Transcript Chapter 15 Local Area Network Overview

ECS 152A
9. Local Area Networks
LAN Applications (1)
• Personal computer LANs
—Low cost
—Limited data rate
• Back end networks
—Interconnecting large systems (mainframes and large
storage devices)
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High data rate
High speed interface
Distributed access
Limited distance
Limited number of devices
LAN Applications (2)
• Storage Area Networks
— Separate network handling storage needs
— Detaches storage tasks from specific servers
— Shared storage facility across high-speed network
— Hard disks, tape libraries, CD arrays
— Improved client-server storage access
— Direct storage to storage communication for backup
• High speed office networks
— Desktop image processing
— High capacity local storage
• Backbone LANs
— Interconnect low speed local LANs
— Reliability
— Capacity
— Cost
Storage Area Networks
LAN Architecture
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Topologies
Transmission medium
Layout
Medium access control
Topologies
• Tree
• Bus
—Special case of tree
• One trunk, no branches
• Ring
• Star
LAN Topologies
Bus and Tree
• Multipoint medium
• Transmission propagates throughout medium
• Heard by all stations
— Need to identify target station
• Each station has unique address
• Full duplex connection between station and tap
— Allows for transmission and reception
• Need to regulate transmission
— To avoid collisions
— To avoid hogging
• Data in small blocks - frames
• Terminator absorbs frames at end of medium
Frame
Transmission
on Bus LAN
Ring Topology
• Repeaters joined by point to point links in closed
loop
—Receive data on one link and retransmit on another
—Links unidirectional
—Stations attach to repeaters
• Data in frames
—Circulate past all stations
—Destination recognizes address and copies frame
—Frame circulates back to source where it is removed
• Media access control determines when station
can insert frame
Frame
Transmission
Ring LAN
Star Topology
• Each station connected directly to central node
—Usually via two point to point links
• Central node can broadcast
—Physical star, logical bus
—Only one station can transmit at a time
• Central node can act as frame switch
Choice of Topology
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Reliability
Expandability
Performance
Needs considering in context of:
—Medium
—Wiring layout
—Access control
Bus LAN
Transmission Media (1)
• Twisted pair
—Early LANs used voice grade cable
—Didn’t scale for fast LANs
—Not used in bus LANs now
• Baseband coaxial cable
—Uses digital signalling
—Original Ethernet
Bus LAN
Transmission Media (2)
• Broadband coaxial cable
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As in cable TV systems
Analog signals at radio frequencies
Expensive, hard to install and maintain
No longer used in LANs
• Optical fiber
— Expensive taps
— Better alternatives available
— Not used in bus LANs
• All hard to work with compared with star topology twisted pair
• Coaxial baseband still used but not often in new
installations
Ring and Star Usage
• Ring
—Very high speed links over long distances
—Single link or repeater failure disables network
• Star
—Uses natural layout of wiring in building
—Best for short distances
—High data rates for small number of devices
Choice of Medium
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Constrained by LAN topology
Capacity
Reliability
Types of data supported
Environmental scope
Media Available (1)
• Voice grade unshielded twisted pair (UTP)
—Cat 3
—Cheap
—Well understood
—Use existing telephone wiring in office building
—Low data rates
• Shielded twisted pair and baseband coaxial
—More expensive than UTP but higher data rates
• Broadband cable
—Still more expensive and higher data rate
Media Available (2)
• High performance UTP
— Cat 5 and above
— High data rate for small number of devices
— Switched star topology for large installations
• Optical fiber
— Electromagnetic isolation
— High capacity
— Small size
— High cost of components
— High skill needed to install and maintain
• Prices are coming down as demand and product range increases
Protocol Architecture
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Lower layers of OSI model
IEEE 802 reference model
Physical
Logical link control (LLC)
Media access control (MAC)
IEEE 802 v OSI
802 Layers Physical
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Encoding/decoding
Preamble generation/removal
Bit transmission/reception
Transmission medium and topology
802 Layers Logical Link Control
• Interface to higher levels
• Flow and error control
Logical Link Control
• Transmission of link level PDUs between two
stations
• Must support multiaccess, shared medium
• Relieved of some link access details by MAC
layer
• Addressing involves specifying source and
destination LLC users
—Referred to as service access points (SAP)
—Typically higher level protocol
LLC Services
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Based on HDLC
Unacknowledged connectionless service
Connection mode service
Acknowledged connectionless service
LLC Protocol
• Modeled after HDLC
• Asynchronous balanced mode to support
connection mode LLC service (type 2 operation)
• Unnumbered information PDUs to support
Acknowledged connectionless service (type 1)
• Multiplexing using LSAPs
Media Access Control
• Assembly of data into frame with address and
error detection fields
• Disassembly of frame
—Address recognition
—Error detection
• Govern access to transmission medium
—Not found in traditional layer 2 data link control
• For the same LLC, several MAC options may be
available
LAN Protocols in Context
Media Access Control
• Where
— Central
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Greater control
Simple access logic at station
Avoids problems of co-ordination
Single point of failure
Potential bottleneck
— Distributed
• How
— Synchronous
• Specific capacity dedicated to connection
— Asynchronous
• In response to demand
Asynchronous Systems
• Round robin
— Good if many stations have data to transmit over extended
period
• Reservation
— Good for stream traffic
• Contention
— Good for bursty traffic
— All stations contend for time
— Distributed
— Simple to implement
— Efficient under moderate load
— Tend to collapse under heavy load
MAC Frame Format
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MAC layer receives data from LLC layer
MAC control
Destination MAC address
Source MAC address
LLS
CRC
MAC layer detects errors and discards frames
LLC optionally retransmits unsuccessful frames
Generic MAC Frame Format
Bridges
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Ability to expand beyond single LAN
Provide interconnection to other LANs/WANs
Use Bridge or router
Bridge is simpler
—Connects similar LANs
—Identical protocols for physical and link layers
—Minimal processing
• Router more general purpose
—Interconnect various LANs and WANs
—see later
Why Bridge?
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Reliability
Performance
Security
Geography
Functions of a Bridge
• Read all frames transmitted on one LAN and
accept those address to any station on the other
LAN
• Using MAC protocol for second LAN, retransmit
each frame
• Do the same the other way round
Bridge Operation
Bridge Design Aspects
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No modification to content or format of frame
No encapsulation
Exact bitwise copy of frame
Minimal buffering to meet peak demand
Contains routing and address intelligence
— Must be able to tell which frames to pass
— May be more than one bridge to cross
• May connect more than two LANs
• Bridging is transparent to stations
— Appears to all stations on multiple LANs as if they are on one
single LAN
Bridge Protocol Architecture
• IEEE 802.1D
• MAC level
— Station address is at this level
• Bridge does not need LLC layer
— It is relaying MAC frames
• Can pass frame over external comms system
— e.g. WAN link
— Capture frame
— Encapsulate it
— Forward it across link
— Remove encapsulation and forward over LAN link
Connection of Two LANs
Fixed Routing
• Complex large LANs need alternative routes
—Load balancing
—Fault tolerance
• Bridge must decide whether to forward frame
• Bridge must decide which LAN to forward frame
on
• Routing selected for each source-destination
pair of LANs
—Done in configuration
—Usually least hop route
—Only changed when topology changes
Bridges and
LANs with
Alternative
Routes
Spanning Tree
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Bridge automatically develops routing table
Automatically update in response to changes
Frame forwarding
Address learning
Loop resolution
Frame forwarding
• Maintain forwarding database for each port
—List station addresses reached through each port
• For a frame arriving on port X:
—Search forwarding database to see if MAC address is
listed for any port except X
—If address not found, forward to all ports except X
—If address listed for port Y, check port Y for blocking
or forwarding state
• Blocking prevents port from receiving or transmitting
—If not blocked, transmit frame through port Y
Address Learning
• Can preload forwarding database
• Can be learned
• When frame arrives at port X, it has come form
the LAN attached to port X
• Use the source address to update forwarding
database for port X to include that address
• Timer on each entry in database
• Each time frame arrives, source address
checked against forwarding database
Spanning Tree Algorithm
• Address learning works for tree layout
—i.e. no closed loops
• For any connected graph there is a spanning
tree that maintains connectivity but contains no
closed loops
• Each bridge assigned unique identifier
• Exchange between bridges to establish spanning
tree
Loop of Bridges
Layer 2 and Layer 3 Switches
• Now many types of devices for interconnecting
LANs
• Beyond bridges and routers
• Layer 2 switches
• Layer 3 switches
Hubs
• Active central element of star layout
• Each station connected to hub by two lines
— Transmit and receive
• Hub acts as a repeater
• When single station transmits, hub repeats signal on outgoing line
to each station
• Line consists of two unshielded twisted pairs
• Limited to about 100 m
— High data rate and poor transmission qualities of UTP
• Optical fiber may be used
— Max about 500 m
• Physically star, logically bus
• Transmission from any station received by all other stations
• If two stations transmit at the same time, collision
Hub Layouts
• Multiple levels of hubs cascaded
• Each hub may have a mixture of stations and other hubs
attached to from below
• Fits well with building wiring practices
— Wiring closet on each floor
— Hub can be placed in each one
— Each hub services stations on its floor
Two Level Star Topology
Buses and Hubs
• Bus configuration
—All stations share capacity of bus (e.g. 10Mbps)
—Only one station transmitting at a time
• Hub uses star wiring to attach stations to hub
—Transmission from any station received by hub and
retransmitted on all outgoing lines
—Only one station can transmit at a time
—Total capacity of LAN is 10 Mbps
• Improve performance with layer 2 switch
Shared Medium Bus and Hub
Shared Medium Hub and
Layer 2 Switch
Layer 2 Switches
• Central hub acts as switch
• Incoming frame from particular station switched
to appropriate output line
• Unused lines can switch other traffic
• More than one station transmitting at a time
• Multiplying capacity of LAN
Layer 2 Switch Benefits
• No change to attached devices to convert bus LAN or
hub LAN to switched LAN
• For Ethernet LAN, each device uses Ethernet MAC
protocol
• Device has dedicated capacity equal to original LAN
— Assuming switch has sufficient capacity to keep up with all
devices
— For example if switch can sustain throughput of 20 Mbps, each
device appears to have dedicated capacity for either input or
output of 10 Mbps
• Layer 2 switch scales easily
— Additional devices attached to switch by increasing capacity of
layer 2
Types of Layer 2 Switch
• Store-and-forward switch
— Accepts frame on input line
— Buffers it briefly,
— Then routes it to appropriate output line
— Delay between sender and receiver
— Boosts integrity of network
• Cut-through switch
— Takes advantage of destination address appearing at beginning
of frame
— Switch begins repeating frame onto output line as soon as it
recognizes destination address
— Highest possible throughput
— Risk of propagating bad frames
• Switch unable to check CRC prior to retransmission
Layer 2 Switch v Bridge
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Layer 2 switch can be viewed as full-duplex hub
Can incorporate logic to function as multiport bridge
Bridge frame handling done in software
Switch performs address recognition and frame
forwarding in hardware
• Bridge only analyzes and forwards one frame at a time
• Switch has multiple parallel data paths
— Can handle multiple frames at a time
• Bridge uses store-and-forward operation
• Switch can have cut-through operation
• Bridge suffered commercially
— New installations typically include layer 2 switches with bridge
functionality rather than bridges
Problems with Layer 2
Switches (1)
• As number of devices in building grows, layer 2 switches
reveal some inadequacies
• Broadcast overload
• Lack of multiple links
• Set of devices and LANs connected by layer 2 switches
have flat address space
— Allusers share common MAC broadcast address
— If any device issues broadcast frame, that frame is delivered to
all devices attached to network connected by layer 2 switches
and/or bridges
— In large network, broadcast frames can create big overhead
— Malfunctioning device can create broadcast storm
• Numerous broadcast frames clog network
Problems with Layer 2
Switches (2)
• Current standards for bridge protocols dictate no closed
loops
— Only one path between any two devices
— Impossible in standards-based implementation to provide
multiple paths through multiple switches between devices
• Limits both performance and reliability.
• Solution: break up network into subnetworks connected
by routers
• MAC broadcast frame limited to devices and switches
contained in single subnetwork
• IP-based routers employ sophisticated routing
algorithms
— Allow use of multiple paths between subnetworks going through
different routers
Problems with Routers
• Routers do all IP-level processing in software
—High-speed LANs and high-performance layer 2
switches pump millions of packets per second
—Software-based router only able to handle well under
a million packets per second
• Solution: layer 3 switches
—Implementpacket-forwarding logic of router in
hardware
• Two categories
—Packet by packet
—Flow based
Packet by Packet or
Flow Based
• Operates insame way as traditional router
• Order of magnitude increase in performance
compared to software-based router
• Flow-based switch tries to enhance performance
by identifying flows of IP packets
—Same source and destination
—Done by observing ongoing traffic or using a special
flow label in packet header (IPv6)
—Once flow is identified, predefined route can be
established
Typical Large LAN Organization
• Thousands to tens of thousands of devices
• Desktop systems links 10 Mbps to 100 Mbps
— Into layer 2 switch
• Wireless LAN connectivity available for mobile users
• Layer 3 switches at local network's core
— Form local backbone
— Interconnected at 1 Gbps
— Connect to layer 2 switches at 100 Mbps to 1 Gbps
• Servers connect directly to layer 2 or layer 3 switches at
1 Gbps
• Lower-cost software-based router provides WAN
connection
• Circles in diagram identify separate LAN subnetworks
• MAC broadcast frame limited to own subnetwork
Typical
Large
LAN
Organization
Diagram
High Speed LANs
• Range of technologies
—Fast and Gigabit Ethernet
—Fibre Channel
—High Speed Wireless LANs
Why High Speed LANs?
• Office LANs used to provide basic connectivity
— Connecting PCs and terminals to mainframes and midrange
systems that ran corporate applications
— Providing workgroup connectivity at departmental level
— Traffic patterns light
• Emphasis on file transfer and electronic mail
• Speed and power of PCs has risen
— Graphics-intensive applications and GUIs
• MIS organizations recognize LANs as essential
— Began with client/server computing
• Now dominant architecture in business environment
• Intranetworks
• Frequent transfer of large volumes of data
Applications Requiring High
Speed LANs
• Centralized server farms
— User needs to draw huge amounts of data from multiple
centralized servers
— E.g. Color publishing
• Servers contain tens of gigabytes of image data
• Downloaded to imaging workstations
• Power workgroups
• Small number of cooperating users
— Draw massive data files across network
— E.g. Software development group testing new software version
or computer-aided design (CAD) running simulations
• High-speed local backbone
— Processing demand grows
— LANs proliferate at site
— High-speed interconnection is necessary
Ethernet (CSMA/CD)
• Carriers Sense Multiple Access with Collision
Detection
• Xerox - Ethernet
• IEEE 802.3
IEEE802.3 Medium Access
Control
• Random Access
— Stations access medium randomly
• Contention
—Stations content for time on medium
ALOHA
• Packet Radio
• When station has frame, it sends
• Station listens (for max round trip time)plus small
increment
• If ACK, fine. If not, retransmit
• If no ACK after repeated transmissions, give up
• Frame check sequence (as in HDLC)
• If frame OK and address matches receiver, send ACK
• Frame may be damaged by noise or by another station
transmitting at the same time (collision)
• Any overlap of frames causes collision
• Max utilization 18%
Slotted ALOHA
• Time in uniform slots equal to frame
transmission time
• Need central clock (or other sync mechanism)
• Transmission begins at slot boundary
• Frames either miss or overlap totally
• Max utilization 37%
CSMA
• Propagation time is much less than transmission time
• All stations know that a transmission has started almost
immediately
• First listen for clear medium (carrier sense)
• If medium idle, transmit
• If two stations start at the same instant, collision
• Wait reasonable time (round trip plus ACK contention)
• No ACK then retransmit
• Max utilization depends on propagation time (medium
length) and frame length
— Longer frame and shorter propagation gives better utilization
Nonpersistent CSMA
1. If medium is idle, transmit; otherwise, go to 2
2. If medium is busy, wait amount of time drawn from
probability distribution (retransmission delay) and
repeat 1
• Random delays reduces probability of collisions
— Consider two stations become ready to transmit at same time
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While another transmission is in progress
— If both stations delay same time before retrying, both will
attempt to transmit at same time
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Capacity is wasted because medium will remain idle
following end of transmission
— Even if one or more stations waiting
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Nonpersistent stations deferential
1-persistent CSMA
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To avoid idle channel time, 1-persistent protocol used
Station wishing to transmit listens and obeys following:
If medium idle, transmit; otherwise, go to step 2
If medium busy, listen until idle; then transmit
immediately
1-persistent stations selfish
If two or more stations waiting, collision guaranteed
— Gets sorted out after collision
P-persistent CSMA
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Compromise that attempts to reduce collisions
— Like nonpersistent
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And reduce idle time
— Like1-persistent
• Rules:
1. If medium idle, transmit with probability p, and delay
one time unit with probability (1 – p)
— Time unit typically maximum propagation delay
2. If medium busy, listen until idle and repeat step 1
3. If transmission is delayed one time unit, repeat step 1
• What is an effective value of p?
Value of p?
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Avoid instability under heavy load
n stations waiting to send
End of transmission, expected number of stations attempting to
transmit is number of stations ready times probability of
transmitting
— np
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If np > 1on average there will be a collision
Repeated attempts to transmit almost guaranteeing more collisions
Retries compete with new transmissions
Eventually, all stations trying to send
— Continuous collisions; zero throughput
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So np < 1 for expected peaks of n
If heavy load expected, p small
However, as p made smaller, stations wait longer
At low loads, this gives very long delays
CSMA/CD
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With CSMA, collision occupies medium for
duration of transmission
Stations listen whilst transmitting
1. If medium idle, transmit, otherwise, step 2
2. If busy, listen for idle, then transmit
3. If collision detected, jam then cease
transmission
4. After jam, wait random time then start from
step 1
CSMA/CD
Operation
Which Persistence Algorithm?
• IEEE 802.3 uses 1-persistent
• Both nonpersistent and p-persistent have
performance problems
• 1-persistent (p = 1) seems more unstable than
p-persistent
—Greed of the stations
—But wasted time due to collisions is short (if frames
long relative to propagation delay
—With random backoff, unlikely to collide on next tries
—To ensure backoff maintains stability, IEEE 802.3 and
Ethernet use binary exponential backoff
Binary Exponential Backoff
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Attempt to transmit repeatedly if repeated collisions
First 10 attempts, mean value of random delay doubled
Value then remains same for 6 further attempts
After 16 unsuccessful attempts, station gives up and
reports error
• As congestion increases, stations back off by larger
amounts to reduce the probability of collision.
• 1-persistent algorithm with binary exponential backoff
efficient over wide range of loads
— Low loads, 1-persistence guarantees station can seize channel
once idle
— High loads, at least as stable as other techniques
• Backoff algorithm gives last-in, first-out effect
• Stations with few collisions transmit first
Collision Detection
• On baseband bus, collision produces much
higher signal voltage than signal
• Collision detected if cable signal greater than
single station signal
• Signal attenuated over distance
• Limit distance to 500m (10Base5) or 200m
(10Base2)
• For twisted pair (star-topology) activity on more
than one port is collision
• Special collision presence signal
IEEE 802.3 Frame Format
10Mbps Specification
(Ethernet)
• <data rate><Signaling method><Max segment length>
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10Base5
Medium Coaxial
Signaling Baseband
Manchester
Topology Bus
Nodes
100
10Base2
10Base-T
10Base-F
Coaxial
Baseband
Manchester
Bus
30
UTP
Baseband
Manchester
Star
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850nm fiber
Manchester
On/Off
Star
33
100Mbps Fast Ethernet
• Use IEEE 802.3 MAC protocol and frame format
• 100BASE-X use physical medium specifications from
FDDI
— Two physical links between nodes
• Transmission and reception
— 100BASE-TX uses STP or Cat. 5 UTP
• May require new cable
— 100BASE-FX uses optical fiber
— 100BASE-T4 can use Cat. 3, voice-grade UTP
• Uses four twisted-pair lines between nodes
• Data transmission uses three pairs in one direction at a time
• Star-wire topology
— Similar to 10BASE-T
100Mbps (Fast Ethernet)
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100Base-TX
• 2 pair, STP
• MLT-3
2 pair, Cat 5 UTP
MLT-3
100Base-FX
100Base-T4
2 optical fiber
4B5B,NRZI
4 pair, cat 3,4,5
8B6T,NRZ
100BASE-X Data Rate and
Encoding
• Unidirectional data rate 100 Mbps over single
link
—Single twisted pair, single optical fiber
• Encoding scheme same as FDDI
—4B/5B-NRZI
—Modified for each option
100BASE-X Media
• Two physical medium specifications
• 100BASE-TX
— Two pairs of twisted-pair cable
— One pair for transmission and one for reception
— STP and Category 5 UTP allowed
— The MTL-3 signaling scheme is used
• 100BASE-FX
— Two optical fiber cables
— One for transmission and one for reception
— Intensity modulation used to convert 4B/5B-NRZI code group
stream into optical signals
— 1 represented by pulse of light
— 0 by either absence of pulse or very low intensity pulse
100BASE-T4
• 100-Mbps over lower-quality Cat 3 UTP
— Taking advantage of large installed base
— Cat 5 optional
— Does not transmit continuous signal between packets
— Useful in battery-powered applications
• Can not get 100 Mbps on single twisted pair
— Data stream split into three separate streams
• Each with an effective data rate of 33.33 Mbps
— Four twisted pairs used
— Data transmitted and received using three pairs
— Two pairs configured for bidirectional transmission
• NRZ encoding not used
— Would require signaling rate of 33 Mbps on each pair
— Does not provide synchronization
— Ternary signaling scheme (8B6T)
100BASE-T Options
Full Duplex Operation
• Traditional Ethernet half duplex
— Either transmit or receive but not both simultaneously
• With full-duplex, station can transmit and receive
simultaneously
• 100-Mbps Ethernet in full-duplex mode, theoretical
transfer rate 200 Mbps
• Attached stations must have full-duplex adapter cards
• Must use switching hub
— Each station constitutes separate collision domain
— In fact, no collisions
— CSMA/CD algorithm no longer needed
— 802.3 MAC frame format used
— Attached stations can continue CSMA/CD
Mixed Configurations
• Fast Ethernet supports mixture of existing 10-Mbps LANs
and newer 100-Mbps LANs
• E.g. 100-Mbps backbone LAN to support 10-Mbps hubs
— Stations attach to 10-Mbps hubs using 10BASE-T
— Hubs connected to switching hubs using 100BASE-T
• Support 10-Mbps and 100-Mbps
— High-capacity workstations and servers attach directly to 10/100
switches
— Switches connected to 100-Mbps hubs using 100-Mbps links
— 100-Mbps hubs provide building backbone
• Connected to router providing connection to WAN
Gigabit Ethernet Configuration
Gigabit Ethernet - Differences
• Carrier extension
• At least 4096 bit-times long (512 for 10/100)
• Frame bursting
Gigabit Ethernet – Physical
• 1000Base-SX
—Short wavelength, multimode fiber
• 1000Base-LX
—Long wavelength, Multi or single mode fiber
• 1000Base-CX
—Copper jumpers <25m, shielded twisted pair
• 1000Base-T
—4 pairs, cat 5 UTP
• Signaling - 8B/10B
Gbit Ethernet Medium Options
(log scale)
10Gbps Ethernet - Uses
• High-speed, local backbone interconnection between large-capacity
switches
• Server farm
• Campus wide connectivity
• Enables Internet service providers (ISPs) and network service
providers (NSPs) to create very high-speed links at very low cost
• Allows construction of (MANs) and WANs
— Connect geographically dispersed LANs between campuses or points of
presence (PoPs)
• Ethernet competes with ATM and other WAN technologies
• 10-Gbps Ethernet provides substantial value over ATM
10Gbps Ethernet - Advantages
• No expensive, bandwidth-consuming conversion
between Ethernet packets and ATM cells
• Network is Ethernet, end to end
• IP and Ethernet together offers QoS and traffic policing
approach ATM
• Advanced traffic engineering technologies available to
users and providers
• Variety of standard optical interfaces (wavelengths and
link distances) specified for 10 Gb Ethernet
• Optimizing operation and cost for LAN, MAN, or WAN
10Gbps Ethernet - Advantages
• Maximum link distances cover 300 m to 40 km
• Full-duplex mode only
• 10GBASE-S (short):
— 850 nm on multimode fiber
— Up to 300 m
• 10GBASE-L (long)
— 1310 nm on single-mode fiber
— Up to 10 km
• 10GBASE-E (extended)
— 1550 nm on single-mode fiber
— Up to 40 km
• 10GBASE-LX4:
— 1310 nm on single-mode or multimode fiber
— Up to 10 km
— Wavelength-division multiplexing (WDM) bit stream across four light
waves
10Gbps Ethernet Distance
Options (log scale)
Token Ring (802.5)
• Developed from IBM's commercial token ring
• Because of IBM's presence, token ring has
gained broad acceptance
• Never achieved popularity of Ethernet
• Currently, large installed base of token ring
products
• Market share likely to decline
Ring Operation
• Each repeater connects to two others via unidirectional
transmission links
• Single closed path
• Data transferred bit by bit from one repeater to the next
• Repeater regenerates and retransmits each bit
• Repeater performs data insertion, data reception, data
removal
• Repeater acts as attachment point
• Packet removed by transmitter after one trip round ring
Listen State Functions
• Scan passing bit stream for patterns
—Address of attached station
—Token permission to transmit
• Copy incoming bit and send to attached station
—Whilst forwarding each bit
• Modify bit as it passes
—e.g. to indicate a packet has been copied (ACK)
Transmit State Functions
• Station has data
• Repeater has permission
• May receive incoming bits
—If ring bit length shorter than packet
• Pass back to station for checking (ACK)
—May be more than one packet on ring
• Buffer for retransmission later
Bypass State
• Signals propagate past repeater with no delay
(other than propagation delay)
• Partial solution to reliability problem (see later)
• Improved performance
Ring Repeater States
802.5 MAC Protocol
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Small frame (token) circulates when idle
Station waits for token
Changes one bit in token to make it SOF for data frame
Append rest of data frame
Frame makes round trip and is absorbed by transmitting
station
• Station then inserts new token when transmission has
finished and leading edge of returning frame arrives
• Under light loads, some inefficiency
• Under heavy loads, round robin
Token Ring
Operation
Dedicated Token Ring
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Central hub
Acts as switch
Full duplex point to point link
Concentrator acts as frame level repeater
No token passing
802.5 Physical Layer
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•
•
•
Data Rate
Medium
Signaling
Max Frame
Access Control
4
16
100
UTP,STP,Fiber
Differential Manchester
4550
18200
18200
TP or DTR TP or DTR DTR
• Note: 1Gbit specified in 2001
— Uses 802.3 physical layer specification
Fibre Channel - Background
• I/O channel
— Direct point to point or multipoint comms link
— Hardware based
— High Speed
— Very short distance
— User data moved from source buffer to destiation buffer
• Network connection
— Interconnected access points
— Software based protocol
— Flow control, error detection &recovery
— End systems connections
Fibre Channel
• Best of both technologies
• Channel oriented
—Data type qualifiers for routing frame payload
—Link level constructs associated with I/O ops
—Protocol interface specifications to support existing
I/O architectures
• e.g. SCSI
• Network oriented
—Full multiplexing between multiple destinations
—Peer to peer connectivity
—Internetworking to other connection technologies
Fibre Channel Requirements
• Full duplex links with two fibers per link
• 100 Mbps to 800 Mbps on single line
— Full duplex 200 Mbps to 1600 Mbps per link
•
•
•
•
•
Up to 10 km
Small connectors
High-capacity utilization, distance insensitivity
Greater connectivity than existing multidrop channels
Broad availability
— i.e. standard components
• Multiple cost/performance levels
— Small systems to supercomputers
• Carry multiple existing interface command sets for existing channel
and network protocols
• Uses generic transport mechanism based on point-to-point links and
a switching network
• Supports simple encoding and framing scheme
• In turn supports a variety of channel and network protocols
Fibre Channel Elements
• End systems - Nodes
• Switched elements - the network or fabric
• Communication across point to point links
Fibre Channel Network
Fibre Channel Protocol
Architecture (1)
• FC-0 Physical Media
— Optical fiber for long distance
— coaxial cable for high speed short distance
— STP for lower speed short distance
• FC-1 Transmission Protocol
— 8B/10B signal encoding
• FC-2 Framing Protocol
— Topologies
— Framing formats
— Flow and error control
— Sequences and exchanges (logical grouping of frames)
Fibre Channel Protocol
Architecture (2)
• FC-3 Common Services
—Including multicasting
• FC-4 Mapping
—Mapping of channel and network services onto fibre
channel
• e.g. IEEE 802, ATM, IP, SCSI
Fibre Channel Physical Media
• Provides range of options for physical medium,
the data rate on medium, and topology of
network
• Shielded twisted pair, video coaxial cable, and
optical fiber
• Data rates 100 Mbps to 3.2 Gbps
• Point-to-point from 33 m to 10 km
Fibre Channel Fabric
• General topology called fabric or switched topology
• Arbitrary topology includes at least one switch to
interconnect number of end systems
• May also consist of switched network
— Some of these switches supporting end nodes
• Routing transparent to nodes
— Each port has unique address
— When data transmitted into fabric, edge switch to which node
attached uses destination port address to determine location
— Either deliver frame to node attached to same switch or
transfers frame to adjacent switch to begin routing to remote
destination
Fabric Advantages
• Scalability of capacity
— As additional ports added, aggregate capacity of network
increases
— Minimizes congestion and contention
— Increases throughput
• Protocol independent
• Distance insensitive
• Switch and transmission link technologies may change
without affecting overall configuration
• Burden on nodes minimized
— Fibre Channel node responsible for managing point-to-point
connection between itself and fabric
— Fabric responsible for routing and error detection
Alternative Topologies
• Point-to-point topology
—Only two ports
—Directly connected, with no intervening switches
—No routing
• Arbitrated loop topology
—Simple, low-cost topology
—Up to 126 nodes in loop
—Operates roughly equivalent to token ring
• Topologies, transmission media, and data rates
may be combined
Five Applications of Fibre
Channel
Fibre Channel Prospects
• Backed by Fibre Channel Association
• Interface cards for different applications available
• Most widely accepted as peripheral device interconnect
— To replace such schemes as SCSI
• Technically attractive to general high-speed LAN
requirements
• Must compete with Ethernet and ATM LANs
• Cost and performance issues should dominate the
consideration of these competing technologies