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  • Network technologies were developed to connect components (in this case individual computer

  • systems) separated by larger distances, i.e., distances measured in meters instead of centimeters.

  • Communicating over these larger distances led to different design tradeoffs.

  • In early networks, information was sent as a sequence of bits over the shared communication

  • medium.

  • The bits were organized into packets, each containing the address of the destination.

  • Packets also included a checksum used to detect errors in transmission and the protocol supported

  • the ability to request the retransmission of corrupted packets.

  • The software controlling the network is divided into a "stack" of modules, each implementing

  • a different communication abstraction.

  • The lowest-level physical layer is responsible for transmitting and receiving an individual

  • packet of bits.

  • Bit errors are detected and corrected, and packets with uncorrectable errors are discarded.

  • There are different physical-layer modules available for the different types of physical

  • networks.

  • The network layer deals with the addressing and routing of packets.

  • Clever routing algorithms find the shortest communication path through the multi-hop network

  • and deal with momentary or long-term outages on particular network links.

  • The transport layer is responsible for providing the reliable communication of a stream of

  • data, dealing with the issues of discarded or out-of-order packets.

  • In an effort to optimize network usage and limit packet loses due to network congestion,

  • the transport layer deals with flow control, i.e., the rate at which packets are sent.

  • A key idea in the networking community is the notion of building a reliable communication

  • channel on top of a "best efforts" packet network.

  • Higher layers of the protocol are designed so that its possible to recover from errors

  • in the lower layers.

  • This has proven to be much more cost-effective and robust than trying to achieve 100% reliability

  • at each layer.

  • As we saw in the previous section, there are a lot of electrical issues when trying to

  • communicate over a shared wire with multiple drivers and receivers.

  • Slowing down the rate of communication helps to solve the problems, but "slow" isn't in

  • the cards for today's high-performance systems.

  • Experience in the network world has shown that the fastest and least problematic communication

  • channels have a single driver communicating with a single receiver, what's called a point-to-point

  • link.

  • Using differential signaling is particularly robust.

  • With differential signaling, the receiver measures the voltage difference across the

  • two signaling wires.

  • Electrical effects that might induce voltage noise on one signaling wire will affect the

  • other in equal measure, so the voltage difference will be largely unaffected by most noise.

  • Almost all high-performance communication links use differential signaling.

  • If we're sending digital data, does that mean we also have to send a separate clock signal

  • so the receiver knows when to sample the signal to determine the next bit?

  • With some cleverness, it turns out that we can recover the timing information from the

  • received signal assuming we know the nominal clock period at the transmitter.

  • If the transmitter changes the bit its sending at the rising edge of the transmitter's clock,

  • then the receiver can use the transitions in the received waveform to infer the timing

  • for some of the clock edges.

  • Then the receiver can use its knowledge of the transmitter's nominal clock period to

  • infer the location of the remaining clock edges.

  • It does this by using a phase-locked loop to generate a local facsimile of the transmitter's

  • clock, using any received transitions to correct the phase and period of the local clock.

  • The transmitter adds a training sequence of bits at the front of packet to ensure that

  • the receiver's phased-lock loop is properly synchronized before the packet data itself

  • is transmitted.

  • A unique bit sequence is used to separate the training signal from the packet data so

  • the receiver can tell exactly where the packet starts

  • even if it missed a few training bits while the clocks were being properly synchronized.

  • Once the receiver knows the timing of the clock edges, it can then sample the incoming

  • waveform towards the end of each clock period to determine the transmitted bit.

  • To keep the local clock in sync with the transmitter's clock, the incoming waveform needs to have

  • reasonably frequent transitions.

  • But if the transmitter is sending say, all zeroes, how can we guarantee frequent-enough

  • clock edges?

  • The trick, invented by IBM, is for the transmitter to take the stream of message bits and re-encode

  • them into a bit stream that is guaranteed to have transitions no matter what the message

  • bits are.

  • The most commonly used encoding is 8b10b, where 8 message bits are encoded into 10 transmitted

  • bits, where the encoding guarantees a transition at least every 6 bit times.

  • Of course, the receiver has to reverse the 8b10b encoding to recover the actual message

  • bits.

  • Pretty neat!

  • The benefit of this trick is that we truly only need to send a single stream of bits.

  • The receiver will be able to recover both the timing information and the data without

  • also needing to transmit a separate clock signal.

  • Using these lessons, networks have evolved from using shared communication channels to

  • using point-to-point links.

  • Today local-area networks use 10, 100, or 1000 BaseT wiring which includes separate

  • differential pairs for sending and receiving, i.e., each sending or receiving channel is

  • unidirectional with a single driver and single receiver.

  • The network uses separate switches and routers to receive packets from a sender and then

  • forward the packets over a point-to-point link to the next switch,

  • and so on, across multiple point-to-point links until the packet arrives at its destination.

  • System-level connections have evolved to use the same communication strategy: point-to-point

  • links with switches for routing packets to their intended destination.

  • Note that communication along each link is independent, so a network with many links

  • can actually support a lot of communication bandwidth.

  • With a small amount of packet buffering in the switches to deal with momentary contention

  • for a particular link, this is a very effective strategy for moving

  • massive amounts of information from one component to the next.

  • In the next section, we'll look at some of the more interesting details.

Network technologies were developed to connect components (in this case individual computer

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20.2.4 點對點通信 (20.2.4 Point-to-point Communication)

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    林宜悉 發佈於 2021 年 01 月 14 日
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