04 - LAN

ucla | CS 118 | 2024-10-15 14:35

LAN

fiber unidirectional 1 to 1
Ethernet is many to many link
however bc its a single link, we need to be able to demux the signal to the correct receiver -> Media Access Control Adress (MAC addr.)
LAN is also known as
- Multi access link - multiple nodes on the same link
- broadcast links - every transmission can be heard by all nodes
- LAN - geographically serviced by a local region 1-10 km(s)
Pros
- cost - connect all devices on a campus on a single link
- bandwidth - ethernet i high bandwidth distributed computing
- statistical multiplexing - time division multiplexing (TDM) is not good when user traffic is busy -> many collisions
  - each user splits the bandwidth, high bandwidth when low traffic

strict - TDM/FDM where user given fix allocation regardless of whether user want to send -> bad
bursty - traffic has aa high and average peak
therefore, strict gives each user $B/N$ bandwidth
stat mux gives $B/x$ where $x$ is number of busy users, $x«N$

ethernet predecessor - multiple ground stations across Hawaii
problem - no collision detection when traffic was high -> similar problems in wireless 802.11
sol - slotted aloha, allocate bandwidth and reduce collision period by half but require shared clock

Bob Metcalfe - inventor of Ethernet at Xerox PARC and CSMA/CD for Harvard PhD thesis

aborts transmission after 64 bytes if a collision is detected while aloha sends full 1500 byte datagram
only handles collision detection (frequent) but not frame corruption recovery
senders cannot detect collisions, only nodes at the intersection of 2 signals can detect collisions at their node
note that collision are between waves in ethernet and 802.11 not mass-based particles

ethernet enforces minimum packet size of 64 bytes for CD
without min packet size - no retransmission
with limited packet size
the min-size of the packet depends on the pipe size = transmission rate $\times$ round trip prop delay
- e.g., if R = 10Mbit/s, One way PD = 25.6$\mu$s; then pipe size = 512 bits = 64 bytes
- pipe size (bandwidth-delay product, B) = transmission rate (B/s) $\times$ round-trip propagation delay (s)

consider 2 colliders, one should wait 1 slot and the other 0 slots, mux collision or time with coin toss -> just pick 1 signal to pass through the detecting node, e.g. if C detects colision of A->B and B->A and order is A->C->B then C just muxes one of the signals
consider 16 colliders - ethernet does Binary Exponential Backoff
- after attempt A, each station randomly picks a number of slots between 0 and $2^A-1$. A slot is 51.2$\mu s$ (because of prev question pipe). after 1 collision pick 0 or 1 slot, after 2 collisions pick up to 4 sots, after 3 up to 8 slots (slots 0…7)
ethernet’s 3 mechanisms
- CS - don’t transmit when someone else is (imagine the detecting node wants to transmit)
- CD - stop frame if CD before 64 bytes of frame
- Exponential backoff - collisions are frequent so must retransmit, random backoff avoids synchronized colllissions, dynamically adjut number of colliders with backoff
terms
- slot time - 2T, where T is 1-way prop delay, limited to 51.2$\mu s$ to allow 64 byte pipe size
- min packet size - 64 bytes to avoid transmision end before CD
- jam - transmit smal number of bits after CD to ensure other tranmitterss also detect collision
- CD - one option is use Manchester encoding averge voltage, e.g. 0 (0V) or 1 (1.5V) has avg 0.75V, collision => average 1.5V

slot time - 2T, where T is 1-way prop delay, limited to 51.2$\mu s$ to allow 64 byte pipe size
min packet size - 64 bytes to avoid transmision end before CD
jam - transmit smal number of bits after CD to ensure other tranmitterss also detect collision
CD - one option is use Manchester encoding averge voltage, e.g. 0 (0V) or 1 (1.5V) has avg 0.75V, collision => average 1.5V

dest for mux, source for mux ack, length in case frame is smaller than min packet size, CRC - error detection hash

limited distance of 2.5 km with 500m wires and 4 repeaters
thin wire or thick wire
physical topology of star (all nodes connected to hub) or tree (bus with main line and branches)

Bandwidth and distance, if speed increased, distance must be shorter, e.g. 100 Mbps ether hass 200m extent
Gigabit ether has 2m extent -> switches, hubs, point to point
therefore, cost of stat mux is why ethernet is limited to LAN
so modern day shift from mainline ethernet to series of point to point switches but keep ethernet header
CSMA/CD to Switches