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Chapter 2 Tidbits

OutlineA little hardwareEthernetRealistic data transfer rates

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Why?

• Host architecture affects “realistic” data transfer rates, which affects protocol design

• It’s a good idea to see a bit of the details of how at least one kind of LAN works– Ethernet is popular (though being replaced by faster

technologies)

– It uses a media access control (MAC) protocol that gives a good introduction to some of the problems encountered in MAC design

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Example Workstation Architecture

I/O bus

(To network)

CPU

Memory

NetworkadaptorCache

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Key Features

• Memory is finite (say 4 – 128 MB)– Memory (along with bandwidth) are two key scarce network resources

that must be carefully managed– Scarce because on a switch or router, packets must be buffered in

memory while awaiting transmission over link

• Each node connects to network via a network adaptor– Sits on system I/O bus and delivers data between memory and network

link– Device driver on workstation manages the adaptor (i.e., where

(memory address) should incoming data be stored)

• While CPU speeds doubling every 18 months, same is not true of memory speeds (only appx. 7% increase per year)– As a network node, a workstation runs at memory, not processor,

speeds to a first approximation!

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The Memory Bottleneck

Text Section 2.9.3!

Memory Bandwidth on a modern PC-class machine

I/O bus235 Mbps 1056 Mbps

CPU

Memory

Crossbar

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The Memory Bottleneck

• Looks like we should be able to transmit frames between network adaptor and host memory at rate of 1056 Mbps– Misleading: the figure measures peak bandwidth!

– Limitation is size of data blocks being transferred across bus

– Ex. Takes 8 clock cycles to acquire bus (regardless of number of bytes transferred), so 64 byte payload takes 24 cycles (bus is 32 bits wide so 16 cycles for data + 8 for bus acquisition) so max bandwidth is

€

16 ÷ (8 +16) ×1056 = 704Mbps

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But There’s More…• Memory/CPU bandwidth (1880 Mbps) is realistic, but

same order of magnitude as range of network bandwidths machine can support.

• So yes, can deliver frames from network to memory and then load into CPU registers at network bandwidth, but…

• It is impractical for device driver, OS, and application to go to memory multiple times for each word of data in a network packet (possibly because it needs to copy data from one buffer to another).– If memory/CPU path crossed n times, then effective throughput is

1880 Mbps/n. – Ex. If copying needs to be done 5 times (not uncommon), then

your peak bandwidth is now 376 Mbps.

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Ethernet (802.3)

• Multiple access network– Nodes send and receive frames over a shared link

• Carrier Sense Multiple Access with Collision Detect (CSMA/CD)– CSMA: all nodes can distinguish between an idle and busy link– CD: nodes listen as they transmit and can therefore detect when

frame it is transmitting has interfered with a frame transmitted by another node

• Evolved philosophically from the Aloha packet radio network (both mediate access to a shared medium—for Aloha it was the atmosphere, for Ethernet a coaxial cable).

Section 2.6 in text!

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Ethernet (802.3)

• DEC, Intel, Zerox joined forces to define 10Mbps Ethernet standard in 1978. – 100Mbps and 1Gps Ethernet are designed for point-to-

point switched network, so not the “true” ethernet algorithm.

• IEEE Standard 802.3 is the “official” protocol, though not every Ethernet follows this standard (slight differences in certain packet header fields, etc).

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Physical Properties

• Implemented on coaxial cable of up to 500 meters in length

• Hosts connect by “tapping” into it. – Taps at least 2.5 meters apart

• Transceiver is small device directly attached to tap– Detects when line is idle and drives signal when host is

transmitting

• All protocol logic implemented in the adaptor (not transceiver)

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Ethernet Transceiver and AdaptorTransceiver

Ethernet cable

Adaptor

Host

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Multiple Ethernet Segments• Multiple Ethernet segments can be joined together

by repeaters– No more than four repeaters between any two hosts (so

Ethernet has a total reach of only 2500 meters).– Like single Ethernet, each host hears every message

sent on network

• An Ethernet is limited, regardless of whether repeaters used to 1024 hosts

• Again, any signal is heard by everyone– Terminators attached to end of each segment absorb

signal and keep if from reflecting back and interfering with trailing signals

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All hosts in thesame collision domain

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Frame Format

• 64 bit preamble for synchronization (unseen by host)

• 48 bit address fields for both destination and source

• Type field (to which higher level protocol should this frame be delivered)

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Frame Format (cont.)

• Minimum of 46 and maximum of 1500 bytes of data– Minimum required to frames long enough for collision

detection

• 32 bit CRC (cyclic redundancy check)• 802.3 format slightly different (16 bit length field

replaces the type field)– Software kludged so that it can handle either

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Addresses

• Typically written in hex notation– Ex. 8:0:2b:e4:b1:2

• Every Ethernet Adaptor in world has unique Ethernet address– Burned into ROM at factory– Each manufacturer has different prefix

• Ex. AMD starts with x080020

• Each frame transmitted on Ethernet received by every adaptor

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Adaptor Accepts:

• Frames addressed to its own address• Frames addressed to the broadcast address (all 1s)• Frames addressed to a multicast address (first bit 1) if

instructed to listen for that address by host• All frames, if it is in promiscuous mode

• It passes to host only the frames that it accepts

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Transmitter Algorithm

• When adaptor has frame to send and line is idle, it transmits frame immediately– Upper bound of 1500 bytes ensures that no adaptor can

occupy the line for too long

• If adaptor has frame to send and line is busy, it waits for line to go idle then transmits immediately– Well, all adaptors wait 9.6s after one frame before

sending next

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Transmitter Algorithm (cont.)

• It is possible for two or more adaptors to begin transmitting at same time (a collision)

• If collision occurs, both adaptors transmit a 32 bit jamming sequence, then stop transmitting.– So in minimal case an adaptor sends 96 bits: 64 bit preamble and

32 bit jamming sequence (this is called a runt frame)

– Runt frame only occurs if senders are near. In worst case they may need to send as many as 512 bits (64) bytes past preamble to detect collision (thus the 46 data byte miminum: 14 bytes of header plus 46 bytes of data plus 4 byte CRC = 64 bytes).

– Why 512 bits?!

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512 Bits…

A sends frame at time t

A’s frame arrives at Bat time t+d

B begins transmitting attime t+d and collides withA’s frame

Host A must transmit for 2d to be sure it detects all possible collisions!

B’s runt (32 bits) arrives atA at time t +2d

Worst case scenario:

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512 Bits (cont.)

• A maximally configured Ethernet is 2500 meters long

• Has max of 4 repeaters• Round trip delay has been determined to be 51.2s

long, which on 10Mbps Ethernet is 512 bits• Bottom line: limits placed on Ethernet are

designed to limit latency on the network, and thus allow the access algorithm to work somewhat efficiently

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Ethernet Success

• Extremely easy to administer and maintain– No switches that can fail

– No routing or configuration tables to maintain

– Easy to add new hosts

• Inexpensive– Cable is cheap

– Only additional cost is network adapter for each host