EE4204 Computer Networks Final Examination Cheat-sheet (AY2018/2019 Semester 1) Niu Yunpeng @ NUS CEG
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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);
