter-arrival time between the calls can be characterised using telephony traffic models. During the active on interval voice generates fixed size packets with a fixed .