Extending LANs, Long-Distance Networking

extending LANs:
- ethernet limitations
- extending Ethernet, repeaters
- bridges
- switches
Digital Telephony:
- Sampling and Quantization
- synchronization
- telephone standards
SONET
ISDN, DSL, cable

Ethernet Limitations

collision detection: minimum packet size 64 bytes says network diameter can be at most 512/2=256 bit periods
for 10Mb/s, this is 25.6 us -- at 200,000 Km/s, this is 5,120 m.
for 100Mb/s, this is 2.56 us -- at 200,000 Km/s, this is 512 m.
another limitation is from the power provided by the transceivers
thickwire ethernet is designed to reach up to 500 m
10BaseT ethernet is limited to 100 m between stations

one way to overcome ethernet distance limitations is to:
- attach a fast modem to the ethernet
- design it to send everything on one side to the other
fiber modems could run faster than the ethernet
might reach several kilometers
not more, since the computer is still doing the collision detection and retransmission

a repeater with more than 2 ports is a hub
CAT-5 wiring (for 10Base-T Ethernet) has the computer send on one pair of wires, and the hub on a different pair
this means a regular CAT-5 wire cannot connect a hub to another hub
instead, use a crossover cable, or the uplink port on the hub

repeaters have no knowledge of Ethernet frames
a computer with two Ethernet interfaces can be programmed to transfer all the (correctly received) frames from one interface to the other
this is a bridge
this simple bridge breaks the collision domain and restores signals, but does not decrease the traffic on each segment
a bridge that has hardware to automatically transfer the frames from one port to another is a switch

a bridge or switch could keep track of the port P on which it receives frames from address A
then, any frames for A can be sent only to port P, not to the other ports
if the frame for A is received on port P, it need not even be forwarded
this can dramatically improve congestion, and somewhat improve security
such a system is a learning bridge or learning switch
broadcast packets must still be sent to all ports

if many bridges connect different segments, we might get a topology loop
if a broadcast packet is sent on this network, it will loop around, consuming resources
as more broadcast packets loop around, regular traffic is delayed or dropped, which is a broadcast storm
avoiding broadcast storms:
- don't have topological loops. If one bridge goes down, the network is partitioned
- have the bridges compute a spanning tree, and only forward packets along that spanning tree

because a bridge only forwards the essential traffic, it is good to have a bridge at each end of a long-distance connection
the long-distance connection is point-to-point, and may have long delay and low bandwidth
the bridge buffers segments, sending only as fast as the link allows

telephone carries voice faithfully up to 3000 Hz, then drops quickly
assuming we don't need above 4,000 Hz, how do we encode that data digitally?
Nyquist theorem: if you signal is band-limited to x Hz, sampling 2x times per second will allow you to exactly reconstruct the signal
the samples are still analog: we divide the possible voltages linearly (logarithmically?) into 256 intervals, and encode to the nearest interval
for the telephone, every second, 8,000 samples of 8 bits, 64,000 b/s

a synchronous network such as the telephone network has some fundamental differences from an asynchronous data network:
- any substantial delay or queueing of data is not an option
- TDM still works: each call has a slot and all calls are served in round-robin order
- a multiplexer may have to delay each payload to be synchronized with the envelopes it sends
everything in the digital telephone network is designed to carry data 8,000 times per second, or one message every 125 us
faster lines can carry more than one byte in that time (there are no lines slower than 64Kb/s)

a Data Service Unit connects to the computer side, a Channel Service Unit connects to the telephone side: together, a DSU/CSU (WAN modem)
T1/DS1: 24 (and 1/8) voice circuits, 1.544 Mb/s. E1: 32 voice circuits, 2.048 Mb/s
T3/DS3: 28 T1 circuits (and 27 voice circuits), 44.763 Mb/s
STS-1/OC-1: 51.840 Mb/s ("Synchronous Optical Signal"/"Optical Carrier")
STS-3/OC-3: 155.520 Mb/s
OC-12: 622.080 Mb/s
OC-48: 2.488320 Gb/s

Framing technology for carrying data (not voice) over digital public lines
very simple header identifies virtual circuit (usually permanently set up -- "provisioned" -- at contract time)
payload delivered to the destination with high likelihood
used for bridging LANs (e.g. between branches of the same company), sometimes for Internet traffic

Synchronous Optical NETwork (Synchronous Digital Hierarchy/SDH)
SONET dictates:
- framing (multiples of 810 bytes, probabilistically identified)
- multiplexing
- payload encoding (payload "floats" in the frame, with part in one frame and part in the successive frame)
frame "header" is 2-dimensional, 3 out of every 90 bytes
very complex
a frame is sent every 125 us, meaning the minimum speed is? (in-class exercise)

1st digital technology to the home ("local loop")
two channels (4 wires), each capable of 64Kb/s, for a total of 128Kb/s
not popular:
- too expensive for most homes (much more expensive than a modem)
- too early for the web (you don't need that much bandwidth for plain-text email)

Asymmetric Digital Subscriber Line
most of the traffic is in the direction from the central office to the home (downstream)
so reserve most of the bandwidth for downstream: up to 6.144 Mb/s (only up to 640Kb/s upstream)
bandwidth is available on the twisted pair of the local loop: don't use baseband (like RS-232), but broadband (like radio channels)
adaptive technology: send more bits through the channels that have high S/N ratio
distance limitations (CO to CPE)

Symmetric DSL/SDSL -- different encoding, symmetric bandwidth
High-Speed DSL/HDSL -- symmetric, 1.544Mb/s in both directions
Very high-Speed DSL/VDSL -- symmetric, up to 52Mb/s in both directions, requires new wiring to bring switch closer to the CPE

the channel in a cable TV system not used for broadcasting TV signals can be used as an ether to broadcast data packets
cable modems are needed at both ends
maximum rate up to 36Mb/s (but depending on traffic and number of channels)
cable not always designed for two-directional communication, e.g. repeaters might only repeat in one direction (can use telephone uplink)
in Hawaii, chiefly road-runner