tcom 513 optical communications networks

TCOM 513Optical Communications

Networks

Spring, 2007

Thomas B. Fowler, Sc.D.

Senior Principal Engineer

Mitretek Systems

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Topics for TCOM 513

Week 1: Wave Division Multiplexing Week 2: Opto-electronic networks Week 3: Fiber optic system design Week 4: MPLS and Quality of Service Week 5: Optical control planes Week 6: The business of optical networking: economics

and finance Week 7: Future directions in optical networking

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Virtual Session

End-to-End Messages

Physical

Presentation Presentation

Session Session

Network Network

Data Link Control

Data Link Control

PhysicalPhysical

Physical Link, e.g. electrical signals

Physical portion of code

Logical portion of

code

Virtual Network ServiceApplicationApplication

End-to-End PacketsTransport Transport

DLC DLC DLC DLC

NetworkNetwork

Bits

Packets

Frames

Physical Physical Physical

Originating site

Terminating site

Subnet node

Subnet node

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Opto-electronic systems and networks

LAN protocols– Fiber distributed data interface (FDDI)– Fiber channel– Gigabit/10 Gigabit Ethernet

SONET/SDH Ethernet over optical networks

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LAN protocols

Layers 1 and 2 Map into OSI reference model

Souce: Cisco

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FDDI

Developed by American National Standards Institute (ANSI) Originally proposed as internal fiber optic I/O channel for

computers Later became generalized to high-speed LAN running at

100 Mbps– Can run on copper as well as fiber– Dual-ring is usual configuration– Can go up to 200 Mbps with single ring

Token ring architecture– Advantage of token-passing networks: deterministic– Possible to calculate maximum time before station can

transmit• Popular in real-time environments

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Characteristics of FDDI

Token ring architecture– Two countercirculating rings– Only one used for data; other for backup

Ring size– Up to 200 km (on multimode fiber, single ring)– Dual ring size up to 100 km– Maximum of 500 stations

• Max distance between stations is 2 km Packet switched: utilizes variable length frames

– Max frame size is 4500 bytes– Frame header contains destination address

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Characteristics of FDDI (continued) Guaranteed bandwidth availability

– Equality of access as in all token-ring systems– Guaranteed bandwidth for synchronous traffic

Token-ring protocol– Similar to IEEE 802.5 token-ring LAN– Differs in that it is dependent on timers

Ring stations– Each may connect to both rings or only primary ring

Ring monitor– Performed cooperatively by all stations rather than by

single active monitor• All look for errors; if found any station can request

reinitialization of ring– Each station does not have to have ring monitor

function

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FDDI ring structure, with/without break

Source: Dutton

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FDDI ring configuration

Source: Dutton

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FDDI token ring protocol operation

Ring access controlled by special frame called a “token”– Only one token present at any time– When a station receives the token it has permission to

send– When station finishes sending it must place token back

on ring Each station on the ring receives and retransmits frames

– Ring is not a node

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Timing on FDDI

3 timers required due to need to handle synchronous traffic– Token rotation timer (TRT)

• Elapsed time since last token received– Target token rotation timer (TTRT)

• Target maximum time between tokens time for token to traverse ring

• 4 msec < TTRT < 165 msec• Optimal value often around 8 msec

– Token holding timer (THT)• Governs max amount of data station may send• Max time allocated for station to send

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Operation

When station receives token it compares time since last token (TRT) with target time (TTRT)– Normal operation: TRT < TTRT

• Station can send multiple frames until TTRT reached• TTRT-TRT = THT

– Overload: 2xTTRT> TRT > TTRT• Synchronous data only permitted

– Error: TRT > TTRT• Must be conveyed to LAN manager

Delays may occur– Stations must be capable of buffering data

Stations must remove data they send when it returns to them

May be many frames on ring, but only one token

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Operation (continued)

When ring initialized, stations cooperate to determine TTRT value– Minimum of all requested TTRT values– Changed only if new station enters ring

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Physical media for FDDI

Multimode fiber– Originally defined mode of operation

Single mode fiber– Included in standard but little used

Twisted pair copper wire– STP = shielded twisted pair

• Not as good as fiber, but cheaper– UTP-5 (=cat 5) unshielded twisted pair standard in 1994

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Media specifications

Medium Fiber Light

sourceDetector Transmit power Receiver

sensitivity

Multimode 62.5/125

50/125

85/125

100/140

LED PIN diode (1) -20 to -14 dBm

(2) -4 to 0 dBm

(1) -31 to -14 dBm

(2) -37 to -15 dBm

Single mode

9 micron LED PIN diode (1)-20 to -14 dBm

(2)-4 to 0 dBm

(1)-31 to -14 dBm

(2)-37 to -15 dBm

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Data encoding and clocking

Four data bits encoded as five bit group– 100 Mbps actually 125 Mbaud on ring– Allows adding of more transitions into bit stream to allow

for problem of too many 1s or 0s Uses Non Return to Zero Inverted (NRZI) encoding Each station has own clock

– Specification is accuracy of 0.005%– Max difference between stations 0.01%– 10 bit buffer inside each station to allow for differences in

clocks between stations• Gives average of 4.5 bit times to smooth out timing

differences– Determines max frame size

4.5 bits/0.01% = 45,000 bits = 9,000 symbols = 4,500 bytes

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Physical layer operation

Source: Dutton

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Comparison with standard token ring networks

Standard TRN uses Manchester encoding– Allows exact recovery of clock, but at cost of doubling

frequency FDDI uses optical signals at higher speed than TRN

– Does not have exact clock recovery, substitutes buffer

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FDDI layers

Source: Dutton

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FDDI layers (continued)

Physical Medium Dependent layer (PMD)– Optical link parameters– Cables and connectors– Optical bypass switch– Power levels

Physical Layer Protocol (PHY)– Access to ring– Clocking, synchronization, buffering– Code conversion– Ring continuity

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FDDI layers (continued)

Media Access Control (MAC)– Tokens and timers– Frame check sequence

Station Management (SMT)– Ring Management (RMT)

• Ensures valid token circulating– Connection Management (CMT)

• Physical connections and topology– Operational Management

• Monitors timers and parameters• Interfaces to external network management software

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SONET overview

SONET = Synchronous Optical Network– Should have been called Synchonous Opto-electronic

network (SOENET) Technology developed in 1980s for long-haul trunks

needed by Telcos– Formulated by Exchange Carriers Standards

Association (ECSA)• Industry group which sets standards for telecoms• 1984 work began

– Expected to serve as basis for Telcos for 20-30 years– Designed from ground up based on 64kbps channels

(DS0—voice channels)• Everything a multiple of this

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SONET (continued)

Emphasis on qualities important to Telcos– Reliability– Availability– Millisecond recovery from outages

Optimal use of bandwidth of secondary concern Not originally intended as bulk data carrier or carrier for

asychronous packets Serves as transport only

– Does not do switching Utilizes optical components only because copper not fast

enough– Otherwise copper or fiber could transmit SONET

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Advantages of SONET

Reduction in equipment Standardization of equipment to allow for plug and play Increased network reliability Provision of overhead and payload bytes Synchronous multiplexing format

– Allows carrying of different loads– Simplifies interfacing to switching equipment

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Basic structure of SONET

Utilizes time division multiplexing to combine large number of individual signals

Structured in fixed-length frames Entire network operates synchronously Synchronous operation requires extremely precise

clocking throughout network– Utilizes Stratum atomic clock

• Known as “Primary Reference Clock” (PRC)• Accurate to 1 part in 1011

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Basic structure of optical part of SONET

Modulator

Input signal Connector Optional optical amplifier

Amplifier Decoder

Output signal

Optical fiber Optical fiber

Light Wavelength = 800-1600 nm

Electricity Electricity

Light source

Detector

Input SONET

signal(time

multiplexed individual

signals)

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Evolving SONET network architecture

Source Encoder(Time

Division Multiplexer)

Modulator/ transmitter

(Wavelength multiplexer)

ReceiverDecoder(Demux)

Receiver/ demodulator

(Demux)

Link

end user services

end userservices

SONET

SONET

DWDM

DWDM

SONET

SONET

end user services

end user services

1

n

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SONET structure

First step in SONET multiplexing process: generation of lowest level or base signal– Referred to as Synchronous Transport Signal level-1, or

STS-1– 51.84 Mbits/second– Higher level signals are multiples of this, giving rise to

STS-N• N is not arbitrary, but restricted to certain values• STS-N signals composed of N byte-interleaved STS-1

signals– Optical counterpart known as “Optical Carrier level-1”

or OC-1

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SONET hierarchy

Source: Tektronix

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SONET frame format 810 bytes

– Logically a 90 column by 9 rows– Order of transmission: row by row, L to R within rows,

most significant byte first

9 rows

90 columns

Source: Tektronix

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SONET frame format (continued)

One frame per 125 sec = 8,000 frames/sec 8,000 frames/sec x 810 bytes/frame x 8 bits/byte = 51,840,000

bits/sec Column = 9 bytes x 8000 per second x 8 bits/byte = 576K bits

SONET frame

TransportOverhead

Synchronous Payload Envelope (SPE)—783 bytes

STS PathOverhead

(POH)—9 bytes

Payload756 bytes(84 cols.)

Fixedstuff

18 bytes

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SONET frame structure: SPE

Source: Tektronix

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SONET frame structure (continued)

SPE does not have to be aligned with STS frame– Can begin anywhere in STS frame– Starting location designated by STS payload pointer in

transport overhead

Source: Tektronix

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Overhead structure

Two types– Transport (27 bytes)

• Section (9 bytes)• Line (18 bytes)

– Path (9 bytes, embedded in SPE)

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Overhead structure (continued)

Source: Tektronix

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Detailed structure of overhead

Source: Tektronix

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Function of overhead

Section (9 bytes)– Performance monitoring (STS-N signal)– Local orderwire– Datacomm channels to carry info for OAM&P– Framing

Line overhead (18 bytes)– Locating SPE in frame– Multiplexing or concatenating signals– Performance monitoring– Automatic protection switching– Line maintenance

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Function of overhead (continued)

Path overhead (9 bytes)– Performance monitoring (STS SPE)– Signal label (contents of STS SPE)– Path status– Path trace

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SONET alarms

Three levels to allow close monitoring of deteriorating conditions– Anomaly: discrepancy between observed and expected

• Does not constitute interruption in service– Defect: density of anomalies reached level where

service is interrupted• May be correctable

– Failure: Inability of function to perform required action (defect) persisted beyond allowable time span

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SONET alarms

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SONET Alarms (continued)

Source: Tektronix

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Tributaries and Virtual Tributaries (VTs)

Need exists to transmit channels slower than full STS– Called tributaries or virtual tributaries– Only certain channel speeds allowed

Tributaries may occupy a number of consecutive columns within payload or be interleaved (time multiplexed) (usual)– US T-1 (1.544 Mbps) uses 3 columns

• Only requires 24 slots, given 27 = 3 slots wasted• Recall that each slot is 64 kbits, x 24 = 1.544 Mbps

– European E-1 (2.048 Mbps) uses 4 columns• Only requires 32, given 36 = 4 slots wasted

– Benefit is that single tributary can be demultiplexed without need to demultiplex entire stream

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VT sizes

Used for T1

Used for E1

Source: Tektronix

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Tributaries (continued)

An SPE carrying VTs is divided into 7 VT groups– Each group consists of 12 columns– 12 x 7 = 84 columns = payload capacity

Columns for each VT type are all factors of 12 Each VT group can carry only one VT type

– Cannot mix VT1.5 and VT3, even though they would fit– Separate VT groups within frame can carry different VT

types– Allowed combinations within a VT group

• 4 VT1.5• 3 VT2• 2 VT3• 1 VT6

Within group, VTs are interleaved (time multiplexed)

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Multiplexing of VTs within group

Source: Tektronix

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Multiplexing of VT groups

Source: Tektronix

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Pointers

Used to compensate for frequency and phase variation Allow transport of synchronous payloads across

plesiosynchronous (almost synchronous) network boundaries

Avoid delays and losses of having to use 125 sec slip buffers

Dynamically and flexibly aligning payloads– Dropping– Inserting– Cross-connecting

Effects of jitter can also minimized

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Pointers (continued)

Byte stuffing used to fix alignment dynamically– Positive: byte added– Negative: byte deleted

Does not affect data

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Pointers (continued)

Source: Tektronix

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Layers of multiplexing in SONET

Time division– (1) Data prior to sending to SONET

• E.g., several slow-speed channels multiplexed to make T1

– (2) Within VT group• E.g., several T1s

– (3) Among VT groups in STS frame– (4) Among STS frames for speeds greater than OC-1

• May be done multiple times, e.g., 4 OC-3 to OC-12, 4 OC-12 to OC-48, 4 OC-48 to OC-192

If WDM used, (5) wavelength multiplexing of SONET signals

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SONET multiplexing (continued)

56K

128K

384K

x1001 Tbps

TDMLevel 1

TDMLevel 2

TDMLevel 3

TDMLevel 4

WDMLevel 5

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SONET network elements

Terminal multiplexers– Level 3 or 4

Regenerator (repeater) Digital loop carrier (DLC)

– Concentrator at level 1 Add/drop multiplexer (ADM)

– Picks off multiplexed signals– Adds new signals

Source: Tektronix

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SONET network elements (continued)

Digital cross-connects (DCS or DCX)– Accesses signals at STS-1 level and switches them– SONET equivalent of DS3 cross connect– Allows overhead to be maintained because network is

synchronous– Can make 2-way connections at DS3, STS-1, STS-Nc

levels• STS-Nc requires contiguous, not interleaved bytes

Source: Tektronix

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SONET network configurations

Point-to-point Point-to-multipoint Hub Ring

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Point-to-point

Two terminal multiplexers connected by optical link– May or may not use repeaters– Simplest SONET application

Source: Tektronix

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Point-to-multipoint

Linear add/drop architecture– Circuits added, dropped along the path

SONET ADM designed for this task– Avoids need to completely demux signal, cross-connect

channels, remux– Typically placed along path to allow adding, dropping

channels where needed

Source: Tektronix

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Hub Concentrates traffic at one or more sites Allows for easy reprovisioning Two implementations

– Cross-connecting tributary services• Requires 2 or more ADMs, cross-connect switch

– Cross-connecting at tributary and SONET level• Requires cross-connect switch

Source: Tektronix

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Ring architecture Most popular architecture

– Used by all major carriers Basic building block is ADM Bi-directional or uni-directional traffic Main advantage: survivability

– If fiber cut, multiplexers canreroute in milliseconds

Source: Tektronix

After cut

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Ring architecture (continued)

Source: Tektronix

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Limitations of SONET ring architecture

SONET ring architecture very complex Main problem is scalability

– To increase capacity or add new locations requires building a new set of rings, which is very expensive

– Mitigated to some extent by DWDM But hardware is standardized and available from multiple

sources– SONET does its job well– Is established and low-risk technology

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SONET and SDH

SDH=Synchronous Digital Hierarchy– Used widely outside of US, Japan– Same 125sec frames– Developed to accommodate different world standards

• T1-based• E1-based

– Original SONET standard changed from bit interleaving to byte interleaving

– SONET is subset of SDH

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SONET/SDH hierarchies

Source: Tektronix

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Non-synchronous hierarchies

Source: Tektronix

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Address for following slides:

http://www.cisco.com/networkers/nw00/pres/pdf2000.htm

Presentation # 3003

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Ethernet

Primarily of interest because of newer, high-speed versions– Gigabit Ethernet (GBE)– 10 Gigabit Ethernet

Fast Ethernet (100 Mbps) can run on fiber, but normally implemented with Cat-5 UTP

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Brief review of Ethernet operation

All stations connected to bus, which is in effect a node Ethernet uses “Carrier Sense Multiple Access with

Collision Detection” (CSMA/CD) to control bus traffic Stations transmit independently and asychronously

– If a frame is received, all stations check to see if it is addressed to them

– If two stations transmit simultaneously or closely in time, a “collision” occurs

No guarantee that data will get through without error– Requires higher level protocol to monitor and indicated

need for retransmission

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Clarification (continued)

Most modern Ethernet network interface cards (NICs) can operate either half duplex (with bus or hub) or full duplex (with switch)

Switches are sold by all major vendors– Improve throughput on slower speed LANs– Not much more expensive than hubs– Allow more devices to be connected to LAN

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Source: Luxpath/IEC

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Operation of Ethernet (continued) Operation of CSMA/CD

– If a station wishes to send, it must listen to see if another station is transmitting

• If so, must wait until bus is free

• If not, it can begin to transmit

– Because of signal propagation delays down the bus, a station may be unaware that another has begun to transmit

• If this occurs, called “collision”, garbage is result

• Transmitting station must listen to bus to monitor for collisions

• If collision detected, transmitting station sends “jamming” signal to improve chance that other station detects collision, then stops transmitting

– If collision occurs, all transmitting stations must cease transmission and wait for (different) random periods before retransmitting

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Ethernet and OSI reference model

Application

Presentation

Session

Transport

Network

Data Link

Physical

TCP

IP

Applications:TelnetFTP

SMTPHTTP

Ethernet (802.3)

LLC SublayerMAC Sublayer

Physical signalingMedia attachment

TCP/IP

ApplicationProtocols

OSI Reference ModelTCP/IP Implementation

Using Ethernet

Source: IBM

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Bus and hub architectures

Source: Dutton

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Half-duplex and full-duplex

Meaning of half duplex (HDX) and full duplex (FDX)– Terms going back to teletype days– Half-duplex = same physical line (or bus) used for both

transmit and receive• Requires special protocol to prevent simultaneous

transmission and reception– Full-duplex = different physical line used for both

transmit and receive• Does not require special protocol, but does require

dedicated (at least temporarily) connection

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Half-duplex and full-duplex (continued)

Original Ethernet: half-duplex because all transmitting and receiving on same bus

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Implementation of Ethernet

Physical bus rarely used anymore– Too difficult to manage and repair– Unwieldy to add or change workstations– Requires coax cable in most cases

Implementations done with hub and Cat-5 UTP– Logically looks like bus

Manchester encoding always used– Signal always has transition with every bit

• Logic 0: 0 to 1 transition at bit center• Logic 1: 1 to 0 transition at bit center

– Effectively doubles frequency

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Implementation of Ethernet (continued)

Example of Manchester encoding

Manchester encoding important for collision detection– Because a 0 level and a 1 level occur for each bit, code

is “balanced”• Average DC level is ½ of logic 1 level

– If collision occurs, signals are “ORed”, which raises average DC level

– Detected and interpreted as collision by transceivers

1 1 10 0 0

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Implementation on fiber

Collision detection– Light pulses converted to electricity in transceivers– Average DC value will also change when light pulses

collide on fiber Uses LEDs at 850 nm

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CSMA/CD performance and propagation delay

Propagation delay is main factor limiting performance of Ethernet– Delay means station may begin transmitting when bus

not free– Also means stations will learn that bus is free at

different times Collisions reduce utilization of Ethernet LAN because they

force two or more retransmissions Maximum utilization (maximum throughput) given by

1/(1+6.44)

where

= end-to-end delay/transmission time

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Ethernet throughput vs. offered load

Source: Dutton

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CSMA/CD performance and propagation delay (continued)

On copper wire, transmission speed about 5.2 sec/km For 10 mbps Ethernet, with 1000 bit frame size, utilization

estimated as• = 2 x 5.2 sec/100 sec = 0.104• Max utilization = 1/(1+6.44x0.104) = 0.60 = 60%

For 100 mbps Ethernet, same frame size,• = 2 x 5.2 sec/10 sec = 1.04• Max utilization = 1/(1+6.44x1.04) = 0.13 = 13%

For 1 Gbps Ethernet, same frame size,• r = 2 x 5.2 sec/1 sec = 10.4• Max utilization = 1/(1+6.44x10.4) = .0147 = 1.5%

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Ways to fix deteriorating high speed performance Changing frame size not practical Only other variable is propagation delay

– Needs to be made shorter– Must shorten maximum length of cabling

Light in fiber takes about 3.3-5 sec to travel 1 km, not that different than electricity pulses in copper wire

Standard Ethernet: 1.6 km max LAN segment length (“collision domain”)

High speed Ethernet: 200 m max LAN segment length

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Ways to fix speed problem (continued)

Gigabit Ethernet: would be ~ 20 m, but 200 was kept as spec– Other changes need to be made– Switches used instead of hubs– Minimum frame size 512 bytes, max same as before,

1524 bytes Switch is layer 2 device Reads addresses of frames and sends frame only to

destination– Reduces chances of collision significantly– Increases utilization seen by stations

Use of switches and routers also allows conventional Ethernet networks to span large areas

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Ways to fix speed problem (continued) 10 Gigabit Ethernet uses only full duplex to avoid timing

problems associated with CSMA/CD protocol– Lower speed versions can use it as well– Requires switch which physically connects two devices

which are communicating– No collisions because both connected devices can

transmit and receive at same time

Terminal 1

Terminal 2 Terminal 3

Terminal 4SwitchT

R

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Gigabit Ethernet standard

Shielded twisted pair up to 500 m UTP cat-5 available

– Requires 5-level encoding– 100 m max length

Cat-7 standard under development– Shielded twisted pairs

Single mode fiber at 1310 nm, up to 2 km Multimode fiber at 780 nm (CD-ROM lasers) or VCSELs at

850 nm– On 62.5/100 MM fiber up to 200m– May be extended to 1 or 2 km

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Cabling standards

Source: 10 Gigabit Ethernet Alliance

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10 Gbit Ethernet Fiber only Full duplex only, in combination with switches, will not

need CSMA/CD protocol required for half-duplex slower Ethernet

Standard called IEEE 802.3ae; see http://grouper.ieee.org/groups/802/3/ae/ for info on the spec

Source: 10 Gigabit Ethernet Alliance

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10 Gbit Ethernet

For further info,

www.10gea.org

Source: 10 Gigabit Ethernet Alliance

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Growth rate anticipated for Ethernet

Source: Luxpath/IEC

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Ethernet over SONET

Ethernet over SONET inefficiencies

Source: Cisco

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Future of Ethernet in WAN Objective is to replace SONET at 10 Gbit (OC-192) level Idea is to use Ethernet switch/routers to deal with failures

– Take advantage of extremely low error and failure rates with modern optical fiber

– Simplified architecture• Fewer network elements• No need for rings

– Not established technology yet, high risk compared to SONET

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Future of Ethernet (continued)

10 Gig Ethernet may replace ATM Ethernet already dominates the LAN, where ATM never

made much headway ATM dominates MAN and WAN

– Ethernet could displace ATM because it would eliminate need to switch protocols

– Has not happened yet In 2004, an 18,500 km 10 Gbit Ethernet link from CERN to

Japan was put into service– Special hardware needed for servers, but switches

could handle the speed 100 Gig Ethernet in very early stages

– Deployment not expected until 2010

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Other trends in Ethernet

All-optical Ethernet switches– Eliminate need for conversion back to electronic form– Useful in 10 Gbit WAN applications

Source: Luxpath/IEC

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Fiber Channel

Developed by ANSI to address problems of existing computer channel interfaces

Main thrust: connecting disk drives or arrays of disk drives with computer systems– Allows systems managers to combine data warehouses

spread over a campus or—with repeaters—a metropolitan area

Primarily within computer, but can also be used as LAN Allows interconnection of computers and peripheral

devices– Point-to-point– Crosspoint switch– Arbitrated loop

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Fiber Channel (continued)

Architecture is neither a channel nor a real network topology– An active intelligent interconnection scheme, called a

Fabric, to connect devices High performance serial link supporting its own, as well as

higher level protocols such as the FDDI, SCSI, HIPPI and IPI Speeds up to 4 Gbit/s (higher speeds planned for future)

– 8 Gbit standard ratified– 10 Gbit used now but only to interconnect switches

Can be converted for Local Area Network technology by adding a switch

Primary application is in storage area networks Can also run on copper twisted-pair

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Fiber channel topologies

Point-to-Point

Crosspoint switch

Arbitrated loop

Source: Dutton

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Characteristics of FDDI topologies

Source: Wikipedia

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Fiber Channel Speeds

133 Mbit/sec 266 Mbit/sec 530 Mbit/sec 1 Gbit/sec 2 Gbit/sec 4 Gbit/sec Highest performance: 10 km at 1 Gbit/sec

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Terminology

N_Port: connection for device to fiber channel F_Port: special connection to crosspoint switch fabric NL_Port: N_port in arbitrated loop FL_Port: F_Port connected to arbitrated loop

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Media for Fiber Channel

Uses single mode or multimode fiber– Single mode

• Lasers at 1300 nm, 1550 nm• Data rates up to 1 Gbps• Distance up to 10 km at 1300, >50 km at 1550

– Multimode• Laser at 780 nm, 850 nm

– Distance up to 2 km• LED at 1300 nm

– Distance up to 1.5 km

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Classes of service

Class 1: Dedicated (connection oriented)– 2 N_Ports– Maximum bandwidth guaranteed

Class 2: Multiplex– Connectionless– Acknowledgement of successful delivery

Class 3: Datagram– Connectionless– Best effort– No acknowledgement