Comp 343/443 Week 8: Oct 15 (Midterm) ============================================================================== Basics of virtual-circuit routing ============================================================================== Virtual circuit concept: create "connection channels" between endpoints. Routers have STATE to handle packets Same path is always used (unless a change is negotiated by the routers) Addresses as Endpoint IDs (EIDs): virtual circuits does NOT do this. EIDs tend to be long. Virtual Circuits works well with *small* packets. Small packets are more appropriate for voice, due to "fill delay". Time to fill packets at voice rate of 8 kB/sec: 1000 bytes: 0.125 sec 100 bytes 12 ms 48 bytes 6 ms (ATM) When one person stops talking and another starts (interactive turnaround), we need to add TWO fill-delays in addition to RTT of the connection itself. How is VC different from Datagram? * routers know about connections * because of this, routers can allocate resources intelligently! * per-packet processing costs *may* be reduced How VC routers work: tables actually: pairs one is inbound; other is outbound Packets are addressed using only a single VCI Each switch makes note of the packet's arrival port and its VCI: Looks up in a table to find the outbound . Sends packet out that new port, after REPLACING the vci with the new value. 3.1.2: virtual circuit switching The road not taken by IP. IP advantages: less state info to manage router crashes and partial connection state loss are not a problem per-connection billing is very difficult VC advantages: connections can get resource guarantees smaller headers / faster throughput 3.3: ATM cell basics Fixed-size cells: ATM (and cell networks in general): small cells (typically 5 bytes header + 48 bytes data) virtual circuits; connection-oriented (28-bit addresses after connection is established) Switched point-to-point links; *some* rings Note ATM *mandates* no cell reordering! No physical b'cast cells: fixed size makes hardware simpler reduce store-and-forward delay reduce latency of high-priority traffic Need only 6 ms to fill a cell with voice, versus 1/8 sec for 1KB fixed length packets (a win, generally, for switching) => 1/2 of last cell is wasted, so small cells are better Forwarding delay & packet size; cut through loss of 1 cell destroys packet; need reliable medium The following was NOT done: error correction of Shacham & McKenney [1990] send N cells and then one of all N XOR'ed together allows recovery from any one lost cell 3.3.2: Segmentation/reassembly and AAL 3/4, AAL 5. SAR/AAL. AAL 1, 2, 3/4, 5 AAL 5: high-level crc; just take cells in order. Addressing: VPI/VCI VCI: local use only? 3.3.3: virtual circuits as applied to ATM store-and-forward of cells v. cut-through for packets 3.4: switching: cut-through v. store-and-forward (not done) Crossbar switch, other switching fabrics