Lecture4 traffic in telephone network

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Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network Lecture4 traffic in telephone network

Bài giảng Mạng Viễn thông (33BB) Trần Xuân Nam Khoa Vô tuyến Điện tư Học viện Kỹ thuật Quân sự Bài Circuit-Switching Networks Traffic and Overload Control in Telephone Networks Traffic Management & Overload Control   Các cuộc gọi điện thoại đến và Hoạt động của người tuân theo các mẫu nhất định:     Các ngày lê    Văn phòng: Giữa buổi sáng & giữa buổi chiều Ở nhà: buổi tối Kỳ nghỉ he Quốc tế Phụ nữ, Giao thừa, Tết … Thiên tai & các sự kiện khác gây biến động đột ngột về traffic Cần có traffic management & overload control Trần Xuân Nam/HVKTQS Traffic concentration Many lines      Fewer trunks Thông thường người thuê bao nội hạt chia sẻ chung đường trung kế tốc độ cao, đắt tiền đến nới khác Số trung kế sư dụng thay đổi theo thời gian, thường nhỏ số đường dây thuê bao Multiplexer được sư dụng để tập trung các yêu cầu kết nối vào các trung kế Mục đích là sư dụng tối đa trung kế, với số % nghẽn xác định Yêu cầu kết nối bị nghẽn không có trung kế rỗi Trần Xuân Nam/HVKTQS Traffic concentration Many lines  Traffic biến động mỗi có cuộc gọi được khởi tạo & kết thúc   Chịu ảnh hưởng của hoạt động người Cung cấp tài nguyên (trunks) cho    Fewer trunks Các yêu cầu cuộc gọi được đáp ứng thì quá đắt Các yêu cầu cuộc gọi được đáp ứng phần lớn thời gian hiệu quả Bài toán thiết kế: chọn số trunks thích hợp cho xác suất nghẽn dưới ngưỡng cho trước Trần Xuân Nam/HVKTQS Biến động về Chiếm Trung kế Number of busy trunks N(t) All trunks busy, new call requests blocked t active Trunk number active active active active active active Trần Xuân Nam/HVKTQS active active active Mô hình hóa Traffic Processes   Yêu cầu kết nối tới trung kế rời rạc, không định trước Có thể đặc trưng bởi quá trình Poisson với tốc độ yêu cầu  requests/s có đặc tính  Trong một khoảng thời gian ngắn , chỉ có trạng thái xảy Có call request với Prob[ new arrivals] =   Không có call request với Prob[no new arrivals] = -  Các call request tới độc lập với   (l T )ke- l T Prob(k arrivals in time T) = k! Trần Xuân Nam/HVKTQS Mô hình hóa Traffic Processes  Thời gian chiếm kênh (Holding time): Thời gian một user giữ kết nối X   Tải sử dụng (Offered load): nhu cầu thuê bao tạo bao gồm cả các cuộc gọi qua hệ thống và cuộc gọi không qua hệ thống:     X là một biến ngẫu nhiên với trung bình E(X) a = λ calls/sec * E(X) seconds/call (Erlangs) Tải vận chuyển (Carried load): lượng traffic mạng phục vụ (tức là số cuộc gọi qua mạng) Tải thất thoát (Lost load)=Offered load – Carried load Một Erlang tương ứng với offered load chiếm trung kế 100% thời gian  Ví dụ: tốc độ tới calls/sec và thời gian chiếm kênh trung bình E[X]=1 chiểm trung kế liên tục Trần Xuân Nam/HVKTQS Grade of Service    GoS đại lượng đánh giá mức độ hài lòng của thuê bao GoS tỉ lệ với tổng số lần muốn nối tới đối tác không thành công thiếu tài nguyên (mạch, trung kế, thiết bị), chính là xác suất nghẽn c = số Trunks, Blocking xảy nếu tất cả các trunks đều bận, tức là, N(t)=c Trần Xuân Nam/HVKTQS Xác suất Blocking & Utilization  Nếu các call requests là Poisson, thì xác suất nghẽn Pb được tính bởi công thức Erlang B (cleared condition)  Hiệu dụng (Utilization) là số trung bình trunk sư dụng Pb = ac c ! c k a � k! k=0 Utilization = λ(1 – Pb) E[X]/c = (1 – Pb) a/c Trần Xuân Nam/HVKTQS Blocking Performance a To achieve 1% blocking probability: a = Erlangs requires 11 trunks a = 10 Erlangs requires 18 trunks Trần Xuân Nam/HVKTQS Multiplexing Gain Load Trunks@1% Utilization 0.20 0.29 0.38 10 0.40 11 0.45 13 0.46 14 0.50 15 0.53 17 0.53 10 18 0.56 30 42 0.71 50 64 0.78 60 75 0.80 90 106 0.85 100 117 0.85 Trần Xuân Nam/HVKTQS   Tại một xác suất Pb, hệ thống trở nên hiệu quả nhờ tận dụng trunk Các dòng traffic tổng cộng cần chia sẻ nguồn tài nguyên chung hiệu quả Hiệu quả này gọi là Multiplexing Gain Ví dụ GoS  Nhóm trung kế: GoS=0.01  Đường dây nối giữa văn phòng: GoS=0.02  Đường quốc tế: GoS=0.01  Các tầng chuyển mạch thuê bao: 0.005  Người gưi code: 0.005 Trần Xuân Nam/HVKTQS Routing Control    Routing: thủ tục gán đường cho các kết nối Các đường có lưu lượng lớn cần qua các đường thẳng để sư dụng hiệu quả resources Tại cùng GoS, các dòng tải lớn sư dụng hiệu quả nên có thể nối trực tiếp với nhau; các dòng tải ít nối qua tandem switchs (a) (b) A D B E C F 10 Erlangs between each pair 17 trunks for 10 Erlangs 9x17=153 trunks Efficiency = 90/153=53% Trần Xuân Nam/HVKTQS Trunk group Tandem switch A B C Tandem switch D E F 90 Erlangs when combined 106 trunks for 90 Erlangs Efficiency = 85% Alternative Routing Tandem switch Alternative route Switch High-usage route     Switch Triển khai các trunks giữa các switch với lượng traffic lớn Phân bố trunks với nghẽn lớn, 10%, để hiệu suất sư dụng cao Đáp ứng 1% yêu cầu bị nghẽn từ end-to-end bằng cách chuyển qua đường dài qua tandem switch Tandem switch xư lý overflow traffic từ các switches khác nên hoạt động hiệu quả Trần Xuân Nam/HVKTQS Typical Routing Scenario Tandem switch Tandem switch Alternative routes for B-E, C-F Switch A Switch D Switch B Switch E High-usage route B-E Switch C Switch F High-usage route C-F Trần Xuân Nam/HVKTQS Dynamic Routing Tandem switch Tandem switch Tandem switch Alternative routes Switch B Switch A High-usage route    Traffic biến động theo ngày, giờ  East coast of North America busy while West coast idle Mạng có thể sư dụng tài nguyên rỗi bằng cách phối hợp động việc chọn tuyến  Route some intra-East-coast calls through West-coast switches Try high-usage route and overflow to alternative routes Trần Xuân Nam/HVKTQS Overload Control Carried load Network capacity Offered load Trần Xuân Nam/HVKTQS Overload Situations  Mother’s Day, Xmas  Catastrophes  Network Faults Strategies  Direct routes first  Outbound first  Code blocking  Call request pacing ...Bài Circuit-Switching Networks Traffic and Overload Control in Telephone Networks Traffic Management & Overload Control   Các cuộc gọi điện... Xuân Nam/HVKTQS Blocking Performance a To achieve 1% blocking probability: a = Erlangs requires 11 trunks a = 10 Erlangs requires 18 trunks Trần Xuân Nam/HVKTQS Multiplexing Gain Load Trunks@1%... Situations  Mother’s Day, Xmas  Catastrophes  Network Faults Strategies  Direct routes first  Outbound first  Code blocking  Call request pacing

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