LANs, Aloha Net

LANs

Same building
Up to a few 100 m (Ethernet) or km (FDDI)
Up to a few hundred hosts
Different LANs interconnected by switches or hosts
Physical Layer: Fiber, twisted pair (CAT5?), coaxial
Generally, logically shared medium: every computer sees every message.
MAC (Media Access Control): provide point-to-point connection functions over shared medium (discards packets not for us)
LLC (Logical Link Control) may provide reliability or error checking

Throughput: B b/s
Latency: d s
Total delay to transfer b bits is d + b/B.
Example: B = 10 Mb/s, d = 40ms, transfer time for 10 MB = 80 Mb is t = 40 ms + 80,000,000 b/10,000 b/ms = 8,040 ms.
Example: B = 10 Mb/s, d = 40ms, transfer time for 10 KB = 80 Kb is t = 40 ms + 80,000 b/10,000 b/ms = 48 ms.
Examples assume delay due to computers is either negligible, or included in d.
Note: throughput shared (assume equally) by all active hosts on a network.

What wires/fibers does it use?
Security: shared media are easy to snoop, switched is harder, fiber is harder.
Reliability: what happens if a link goes down? FDDI works well, Ethernet may work well.
Manageability: how hard is it to find a problem? Ethernet works well, FDDI may be harder.

Precursor to Ethernet
University of Hawaii at Manoa
Norm Abrahmson
Connection to stations on other islands
Radio based
Base station and other stations
Different frequencies for transmission from outlying to base station and from base station to other stations

Figure 4.17

No collision between incoming and outgoing (different frequencies)
Possible collision between incoming packets
No collision detection possible:
- transmitter has finished sending before it can get the colliding packet
- transmitter isn't even listening on that frequency
Base station detects collision by garbled packets
Senders eventually time out and retransmit

Slotted: transmission can only begin at time nT, where T is the slot duration (requires transmitter synchronization)
Pure Aloha: transmission can begin any time
Distinction: pure Aloha offers more possibility for collision, achieves lower throughput
Aloha performance is great when only one outlying station is transmitting
Aloha performance is terrible if many stations are trying to transmit at once.
Aloha with reservations: use a separate channel or a distinct time slot to "reserve" slots

Note no collision and 100% utilization on the channel from the base to the outlying stations.
Book states Pure Aloha throughput is at most 18% of bandwidth, Slotted Aloha throughput is at most 36% of bandwidth,
Assumptions:
- very large number n of stations
- at any time t, each station transmits independently with identical probability p
- adjust p to obtain maximum performance