Packet switching networks (MẠNG VIỄN THÔNG SLIDE)

Bài giảng Mạng Viễn thông (33BB) Chương Packet-Switching Networks Network Services and Internal Network Operation Packet Network Topology Datagrams and Virtual Circuits Routing in Packet Networks Shortest Path Routing Chương Packet-Switching Networks Network Services and Internal Network Operation Network Layer  Network Layer: Lớp phức tạp nhất  Yêu cầu các điều phối của các phần tử mạng phân tán theo địa lý (switches & routers)  Cần phải làm việc với phạm vi rất rộng  Hàng tỉ người dùng (người & thiết bị liên lạc)  Các thách thức lớn nhất  Addressing: thông tin cần chuyển tới đâu?  Routing: sử dụng đường nào để đưa thông tin tới đích? 4/96 Packet Switching t1 t0 Network    Truyền thông tin ở dạng một gói đơn hay nhiều gói nho Truyền gói đơn yêu cầu về độ chính xác và trễ truyền Truyền nhiều gói nho, ngoài yêu cầu độ chính xác, trễ truyền còn yêu cầu thêm về thứ tự các gói và đảm bảo quan hệ thời gian giữa các gói nho 5/96 Network Service Messages Messages Segments Transport layer Transport layer Network service Network service Network layer Network layer Network layer Network layer Data link layer Data link layer Data link layer Data link layer layer Physical layer Physical layer Physical layer End system Physical α    End system β Network layer có thể cung cấp nhiều dịch vụ cho transport layer Connection-oriented service hay connectionless service Best-effort hay delay/loss guarantees Connection Oriented Service  Transport layer không thể yêu cầu truyền cho đến đã thiết lập xong kết nối  Network layer được thông báo trước về luồng dữ liệu gửi vào mạng  Trong quá trình thiết lập kết nối có thoa thuận về sử dụng và QoS, và trao đổi tài nguyên mạng  Yêu cầu có ngắt kết nối  Phức tạp connectionless service 7/96 Connectionless Service  Đơn giản  Chỉ cần hai thao thác giữa network layer và transport layer  Phía phát: yêu cầu gửi packet  Phía thu: chỉ thị đã có gói tới  Giao trách nhiệm về kiểm soát lỗi, thứ tự packets, điều khiển luồng cho E2E transport layer 8/96 Network Internal Operation   Connectionless operation  Packets truyền qua mạng ở dạng datagram  Mỗi packet được định tuyến độc lập  Các packets có thể được truyền qua các đường khác và có thể đến không theo thứ tư Connection-oriented operation  Packet qua các đường ảo (virtual circuits) đã được thiết lập từ nguồn tới đích  Virtual circuit được thiết lập một lần và các packet được chuyển tiếp qua đường thiết lập đó  Có thể đảm bảo băng thông, trể và tổn hao 9/96 Network Service vs Operation Network Service  Connectionless   Datagram Transfer Connection-Oriented  Internal Network  Connectionless Tin cậy và có thể truyền ứng dụng có tốc độ bít không đổi (constant bit rate)   IP Connection-Oriented   Telephone connection ATM Các kết hợp có thể có  Connection-oriented service over Connectionless operation  Connectionless service over Connection-Oriented operation  Tùy trường hợp & yêu cầu xác định 10 Bảng định tuyến Datagram Networks Destination address 0785 Output port 1345 12 1566     2458 12 Route được xác định bởi bảng tra Quyết định định tuyến liên quan đến việc tìm chặng tiếp theo ở tuyến tới đích xác định Bảng định tuyến có một giá trị cho mỗi đích xác định cổng tới chặng tiếp theo Kích thước của bảng trở nên quá lớn số trạm đích rất lớn 31 Ví dụ: Internet Routing    Giao thức IP sử dụng chuyển mạch gói datagram qua các mạng  Các mạng được coi các đường liên kết dữ liệu Các hosts có địa chỉ IP hai phần:  Network address + Host address Các routers tra bảng theo network address  Giảm kích thước của routing table 32/96 Packet Switching – Virtual Circuit Packet Packet Packet Packet Virtual circuit      Thiết lập một đường cố định giữa nguồn và đích trước truyền dữ liệu Tất cả các packets một kết nối theo cùng một đường Có thể sử dụng cho dịch vụ variable bit rates Trễ biến đổi ít, vẫn còn lớn so với circuit switching Có thể phân phối tài nguyên mỗi tuyến theo yêu cầu 33 Connection Setup Connect request Connect confirm      SW Connect request Connect confirm SW … SW n Connect request Connect confirm Các bản tin báo hiệu lan truyền chọn được tuyến Các bản tin báo hiệu xác định kết nối và thiết lập các bảng chuyển tiếp switches Tại đầu vào mỗi switch, kết nối được xác định bởi một nhãn gọi là Virtual Circuit Identifier (VCI) Mỗi switch chỉ cần biết quan hệ giữa nhãn (VCI) đầu vào và nhãn (VCI) đầu tới switch tiếp theo Khi bảng chuyển tiếp thiết lập, packets có thể theo dọc đường đã thiết lập 34/96 Trễ thiết lập kết nối t Connect request CC CR CC CR    Connect confirm Release t t t Trễ thiết lập kết nối xảy trước truyền packet Trễ có thể chấp nhận được truyền một số lượng lớn các packets Trễ không thể bo qua truyền số lượng packet ít 35/96 Bảng Chuyển tiếp của Virtual Circuit Input VCI Output port Output VCI  12 13 44  15 15 23  27 13 16  58 34  Mỗi input port của switch có một bảng chuyển tiếp Tìm VCI của header của packet tới Xác định output port (next hop) và chèn VCI cho tuyến tiếp theo Tốc độ xử lý cao độ dài VCI ngắn (số kết nối max cho input port) Bảng có thể bao gồm cả ưu tiên hay các thông tin khác 36 Cut-Through switching Source Switch Switch t t t Destination   3 t Minimum delay = 3τ + T Một số mạng chỉ thực hiện kiểm soát lỗi ở header nên packet có thể chuyển tiếp sau thu và xử lý xong header Cho phép giảm trễ 37/96 Message vs Packet Minimum Delay    Message: Lτ + LT = L τ + (L – 1) T + T Packet L τ + L P + (k – 1) P = L τ + (L – 1) P + T Cut-Through Packet (Chuyển tiếp sau nhận header) = Lτ+ T Chưa tính đến trễ xử lý header 38/96 Ví dụ: ATM Networks  Tất cả thông tin ánh xạ thành các packet có độ dài cố định (53bytes) gọi là cells  Thiết lập kết nối qua mạng  Thiết lập virtual circuits qua mạng  Thiết lập bảng tại các ATM switches  Các dịch vụ  Constant bit rate connections  Variable bit rate connections 39/96 Chapter Packet-Switching Networks Datagrams and Virtual Circuits Structure of a Packet Switch 40 1 2    N    ••• ••• ••• Packet Switch: Intersection where Traffic Flows Meet    N Inputs contain multiplexed flows from access muxs & other packet switches Flows demultiplexed at input, routed and/or forwarded to output ports Packets buffered, prioritized, and multiplexed on output lines 41 Generic Packet Switch “Unfolded” View of Switch  Ingress Line Cards  Controller   N Line card Line card  Line card  Line card Line card  N  Data path Control path Output ports Transfer packets between line cards Egress Line Cards  Input ports Routing in small switches Signalling & resource allocation Interconnection Fabric  … Line card Controller … … Line card Interconnection fabric Line card …  Header processing Demultiplexing Routing in large switches  Scheduling & priority Multiplexing (a) 42 Line Cards Framer Network processor Transceiver To physical ports Backplane transceivers Framer To switch fabric To other line cards Folded View  circuit board is ingress/egress line card  Physical layer processing  Data link layer processing  Network header processing  Physical layer across fabric + framing Interconnection fabric Transceiver 43 Shared Memory Packet Switch Ingress Processing Connection Control Output Buffering 1 Queue Control 2 N Shared Memory … … N Small switches can be built by reading/writing into shared memory44/96 Crossbar Switches (b) Output buffering (a) Input buffering Inputs Inputs 83 … … N N …    Outputs … N Outputs N Large switches built from crossbar & multistage space switches Requires centralized controller/scheduler (who sends to whom when) Can buffer at input, output, or both (performance vs complexity)45 ... 11/96 Chapter Packet- Switching Networks Packet Network Topology 12 Đặc điểm mạng chuyển mạch gói   Packet networks rất khác biệt so với telephone networks Các dòng packet mang tính...Chương Packet- Switching Networks Network Services and Internal Network Operation Packet Network Topology Datagrams and Virtual Circuits Routing in Packet Networks Shortest Path Routing Chương Packet- Switching. .. Network 23/96 Packet Switching Network User Transmission line Network Packet switch Packet switching network  Chuyển packets giữa các người sử dụng  Các đường truyền dẫn + packet switches