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Department of Computer and IT Engineering University of Kurdistan Computer Networks II Virtual Circuit Switching and ATM By: Dr. Alireza Abdollahpouri A Taxonomy of Communication Networks Communication Network Switched Communication Network Circuit-Switched Communication Network Broadcast Communication Network Packet-Switched Communication Network Datagram Network Virtual Circuit Network Broadcast vs. Switched Communication Networks Broadcast communication networks information transmitted by any node is received by every other node in the network examples: usually in LANs (Ethernet, Wavelan) Problem: coordinate the access of all nodes to the shared communication medium (Multiple Access Problem) Switched communication networks information is transmitted to a sub-set of designated nodes examples: WANs (Telephony Network, Internet) Problem: how to forward information to intended node(s) this is done by special nodes (e.g., routers, switches) running routing protocols A Taxonomy of Communication Networks Communication Network Switched Communication Network Circuit-Switched Communication Network Broadcast Communication Network Packet-Switched Communication Network Datagram Network Virtual Circuit Network Circuit Switching Three phases 1. circuit establishment 2. data transfer 3. circuit termination If circuit not available: “Busy signal” Examples Telephone networks ISDN (Integrated Services Digital Networks) Timing in Circuit Switching Host 1 Node 1 Node 2 Host 2 processing delay at Node 1 propagation delay between Host 1 and Node 1 Circuit Establishment propagation delay between Host 2 and Node 1 Data Transmission DATA Circuit Termination A Taxonomy of Communication Networks Communication Network Switched Communication Network Circuit-Switched Communication Network Broadcast Communication Network Packet-Switched Communication Network Datagram Network Virtual Circuit Network Packet Switching Data are sent as formatted bit-sequences, so-called packets. Packets have the following structure: Header Data Trailer Header and Trailer carry control information (e.g., destination address, check sum) Each packet is passed through the network from node to node along some path (Routing) At each node the entire packet is received, stored briefly, and then forwarded to the next node (Store-and-Forward Networks) Typically no capacity is allocated for packets A Taxonomy of Communication Networks Communication Network Switched Communication Network Circuit-Switched Communication Network Broadcast Communication Network Packet-Switched Communication Network Datagram Network Virtual Circuit Network Datagram Packet Switching Each packet is independently switched each packet header contains destination address No resources are pre-allocated (reserved) in advance Example: IP networks Timing of Datagram Packet Switching Host 1 transmission time of Packet 1 at Host 1 Node 1 Packet 1 Host 2 Node 2 propagation delay between Host 1 and Node 2 Packet 2 Packet 1 Packet 3 processing delay of Packet 1 at Node 2 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 Datagram Packet Switching Host C Host D Host A Node 1 Node 2 Node 3 Node 5 Host B Node 6 Node 4 Node 7 Host E A Taxonomy of Communication Networks Communication Network Switched Communication Network Circuit-Switched Communication Network Broadcast Communication Network Packet-Switched Communication Network Datagram Network Virtual Circuit Network Virtual-Circuit Packet Switching Hybrid of circuit switching and packet switching data is transmitted as packets all packets from one packet stream are sent along a pre-established path (=virtual circuit) Guarantees in-sequence delivery of packets However: Packets from different virtual circuits may be interleaved Example: ATM networks Virtual-Circuit Packet Switching Communication with virtual circuits takes place in three phases 1. VC establishment 2. Data transfer 3. VC disconnect Note: packet headers don’t need to contain the full destination address of the packet Timing of Virtual-Circuit Packet Switching Host 1 Node 1 Host 2 Node 2 propagation delay between Host 1 and Node 1 VC establishment Packet 1 Packet 2 Packet 1 Data transfer Packet 3 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 VC termination Virtual-Circuit Packet Switching Host C Host D Host A Node 1 Node 2 Node 3 Node 5 Host B Node 6 Node 4 Node 7 Host E Virtual Circuit Connection setup establishes a path through switches A virtual circuit ID (VCI) identifies path Uses packet switching, with packets containing VCI VCIs are often indices into per-switch connection tables; change at each hop VC1 VC2 1 2 VC1 In, VC Out, VC 1, 1 4, 1 1, 2 4, 3 2, 1 4, 2 4 VC1 VC2 3 1 VC3 2 VC1 4 3 VC2 VC1 Virtual Circuits Examples In-port Lookup table for Switch R1 Packet Sender In-VCI Out-port Out-VCI 1 5 3 7 4 11 0 8 5 7 1 2 2 R1 R2 4 3 1 4 1,7 4,2 3 2 1,5 3,7 2 1 R3 4 3 2,2 3,6 Subsequence packets follow the same circuit Receiver 6 Virtual circuit wide area network VCI Switch and table Source-to-destination data transfer SVC setup request SVC setup acknowledgment Case Study: ATM Multiplexing using different frame sizes Note: A cell network uses the cell as the basic unit of data exchange. A cell is defined as a small, fixed-sized block of information. Multiplexing using cells ATM multiplexing Architecture of an ATM network UNI = User-to-Network Interface NNI = Network-to-Network Interface Example of VPs and VCs VP: Virtual Path VC: Virtual Channel Note: Note that a virtual connection is defined by a pair of numbers: the VPI and the VCI. Connection identifiers Virtual connection identifiers in UNIs and NNIs An ATM cell Why 53? Routing with a switch ATM layers (SONET/SDH) Figure 19-21 Layers in End-Point Devices and ATMATM layers in endpoint devices andSwitches switches User information McGraw-Hill User information ©The McGraw-Hill Companies, Inc., 2001 ATM layer ATM headers ATM Cell Payload Type (2 bits) identifies cell category 00 = user information (data) reserved for later definition (1 bit) 8 7 6 5 4 3 2 1 Virtual Path Identifier VPI (cont) Virtual Channel Identifier VCI (cont) VCI (cont) PT Header Error Control res CLP ATM Cell Cell Loss Priority Bit 0 if sending within committed information rate 1 if sending faster: switches will discarded if congestion occurs 8 7 6 5 4 3 2 1 Virtual Path Identifier VPI (cont) Virtual Channel Identifier VCI (cont) VCI (cont) PT Header Error Control res CLP ATM Cell Header Error Control Field To check the header (not the payload) for errors Discard cell if an error is found 8 7 6 5 4 3 2 1 Virtual Path Identifier VPI (cont) Virtual Channel Identifier VCI (cont) VCI (cont) PT Header Error Control res CLP ATM Adaptation Layers (AAL) ATM Adaptation Layer (AAL) A protocol for converting between higher layer protocol data units (PDU’s), such as TCP packets, IP packets, or JPEG images, and ATM cells for actual transmission Defines procedures for segmentation and reassembly (SAR) segmentation: packets to cells (done by sender) reassembly: cells to packets (done by receiver) AAL present only in end systems, not in switches ATM Services • • • • • Constant Bit Rate (CBR) Real-time Variable Bit Rate (rt-VBR) Non-real-time Variable Bit Rate (nrt-VBR) Available Bit Rate (ABR) Unspecified Bit Rate (UBR): best effort service 46 ATM Services • CBR : User declares required rate. Throughput, delay and delay variation guaranteed. • VBR : User declares average and max rate. rt-VBR : Conferencing. Max delay and delay variation guaranteed. nrt-VBR : Stored video. • ABR (Available bit rate): Follows feedback instructions. Network gives maximum throughput with minimum loss. • UBR (Unspecified bit rate): User sends whenever it wants. No feedback mechanism. No guarantee. Cells may be dropped during congestion. 47 AAL1 AAL1, a connection-oriented service, is suitable for handling constant bit rate sources (CBR), such as uncompressed audio and videoconferencing AAL2 Protocol standard for supporting time-dependent slow or Variable Bit Rate (rt-VBR) connection-oriented traffic (e.g. packetized and compressed audio and MPEG video). AAL3/4 Protocol standard for supporting both Connectionless and connection-oriented Variable Bit Rate (nrt-VBR); e.g., airline reservation, banking transactions. AAL5 AAL5 is similar to AAL3/4, but has a simplified information header scheme that requires only one header per data unit. Examples of this service include datagram traffic and data network applications where no connection is set up before data is transferred. (UBR, ABR). IP-Over-ATM app transport IP Eth phy IP AAL Eth ATM phy phy ATM phy ATM phy app transport IP AAL ATM phy Questions 53