Direct link networks
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Direct Link Networks. Encoding and Framing Error Detection/Correction Reliable Transmission will be covered later (with TCP) Media Access Control (Wired) LAN Technologies Readings Chapter 2 except Section 2.5, which is delayed to the discusson on Transport layer. Bits. 0. 0. 1. 0. 1.

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Direct link networks

Direct Link Networks

  • Encoding and Framing

  • Error Detection/Correction

  • Reliable Transmission will be covered later (with TCP)

  • Media Access Control

  • (Wired) LAN Technologies

  • Readings

    • Chapter 2 except Section 2.5, which is delayed to the discusson on Transport layer.

Direct Link networks


Encoding

Bits

0

0

1

0

1

1

1

1

0

1

0

0

0

0

1

0

NRZ

Encoding

  • Signals propagate over a physical medium

    • modulate electromagnetic waves (on amplitude, frequency, and phase): the process of encoding source data onto a carrier signal with frequency f_c

    • e.g., vary voltage (amplitude)

  • Encode binary data onto signals

    • e.g., 0 as low signal and 1 as high signal

    • known as Non-Return to zero (NRZ)

Direct Link networks


Problems consecutive 1s or 0s

Problems: Consecutive 1s or 0s

  • Low signal (0) may be interpreted as no signal

  • High signal (1) leads to baseline wander

    • Receiver keeps an average of the signal it has seen so far and uses this average to distinguish between low and high signals

  • Unable to recover clock

    • Clocks at the sender and receiver use signal transition to synchronize each other

Direct Link networks


Alternative encodings

Alternative Encodings

  • Non-return to Zero Inverted (NRZI)

    • make a transition from current signal to encode a one; stay at current signal to encode a zero

    • solves the problem of consecutive ones

  • Manchester

    • transmit XOR of the NRZ encoded data and the clock

    • only 50% efficient.

Direct Link networks


Encodings cont

Encodings (cont)

  • 4B/5B

    • every 4 bits of data encoded in a 5-bit code

    • 5-bit codes selected to have no more than one leading 0 and no more than two trailing 0s

    • thus, never get more than three consecutive 0s

    • resulting 5-bit codes are transmitted using NRZI

    • achieves 80% efficiency

Direct Link networks


Encodings cont1

Bits

0

0

1

0

1

1

1

1

0

1

0

0

0

0

1

0

NRZ

Clock

Manchester

NRZI

Encodings (cont)

Direct Link networks


Framing

Bits

Node A

Adaptor

Adaptor

Node B

Frames

Framing

  • Break sequence of bits into a frame – why? determine where a frame starts and ends

  • Typically implemented by network adaptor

Direct Link networks


Approaches

8

16

16

8

Beginning

Ending

Header

Body

CRC

sequence

sequence

Approaches

  • Sentinel-based

    • delineate frame with special pattern: 01111110

    • e.g., HDLC, SDLC, PPP

    • problem: special pattern appears in the payload

    • solution: bit stuffing

      • sender: insert 0 after five consecutive 1s

      • receiver: delete 0 that follows five consecutive 1s

      • What if the receiver sees a 1?

Direct Link networks


Approaches cont

Approaches (cont)

  • Couter-based

    • include payload length in header

    • e.g., DDCMP

    • problem: count field corrupted

    • solution: catch when CRC fails

    • How many frames are affected when counter or sentinel is corrupted?

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Approaches cont1

Approaches (cont)

  • Clock-based

    • each frame is 125us long

    • e.g., SONET: Synchronous Optical Network

Special bit pattern

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Handling errors

Handling Errors

  • Data can be corrupted during transmission

    • Bit values changed

  • Frame includes additional information

    • Set by sender

    • Checked by receiver

  • Error-detection vs error-correction

    • Both need redundant information

    • Detection: error exists or not.

    • Correction: repair if there was an error

  • Statistical guarantee

Direct Link networks


Error detecting and correcting codes

Error Detecting and Correcting Codes

  • How many check/redundancy bits?

    • To detect single-bit error

      • 1 bit: even/odd parity

    • To correct a single-bit error in m-bit message

      • Need a minimum of r bits such that

      • (m + r + 1)  2r

      • Example: 3-bit message needs 3-bit redundancy

  • Correction or detection+retransmission?

Direct Link networks


Error detection techniques

Error Detection Techniques

  • Checksum

    • Treat data as sequence of integers

    • Compute and send arithmetic sum

    • Detects some multiple bit errors not all

    • Internet Checksum

  • Cyclic Redundancy Check

    • Mathematical function of data

    • More complex to compute

    • Handles more errors

Direct Link networks


Cyclic redundancy check

Cyclic Redundancy Check

  • Add k bits of redundant data to an n-bit message

    • want k << n

    • e.g., k = 32 and n = 12,000 (1500 bytes)

  • Represent n-bit message as n-1 degree polynomial

    • e.g., MSG=10011010 as M(x) = x7 + x4 + x3 + x1

  • Let k be the degree of some divisor polynomial

    • e.g., C(x) = x3 + x2 + 1

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Crc cont

CRC (cont)

  • Transmit polynomial P(x) that is evenly divisible by C(x)

    • shift left k bits, i.e., M(x)xk

    • subtract remainder of M(x)xk / C(x) from M(x)xk

  • Receiver polynomial P(x) + E(x)

    • E(x) = 0 implies no errors

  • Divide (P(x) + E(x)) by C(x); remainder zero if:

    • E(x) was zero (no error), or

    • E(x) is exactly divisible by C(x)

Direct Link networks


Crc example

11111001

Generator

Message

1101

10011010000

1101

1001

1101

1000

1101

1011

1101

1100

1101

1000

1101

101

Remainder

CRC Example

Direct Link networks


Choice of c x

Choice of C(x)

  • Want to ensure C(x) doesn’t divide E(x)

  • We can detect

    • All single-bit errors if

      • C(x) has at least 2 terms

    • All double-bit errors if

      • C(x) doesn’t divide xj + 1

        • X15 + x14 + 1 doesn’t divide xj + 1 for any j below 32768

    • Any odd number of errors if

      • C(x) contains the factor x+1

    • Any “burst” error of length less than or equal to k bits

    • Most burst errors of length greater than k bits

Direct Link networks


Error detection summary

Error Detection Summary

  • To detect data corruption

    • Sender adds additional information to packet

    • Receiver checks

  • Techniques

    • Parity bit

    • Checksum

    • Cyclic Redundancy Check

Direct Link networks


Hamming error correcting code

Hamming Error Correcting Code

  • Achieve the theoretical lower limit of the number of redundant bits

  • Correcting single-bit errors

  • Method:

    • Put information bits at positions that are not equal to powers of two

    • Compute the check bits at positions that are power of two

      • A check bit at position 2^j, where j=0, 1, …, checks an information bit at position i if and only if the binary representation of i contains a 1 at position j.

      • The value of an “even” check bit equals the xor of all checked information bits

      • At the receiver side, all checked bits and the check bit are xored and a zero indicates error free; otherwise, the sum of the positions of the check bits with a non-zero value indicates the bit that is in error

  • Example

    • Information bits: 1001000

    • Coded bits: 00110010000

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Error recovery

Error Recovery

  • Reliable delivery over unreliable channel

    • How to recover from corrupted/lost packets

  • Error detection and retransmission

    • With acknowledgements and timeouts

    • Also called Automatic Repeat Request (ARQ)

    • Retransmission incurs round trip delay

  • Error correcting codes

    • Also called Forward Error Correction (FEC)

    • No sender retransmission required

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Summary

Summary

  • Encoding

  • Framing

  • Error correction/detection codes

    • Parity/Checksum/CRC

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Shared media and local area networks

Shared Media andLocal Area Networks

  • Shared Media Access Problem

    • Media access control (MAC) and LAN

    • MAC addresses and network adaptors (NICs)

    • MAC protocols: random access vs. controlled access

  • Ethernet

  • Token Ring and FDDI (read yourself)

  • 802.11 Wireless LAN (WiFi and WiMAX) (next lecture)

Direct Link networks


Multiple access links and lans

Multiple Access Links and LANs

Two types of “links”:

  • point-to-point, e.g.,

    • PPP for dial-up access, or over optical fibers

  • broadcast (shared wire or medium), e.g.

    • traditional Ethernet

    • 802.11 wireless LAN

Direct Link networks


Typical lan structure

RAM

RAM

Typical LAN Structure

  • Transmission Medium

  • Network Interface Card (NIC)

  • Unique MAC “physical” address

Ethernet Processor

ROM

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Adaptors communicating

link layer implemented in “adaptor” (aka NIC), with “transceiver” in it

Ethernet card, dial-up modem, 802.11 wireless card

sending side:

encapsulates packet in frame

adds error checking bits, flow control, reliable data transmission, etc.

receiving side

looks for errors, flow control, reliable data transmission, etc

extracts packet, passes to receiving node

data link & physical layers are closely coupled!

frame

frame

Adaptors Communicating

network packet

rcving

node

link layer protocol

sending

node

adapter

adapter

Direct Link networks


Mac physical addresses

MAC (Physical) Addresses

  • Addressing needed in shared media

    • MAC (media access control) or physical addresses

    • To identify source and destination interfaces and get frames delivered from one interface to another physically-connected interface (i.e., on same physical local area network!)

  • 48 bit MAC address (for most LANs)

    • fixed for each adaptor, burned in the adapter ROM

    • MAC address allocation administered by IEEE

      • 1st bit: 0 unicast, 1 multicast.

      • all 1’s : broadcast

  • MAC flat address -> portability

    • can move LAN card from one LAN to another

Direct Link networks


Mac physical or lan addresses

MAC (Physical, or LAN) Addresses

MAC addressing operations on a LAN:

  • each adaptor on the LAN “sees” all frames

  • accept a frame only ifdest. (unicast) MAC address matches its own MAC address

  • accept all broadcast (MAC= all 1’s) frames

  • accept all frames if set in “promiscuous” mode

  • can configure to accept certain multicast addresses (first bit = 1)

Direct Link networks


Mac sub layer

OSI

IEEE 802

Network layer

Network layer

802.2 Logical link control

LLC

Data link

layer

802.11

Wireless

LAN

Other

LANs

802.3

CSMA-CD

802.5

Token Ring

MAC

Physical

layer

Various physical layers

Physical

layer

MAC Sub-layer

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Broadcast links multiple access

Broadcast Links: Multiple Access

Single shared communication channel

  • Only one can send successfully at a time

  • Two or more simultaneous transmissions

    • interference!

  • How to share a broadcast channel

    • media access control uses same shared media

  • Humans use multi-access protocols all the time

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Random access

Random Access

  • Stations contend for channels

  • Overlapping transmissions (collisions) can occur

    • Carrier sensing?

    • Collision detection?

  • Protocols

    • Aloha

    • Slotted Aloha

    • Carrier Sense Multiple Access: Ethernet

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Controlled access

Controlled Access

  • Stations reserve or are allocated channel

    • No collisions

    • Allocation: static or dynamic

  • Protocols

    • Static channel allocation

      • Time division multiple access

    • Demand adaptive channel allocation

      • Reservation protocols

      • Token passing (token bus, token ring)

Direct Link networks


Taxonomy of mac protocols

Taxonomy of MAC Protocols

WiFi (802.11)

Direct Link networks


Pure unslotted aloha

Pure (unslotted) Aloha

  • Simpler, no synchronization

  • Just send: no waiting for beginning of slot

Direct Link networks


Slotted aloha

Slotted Aloha

  • Time is divided into equal size slots

  • Nodes transmit at the beginning of a slot

  • If collision, retransmit later

Success (S), Collision (C), Empty (E) slots

Direct Link networks


Performance of aloha protocols

0.4

0.3

Slotted Aloha

0.2

0.1

Pure Aloha

1.5

2.0

0.5

1.0

G = offered load = Np

Performance of Aloha Protocols

S = throughput = “goodput”

(success rate)

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Carrier sense multiple access

Carrier Sense Multiple Access

  • Aloha is inefficient (and rude)

    • Doesn’t listen before talking

  • CSMA: Listen before transmit

    • If channel idle, transmit entire packet

    • If busy, defer transmission

      • How long should we wait?

    • Human analogy: don’t interrupt others

  • Can carrier sense avoid collisions completely?

Direct Link networks


Persistent and non persistent csma

Persistent and Non-persistent CSMA

  • Non-persistent

    • If idle, transmit

    • If busy, wait random amount of time

    • If collision, wait random amount of time

  • p-persistent

    • If idle, transmit with probability p

    • If busy, wait till it becomes idle

    • If collision, wait random amount of time

Direct Link networks


Csma cd

CSMA/CD

CSMA with collision detection (CD)

  • Listen while talking

  • Stop transmitting when collision detected

    • Compare transmitted and received signals

    • Save time and bandwidth

    • Improvement over persistent and nonpersistent protocols

  • Human analogy

    • Polite conversationalist

  • Worst case time to detect a collision?

Direct Link networks


Collisions

Collisions

A

B

A

B

A

B

A

B

Direct Link networks


Worst case collision detection time

Worst Case Collision Detection Time

Direct Link networks


Csma cd illustration

CSMA/CD: Illustration

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Ethernet overview

64

48

48

16

32

Src

Dest

Preamble

Type

Body

CRC

addr

addr

Ethernet Overview

  • History

    • developed by Xerox PARC in mid-1970s

    • roots in Aloha packet-radio network

    • standardized by Xerox, DEC, and Intel in 1978

    • similar to IEEE 802.3 standard

  • CSMA/CD

    • carrier sense

    • multiple access

    • collision detection

  • Frame Format

Direct Link networks


Ethernet

Ethernet

“Dominant” LAN technology:

  • cheap $20 for 100Mbs!

  • first widely used LAN technology

  • Simpler, cheaper than token ring LANs and ATM

  • Kept up with speed race: 10, 100, 1000 Mbps

Metcalfe’s Ethernet

sketch

Direct Link networks


Ethernet frame format

8 bytes

6

6

2

0-1500

4

Src

Dest

Type

Data

Preamble

CRC

addr

addr

8 bytes

6

6

2

0-1500

4

Src

Dest

Length

Data

Preamble

CRC

addr

addr

Ethernet Frame Format

DIX frame format

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

IEEE 802.3 format

  • Ethernet has a maximum frame size: data portion <=1500 bytes

  • It imposes a minimum frame size: 64 bytes (excluding preamble)

  • If data portion <46 bytes, pad with “junk” to make it 46 bytes

  • Q: Why minimum frame size in Ethernet?

Direct Link networks


Fields in ethernet frame format

Fields in Ethernet Frame Format

  • Preamble:

    • 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 (SoF: start-of-frame)

    • used to synchronize receiver, sender clock rates, and identify beginning of a frame

  • Addresses: 6 bytes

    • if adapter receives frame with matching destination address, or with broadcast address (eg ARP packet), it passes data in frame to network layer protocol (specified by TYPE field)

    • otherwise, adapter discards frame

  • Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk

    • 802.3: Length gives data size; “protocol type” included in data

  • CRC: checked at receiver, if error is detected, the frame is simply dropped

Direct Link networks


Ieee 802 3 mac ethernet

IEEE 802.3 MAC: Ethernet

MAC Protocol:

  • CSMA/CD

  • Truncated binary exponential backoff

    • for retransmission n: 0 < r < 2k *ASlotTime, where k=min(n,10)

    • give up after 16 retransmissions

  • Slot Time is the critical system parameter

    • upper bound on time to detect collision

    • upper bound on time to acquire channel

    • upper bound on length of frame segment generated by collision

    • quantum for retransmission scheduling

    • max{round-trip propagation, MAC jam time}

Direct Link networks


Ieee 802 3 parameters

IEEE 802.3 Parameters

  • 1 bit time = time to transmit one bit

    • 10 Mbps  1 bit time = 0.1 s

  • Maximum network diameter  2.5km

    • Maximum 4 repeaters

  • “Collision Domain”

    • Distance within which collision can occur and be detected

    • IEEE 802.3 specifies:

      worst case collision detection time: 51.2 s

  • Slot time

    • 51.2 s = 512 bits = 64 bytes

  • Why minimum frame size?

    • 51.2 s => minimum # of bits can be transited at 10Mpbs is 512 bits => 64 bytes is required for collision detection

Direct Link networks


Ethernet mac protocol basic ideas

Ethernet MAC Protocol: Basic Ideas

1-persistent CSMA/CD

  • Carrier sense: station listens to channel first

    • Listen before talking

  • If idle, station may initiate transmission

    • Talk if quiet

  • Collision detection: continuously monitor channel

    • Listen while talking

  • If collision, stop transmission

    • One talker at a time

  • Exponential binary back-off algorithm

Direct Link networks


Ethernet csma cd alg flow chart

Ethernet CSMA/CD Alg. Flow Chart

Direct Link networks


Ethernet csma cd algorithm

1. Adaptor gets datagram and creates frame

2. If adapter senses channel idle, it starts to transmit frame. If it senses channel busy, waits until channel idle and then transmits

3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame ! Signal to network layer “transmit OK”

4. If adapter detects another transmission while transmitting, aborts and sends jam signal

5. After aborting, adapter enters exponential backoff: after the mth collision, adapter chooses a K at random from {0,1,2,…,2m-1}. Adapter waits K*512 bit times and returns to Step 2

6. Quit after 16 attempts, signal to network layer “transmit error”

Ethernet CSMA/CD Algorithm

Direct Link networks


Ethernet s csma cd more

Jam Signal: make sure all other transmitters are aware of collision; 32 bits;

Bit time: .1 microsec for 10 Mbps Ethernet ;for K=1023, wait time is about 50 msec

Exponential Backoff:

Goal: adapt retransmission attempts to estimated current load

heavy load: random wait will be longer

first collision: choose K from {0,1}; delay is K x 512 bit transmission times

after second collision: choose K from {0,1,2,3}…

after ten collisions, choose K from {0,1,2,3,4,…,1023}

Ethernet’s CSMA/CD (more)

Direct Link networks


Csma cd efficiency

CSMA/CD Efficiency

Relevant parameters

  • cable length, signal speed, frame size, bandwidth

  • tprop = max prop between 2 nodes in LAN

  • ttrans = time to transmit max-size frame

  • Efficiency goes to 1 as tprop goes to 0

  • Goes to 1 as ttrans goes to infinity

  • Much better than ALOHA, but still decentralized, simple, and cheap

  • Direct Link networks


    Ieee 802 3 physical layer

    (b)

    (a)

    transceivers

    IEEE 802.3 Physical Layer

    IEEE 802.3 10 Mbps medium alternatives

    Hubs & Switches!

    Thick Coax: Stiff, hard to work with

    T connectors flaky

    Direct Link networks


    Ethernet technologies 10base2

    Ethernet Technologies: 10Base2

    • 10: 10Mbps; 2: under 200 meters max cable length

    • thin coaxial cable in a bus topology

    • repeaters used to connect up to multiple segments

    • repeater repeats bits it hears on one interface to its other interfaces: physical layer device only!

    • has become a legacy technology

    Direct Link networks


    10baset

    nodes

    hub

    10BaseT

    • 10 Mbps rate

    • T stands for Twisted Pair

    • Nodes connect to a hub: “star topology”; 100 m max distance between nodes and hub

    • Hubs are essentially physical-layer repeaters:

      • bits coming in one link go out all other links

      • no frame buffering

      • no CSMA/CD at hub: adapters detect collisions

      • provides net management functionality

    Direct Link networks


    Ethernet hubs switches

         

    Single collision domain

    High-Speed backplane or interconnection fabric

    (b)

    (a)

       

    Ethernet Hubs & Switches

    Twisted Pair Cheap

    Easy to work with

    Reliable

    Star-topology CSMA-CD

    Twisted Pair Cheap

    Bridging increases scalability

    Separate collision domains

    Full duplex operation

    Direct Link networks


    Evolution of ethernet

    Evolution of Ethernet

    From early 80’s 10Base Ethernet to 90’s 100Base (Fast) Ethernet

    to today’s Gigabit Ethernet to 10 Gigabit Ethernet, ……

    IEEE 802.3 Original Parameters

    • transmission Rate: 10 Mbps

    • Min Frame: 512 bits = 64 bytes

    • slot time: 512 bits/10 Mbps = 51.2 msec

      • 51.2 msec x 2x105 km/sec =10.24 km

      • 5.12 km round trip distance

    • max Length: 2500 meters + 4 repeaters

    • For compatibility, desire to maintain same frame format!

      • Each x10 increase in bit rate, must be accompanied by x10 decrease in distance ?!

    Direct Link networks


    100base t fast ethernet issues

    100Base T (Fast) Ethernet: Issues

    • 1 bit time = time to transmit one bit

      • 100 Mbps  1 bit time = 0.01 s

    • If we keep the same “collision domain”, i.e.,

    • worst case collision detection time kept at 51.2 s

      • Q: What will be the minimum frame size?

      • 51.2 s => minimum # of bits can be transited at 100Mpbs is 5120 bits => 640 bytes is required for collision detection

      • This requires change of frame format and protocol!

    • Or we can keep the same minimum frame size, but reduce “collision domain” or network diameter!

      • slot time from 51.2 s to 5.12 s!

      • maximum network diameter  100 m!

    Direct Link networks


    Fast 100mbps ethernet

    Fast (100Mbps) Ethernet

    IEEE 802.3 100 Mbps Ethernet medium alternatives

    • To preserve compatibility with 10 Mbps Ethernet:

    • Same frame format, same interfaces, same protocols

    • Hub topology only with twisted pair & fiber

    • Bus topology & coaxial cable abandoned

    • Category 3 twisted pair (ordinary telephone grade) requires 4 pairs

    • Category 5 twisted pair requires 2 pairs (most popular)

    • Most prevalent LAN today

    Direct Link networks


    Gigabit ethernet

    Gigabit Ethernet

    Gigabit Ethernet Physical Layer Specification

    (IEEE 802.3 1 Gigabit Ethernet medium alternatives)

    Direct Link networks


    Gigabit ethernet1

    Gigabit Ethernet

    • use standard Ethernet frame format

    • allows for point-to-point links and shared broadcast channels

    • in shared (half-duplex hub) mode, CSMA/CD is used

      - slot time increases to 512 bytes

      - small frames need to be extended to 512 B

      - carrier extension:

      • frame bursting: allow stations to transmit burst of short frames

    • Commonly used today: Gigabit switches!

      • Full-Duplex at 1 Gbps for point-to-point links

      • Frame structure preserved but CSMA-CD essentially abandoned

      • Extensive deployment in backbone of enterprise data networks and in server farms

    Direct Link networks


    Carrier extension

    RRRRRRRRRRRRR

    Frame

    Carrier Extension

    512 bytes

    Carrier Extension

    • For 10BaseT : 2.5 km max; slot time = 64 bytes

    • For 1000BaseT: 200 m max; slot time = 512 bytes

    • Carrier Extension : continue transmitting control characters [R] to fill collision interval.

    • This permits minimum 64-byte frame to be handled.

    • Control characters discarded at destination.

    • For small frames net throughput is only slightly better than Fast Ethernet.

    Direct Link networks


    Frame bursting

    Frame

    Extension

    Frame

    Frame

    Frame

    512 bytes

    Frame burst

    Frame Bursting

    • Source sends out burst of frames without relinquishing control of the network.

    • Uses Ethernet Interframe gap filled with extension bits (96 bits)

    • Maximum frame burst is 8192 bytes

    • Three times more throughput for small frames.

    Direct Link networks


    10 gigabit ethernet

    10 Gigabit Ethernet

    IEEE 802.3 10 Gbps Ethernet medium alternatives

    • Frame structure preserved

    • CSMA-CD protocol officially abandoned

    • LAN PHY for local network applications

    • WAN PHY for wide area interconnection using SONET OC-192c

    • Extensive deployment in metro networks anticipated

    Direct Link networks


    Typical ethernet deployment

    Server farm

    Server

    Server

    Server

    Gigabit Ethernet links

    Switch/router

    Switch/router

    Gigabit Ethernet links

    Ethernet switch

    Server

    Ethernet switch

    Ethernet switch

    Server

    Server

    100 Mbps links

    100 Mbps links

    100 Mbps links

    Hub

    Hub

    Hub

    10 Mbps links

    10 Mbps links

    10 Mbps links

    Department C

    Department B

    Department A

    Typical Ethernet Deployment

    Direct Link networks


    Ethernet summary

    Ethernet Summary

    • 1-persistent CSMA/CD

    • 51.2 s to seize the channel

    • Collision not possible after 51.2 s

    • Minimum frame size of 64 bytes

    • Binary exponential backoff

    • Works better under light load

    • Delivery time non-deterministic

    Direct Link networks


    Summary continued

    Summary (continued)

    • Address

      • Unique number assigned to station

      • Put in frame header

      • Recognized by hardware

    • Address forms

      • Unicast

      • Broadcast

      • Multicast

    Direct Link networks


    Summary continued1

    Summary (continued)

    • Type information

      • Describes data in frame

      • Set by sender

      • Examined by receiver

    • Frame format

      • Header contains address and type information

      • Payload contains data being sent

    Direct Link networks


    Summary continued2

    Summary (continued)

    • LAN technologies

      • Ethernet (bus)

      • Token Ring

      • FDDI (ring)

      • Wireless 802.11

    • Wiring and topology

      • Logical topology and Physical topology (wiring)

      • Hub allows

        • Star-shaped bus

        • Star-shaped ring

    Direct Link networks


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