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CSS432 Shared Access NetworksTextbook Ch2.6 - 2.7

Professor: Munehiro Fukuda

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Ethernet Overview History

Developed by Xerox PARC in mid-1970s Similar to IEEE 802.3 standard

CSMA/CD Carrier sense multiple access with collision detection

Frame Format

MAC (Media Access Control) Addresses unique, 48-bit unicast address assigned to each adapter example: 8:0:e4:b1:2 broadcast: all 1s, multicast first bit is 1

Bandwidth: 10Mbps (10Base2=Thin Coax 200m, 10B5=Yellow Thick Coax 500m, 10BT=Twisted pair 100m), 100Mbps(10BaseT), 1Gbps

Length: 2500m (500m segments with 4 repeaters)

Destaddr

8 6 4

CRCPreamble Srcaddr

Type Body

26

Nextframe

Inter-frame gapbytes 46 ~ 1500

Min: 64bytes (512bits) ~ Max: 1518bytes

Used by layer 3IP: 0x0800

ARP: 0x0806IPv6: 0x86DD

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Ethernet Transmit Algorithm If line is idle…

Send immediately Upper bound message size of 1500 bytes: MTU

(Maximum Transmission Unit) Must wait 9.6usec between back-to-back frames

Why? (See the next slide.)

If line is busy… Wait until idle and transmit immediately Called 1-persistent

Transmit a packet with probability 1. (special case of p-persistent: transmitting a

packet with P percent, where 0 < p ≤ 1)

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Transmit Algorithm (cont) If collision…

Send a 32-bit jamming sequent, and then stop transmitting frame (64bit preamble + 32bit jam = 96bits)

Minimum frame is 64 bytes (header + 46 bytes of data) = 512bits Why? (See the next slide.)

Delay and try again 1st time: 0 or 51.2us 2nd time: 0, 51.2, or 102.4us 3rd time51.2, 102.4, or 153.6us nth time: k x 51.2us, for randomly selected k=0..2n -

1 give up after several tries (usually 16) exponential backoff

10Mbps means 10bits/usec.96bis needs 9.6usec

1010101… 64bits1010.. 32bits

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Collisions

A B

A B

A B

A B

500m x 5 = 2500m (with 4 repeaters)

A collision occurred

Time t

Time t + d(d = 25.6us: approx. 0.01us/m)

Time t + 2d(2d = 51.2us)

Jam seq

10Mbps x 51.2us = 10 x 106 x 51.2 x 10-6 = 512bits = (64bytes)

Bandwidth 10Mbps 100Mbps 1Gbps

Bits/usec 10bits/usec 100bits/usec 1000bits/usec

Jamming sequence 96bits => 9.6usec 96bits => 0.96usec 96bits => 0.096usec (96nsec)

Max delay (RTT) 512bits => 51.2usec 512bits => 5.12usec 512bits => 0.512usec

Speed 51.2usec/5000m = 10.2nsec/m 51.2usec/200~400m = 12.8nsec/m 0.512usec/25m (cupper) = 20.5nsec/m

latency

Network load30%

Ethernet

Token Ring

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Token Ring Overview Examples

16Mbps IEEE 802.5 (based on earlier IBM ring) 100Mbps Fiber Distributed Data Interface (FDDI)

Idea Frames flow in one direction: upstream to downstream Special bit pattern (token) rotates around ring Must capture token before transmitting Release token after done transmitting

Immediate release: FDDI Delayed release (after a sent frame came back) IEE802.5

Remove your frame when it comes back around Stations get round-robin service

Frame Format

Control

8 8 8 24

CRCStart offrame

End offrame

Destaddr

Body

4848

Srcaddr

Status

32

Priority bits

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Token Maintenance Lost Token

No node emits a token when initializing ring. A bit error corrupts token pattern. A node holding token crashes.

Lost Token Detection IBM Token Ring: No more presence message from a monitor station FDDI: No more message for more than 2.5ms

Election Procedure A station transmits a claim token with its MAC address if doubting the

existence of a monitor station. The highest address wins. FDDI: the largest TTRT(Target Token Rotation Time) wins.

Token Generation Wait for NumStations x THT(Token Hold Time) + RingLatency

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Wireless Network

Ad-hoc Network WiFi

Base Stations WiFi, Bluetooth, Cellular

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Wireless Network Bluetooth (802.15.1)

License-exempt band at 2.45GHz

Wi-Fi (802.11)

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Cell Phone

master

activeslave

activeslave

parkedslave

parkedslave

7 255: :

Request

Reponses

o e o e o e o eTDM

A B C

DA B C

Hidden nodes: A and C are unaware ofeach other but their frames collide at B.

Exposed node problem: B sends A a frame which C hears.This concludes C can’t send D a frame, which is actually possible.

MACA: Multiple Access with Collision Avoidance: RTS: a sender sends a request to send. CTS: a receiver responds a clear to send. ACK: a receiver sends an ack upon a successful completion.

Distribution system

AP-1 AP-3AP-2 AP: Access point

DA

B

C

probe

Probe response Assoc request

E

1 2 3Base satations

P

Call

P

Handoff

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Wireless Network

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Direct sequence: n (e.g. 4)-bit chipping sequence

CDMA: Code Division Multiple Access (Cellular-3G) Each cell phone sends data with a different (but pre-

assigned chipping code.) A base station distinguish many cell phones using their

unique chipping code.

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Network Adapters

CPU

Memory

I/O busController

BusInterface

LinkInterface

FIFO

FIFO

DMA

NetworkProcessor memory

Interrupt

System Bus PCIBus

NetworkLink

Host Computer

Network Interface Card (NIC)

Programmed I/O

32bit, 33MHz = 1056Mbps633Mbps STS-121000Mbps Ehternet

Example Myrinet Lanai series

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Performance Considerations in Layer 2: Data Link Layer

DMA Initializing DMA channels versus programming CPU I/O

Frame Size Stuffing a full frame versus distinguishing one-time small-frame

transfer and burst frame transfer channels Frame Transfer Strategy

Individual transfers versus pipelined transfers Fragmentation/Aggregation

Frame fragmentation versus frame aggregation Multicast through a switch

Software emulation or hardware implementation

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Memory Bottleneck PCI(33bit, 33MHz) = 1056Mbps System Bus = 235MBps = 1880Mbps (Text example) ≈ real

throughput Memory Bus Controller

Arbitration among CPU and DMA DRAM setup time

Data copy: Application memory space to OS OS memory space to NIC DMA-manageable space

Zero copy/pin-downed communication Bypassing OS

Reducing memory-copying overhead Reducing interrupts

Application

DMA-manageable memory

OS

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Reviews Ethernet: k-persistent, exponential back off, and the

relationship between the minimum frame size and collisions.

Token ring network: immediate/delayed release, THT, TRT, and TTRT

Network adapters: writev/readv and memory bottleneck

Exercises in Chapter 2 Ex. 42 (Ethernet) Ex. 46 (Ethernet) Ex. 53 (Wi-Fi) Ex. 54 (Wi-Fi)