Computer Network 實踐資管 Wang-Jiunn Cheng 2004 PART VII-2 Wide Area Networks (WANs), Routing, and Shortest Paths

Computer Network 實踐資管 Wang-Jiunn Cheng 2004 Early WAN Technologies ARPANET –Historically important in packet switching –Fast when invented; slow by current standards X.25 –Early commercial service –Still used –More popular in Europe

Computer Network 實踐資管 Wang-Jiunn Cheng 2004 Recent WAN Technologies SMDS –Offered by phone companies –Not as popular as Frame Relay Frame Relay –Widely used commercial service –Offered by phone companies ATM

Computer Network 實踐資管 Wang-Jiunn Cheng 2004 Asynchronous Transfer Mode (ATM) Designed by phone companies Single technology meant to handle –Voice –Video –Data Intended as LAN or WAN Goal: replacement for Internet

Computer Network 實踐資管 Wang-Jiunn Cheng 2004 ATM Characteristics End-to-end (application to application) Connection-oriented interface: –Establish connection –Send data –Close connection Performance guarantees (statistical) Uses cell switching

Computer Network 實踐資管 Wang-Jiunn Cheng 2004 ATM Cell Fixed size packet (for highest speed electronics) Size chosen as compromise between voice (small) and data (large) –5 octet header –48 octet payload Note: size not optimal for any application

Computer Network 實踐資管 Wang-Jiunn Cheng 2004 ATM Cell Header

Computer Network 實踐資管 Wang-Jiunn Cheng 2004 ATM Switch Building block of ATM network Connections to –Computers –Other ATM switches Accepts and forwards cells

Computer Network 實踐資管 Wang-Jiunn Cheng 2004 Cell Forwarding Performed directly by hardware Incoming cell sent to an outgoing interface Uses label in cell Motivation: highest speed

Computer Network 實踐資管 Wang-Jiunn Cheng 2004 Label Switching ATM connection identified with 24-bit binary value –Known as Virtual Path Identifier / Virtual Channel Identifier (VPI / VCI) –Generically called label VPI / VCI rewritten at each switch

Computer Network 實踐資管 Wang-Jiunn Cheng 2004 Example Of VPI/VCI Rewriting

Computer Network 實踐資管 Wang-Jiunn Cheng 2004 ATM Quality Of Service Fine-grained (per connection) Specified when connection established Endpoint specifies –Type of data transfer –Throughput desired –Maximum packet burst size –Maximum delay tolerated

Computer Network 實踐資管 Wang-Jiunn Cheng 2004 Type Of Data Transfer Constant Bit Rate (CBR) –Example: audio Variable Bit Rate (VBR) –Example: video with adaptive encoding Available Bit Rate (ABR) –Example: data Unspecified Bit Rate (UBR) Each type has detailed parameters (e.g., mean, max, burst duration)

Computer Network 實踐資管 Wang-Jiunn Cheng 2004 Sending Data Over ATM Uses ATM Adaptation Layer 5 (AAL5) Accepts and delivers large, variable-size packets AAL5 divides into cells for transmission –Called segmentation and reassembly

Computer Network 實踐資管 Wang-Jiunn Cheng 2004 Critique Of ATM Expense Connection setup latency Cell tax: 10% Specification of service requirements –An application may not know what to specify Lack of efficient broadcast –Non broadcast multiple access network (NBMA) Complexity of Qos –Cumbersome and difficult in implementation Assumption of homogeneity –A single, universal networking system –Multiple technologies will exists

Computer Network 實踐資管 Wang-Jiunn Cheng 2004 Summary Wide Area Networks ( WANs) –Span long distances –Connect many computers –Built from packet switches –Use store-and-forward WAN addressing –Two-part address –Switch/computer

Computer Network 實踐資管 Wang-Jiunn Cheng 2004 Summary (continued) Routing –Each switch contains routing table –Table gives next-hop for destination Routing tables created –Manually –Automatically Two basic routing algorithms –Distance vector –Link state

Computer Network 實踐資管 Wang-Jiunn Cheng 2004 Summary (continued) Example WAN technologies –ARPANET –X.25 –SMDS –Frame Relay –ATM