Download EE4204 Computer Networks Final Examination Cheat-sheet and more Study notes Computer Networks in PDF only on Docsity! EE4204 Computer Networks Final Examination Cheat-sheet (AY2018/2019 Semester 1) Niu Yunpeng @ NUS CEG Page 1 Good Luck! EE4204 Final Examination Cheat-sheet 1. Introduction & Basis 1) ISO-OSI seven layers architecture: physical layer, data link layer, IP layer, transport layer, session layer, presentation layer, application layer. 2) IETF five layers: (hourglass design) physical layer, data link layer – frame, IP layer – datagram, transport layer – segment, application layer – message. 3) Layering: ensure encapsulation and fragmentation, protocols provide service interface and peer-to-peer interface (cross layer design, possible?). 3) Two kinds of packet switches: router (IP layer), switch (data link layer). 4) Network components: core network (ISP), access network (telephone-based, cable-based, fiber-based, wired, wireless), network edges (hosts + servers). a. Digital subscriber line (DSL): existing telephone, < 2.5/2.4 Mbps up/down; b. Hybrid fiber coax (HFC): frequency multiplexing, < 2/30 Mbps up/down; c. Fiber to the home (FTTH), passive optical network (PON); d. Wi-Fi 802.11b/g < 11.54 Mbps (local), 3G/4G LTE 1 – 10 Mbps (wide). 5) Link performance: bandwidth (Hz), data rate (bps), channel capacity (noise). 6) In local area networks: broadcast link, point-to-point link, token ring. 7) Multiplexing methods: time division multiplexing (fixed – FTDM, statistical – STDM), frequency division multiplexing. 8) Switching methods: circuit switching (fixed TDM), packet switching (store and forward, statistical TDM). 9) Address translation: domain name to IP address – DNS (over UDP), IP address to MAC address – ARP (under the same LAN). 10) Delays: transmission delay (Tt), propagation delay (Tp), queuing delay (Tq), processing delay, packetization delay, etc. 11) Transmission speed: one-way unacknowledged transfer – , one-way acknowledged transfer – . 12) Delay (D) and bandwidth (B) product = amount of data “in the pipe”. 13) Effective throughput: . 2. Data Link Layer 1) When a packet is transferred around in the network, the source/destination MAC address changes between each two hops, while IP address remains the same (always the initial source or eventual destination address). 2) Link layer ensures channel reliability; transport layer ensures end-to-end reliability. 3) Shannon’s capacity theorem: . 4) Framing approaches: a. sentinel-based: delineate with byte 7E, bit staffing in HDLC– insert 0 after five consecutive 1s, byte staffing in PPP – use 7D as escape character; b. counter-based: count field in header, back-to-back frames could be affected; c. clock-based: 810 bytes per 125 μs = 51.84 Mbps (STS-n = n * 51.94 Mbps). 5) Cyclic Redundancy Check (CRC): represent the message and divisor as polynomial, perform modulo-2 arithmetic (binary addition with no carry). 6) Flow control ensures that the sender does not overwhelm the receiver (stop and wait, sliding window with ACK n or RR n). 7) Automatic repeat request (ARQ): introduce NACK, REJ, SREJ. a. Stop and wait: TIMEOUT mechanism, alternate between ACK0 and ACK1; b. Go back N: ACK n or RR n, REJ i will trigger sender to go back to i; c. Selective reject: ACK n or RR n, SREJ i will trigger sender to re-transmit i. 8) Performance: let represent the number of frames held in the link. a. Stop and wait: link utilization ; b. Sliding window (error-free): assume window size is W, if or if ; c. Selective reject: if else ; d. Go back N: if else . 9) Ethernet: max 2500m by 5 segments (separated by 4 repeaters). a. Collision detection: carrier sense multiple access (CSMA), use exponential back-off algorithm (randomly wait [0, 2n-1] slots at nth collision, give up after); b. Minimum frame size: 64 bytes (512 bits for 10 Mbps link = 51.2 μs RTT); c. LAN connection: bus (single collision domain), hub (copy frames to all other ports) and switch (store and forward, port to port);
