comp312
TRANSCRIPT
Local Area Networks
COMP312
Richard [email protected]
http://www.cs.waikato.ac.nz
Department of Computer Science
University of Waikato
COMP312 - Local Area Networks – p. 1/58
OSI Protocol Model
7. Application
6. Presentation
5. Session
4. Transport
3. Network
2. Link
1. Physical
LANs are L2 networks
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Lecture Outline
• Protocols and Standards.• Hubs, Bridges and Switches• Modern Ethernet.
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Protocols and Standards
• LAN Protocols.• LAN Standards.• Ethernet Frame Format.
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LAN Protocols
1. Physical
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LAN Framing
Data
Network DataNetwork Header
LLC DataLLC Header
MAC DataMAC Header
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IEEE 802.2
• Most MAC Layers provide and unreliable Datagram service• IEEE 802.2 provides a link layer service based on HDLC• There are three classes of service
◦ Unacknowledged connectionless-mode service only.◦ Connection-mode service plus service◦ Acknowledged connectionless service
• 802.2 Also provides◦ Addressing.◦ Service Access Points
• 802.2 Uses Sliding Window flow control and Go-Back-N ARQerror control
Return to Section ToC
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LAN Standards
• Most LAN Standards are set by the IEEE• Most LANs started outside the IEEE in industry or academia but
later got taken to the IEEE• The IEEE 802 committee is responsible for LANs.
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IEEE 802 Standards
IEEE Number Name
802.1 Bridging and Management
802.2 Logical Link Control
802.3 CSMA/CD - Ethernet
802.4 Token Bus - ARCNet
802.5 Token Ring
802.6 MANs - DQDB
802.10 Security
802.11 Wireless LANs
802.12 Demand Priority Access
802.15 PANs (Bluetooth)
802.16 Broadband Wireless MANs
802.17 Resilient Packet Ring
802.20 Mobile Broadband Access
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IEEE 802 Standards
PHY802.3
CSMA/CD802.3
PHY802.4
802.4Token Bus
PHY
802.5
802.5
Token Ring
PHY PHY PHY PHY802.6 802.11 802.12 802.15
PHY802.16
802.6 802.11 802.12 802.15 802.16DQDB W−LAN WB−MANW−PANDPA
802.1 Bridging
802.2 Logical Link Control
PhysicalLayer
DataLink Layer
Return to Section ToC
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Ethernet Frame Format
There are four different Ethernet frame formats
1. Ethernet Version II. This is from the original Ethernet specificationreleased by Xerox, Intel and DEC.
2. Novell Proprietary ("802.3 Raw"). This format was used by NovellNetware and was based on an early version of the 802.3specification.
3. 802.3. The 802.3 standard specifies a header that includes the802.2 LLC fields.
4. 802.3 SNAP. This provides an extended header that allowsbackwards compatibility with the original Version II header.
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Ethernet (802.2) Addresses
• Addresses are six bytes long, Normally written as hyphenated hexe.g. AB-CD-EF-12-34-56.
• The first three bytes are an IEEE assigned OrganizationallyUnique Identifier (OUI)
• The second three bytes are assigned by the manufacturer.• Properly assigned addresses are globally unique.• Some hardware allows manually assigned addresses.• Destination address of all ones is the broadcast address.• Some addresses are reserved for multicast applications (normally
specific addresses are assigned for specific protocols).
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Ethernet Version II
Dst Src Etype Data FCS
• Dst: Destination Address - 6 bytes.
• Src: Source Address - 6 bytes.
• Ethertype: Specifies the protocol being carried within the data section. Used formultiplexing protocols. Ethertypes are all greater than 1536 and are assigned byXerox- 2 bytes.
• Data: Variable length payload. Must be padded if less than the minimum length -46-1500 bytes.
• FCS - Frame Check Sequence used for CRC - 4 bytes.
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Novell Proprietary
Dst Src Length Data FCS
• Dst: Destination Address - 6 bytes.
• Src: Source Address - 6 bytes.
• Length: The length of the entire frame not including the preamble or CRC - 2 bytes.
• Data: Variable length payload. Netware Packets always start with 0xFFFF. Must bepadded if less than the minimum length - 46-1500 bytes.
• FCS - Frame Check Sequence used for CRC - 4 bytes.
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802.3
Dst Src Length DSAPSSAP Ctrl Data FCS
802.2 LLC Header
• DSAP: Destination Service Access Point. References the process associated withthe protocol of data section of the packet at the receiving station - 1 byte.
• SSAP: Source Service Access Point. References the process associated with theprotocol of data section of the packet at the sending station - 1 byte.
• Ctrl: Specifies the type of packet as used by the LLC protocol. May beInformational, Control or Data.
Common DSAP/SSAP values include:
04 - IBM SNA 06 - IP
80 - 3Com AA - SNAP
BC - Banyan E0 - Novell
F4 - Lan Manager
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802.3 SNAP
Dst Src Length DSAPSSAP Ctrl SNAP Data FCS
802.2 LLC Header
• SNAP: SubNetwork Access Protocol - 5 bytes. The first three bytes carry theOrganisation Unique Identifier and are usually the same as the first three bytes ofthe source address. The last two bytes carry a protocol identifier that is usually anEthertype.
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Protocols and Standards - Summary
• The IEEE has separated Link Layer functions from Media Access• This makes all IEEE standard LANs compatible in terms of the
services they offer.• The Ethernet Frame format has evolved as the standard has
progressed.
Return to Section ToC Return to ToC
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Hubs, Bridges and Switches
• Hubs.• Bridges.• Switches.
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Hubs
• A hub is a collapsed broadcast bus.• stations connected to hubs must run CSMA/CD.• Hubs are normally used with UTP wiring and provide digital
regeneration of the signal.• Stations connected to hubs see all packets and select those with
addresses that are of interest.
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Broadcast Bus
A B C D
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Hub
A B C D
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Bridges
• Bridges are intelligent repeaters. They forward packets withoutchanging them.
• Bridges are Layer 2 devices so they are transparent to endstations.
• Identical L2 protocols are required on both sides of a bridge (maybe LLC identical).
• They divide up collision domains so CSMA/CD runs either side ofa bridge, but not across it.
• Bridges can buffer packets to ensure they are not lost without theoriginal transmitter knowing.
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Bridge Advantages
• LAN coverage by increasing the distance that packets can cover.• Performance by reducing collision domain size and so lowering
the rate of collisions.• Throughput and security by not forwarding packets that do not
need to be forwarded.• Bridges may be used to connect incompatible media (e.g. coax to
UTP) or even networks that are use incompatible MACs, butcompatible LLC layers (e.g. WLANs are normally bridged toEthernet networks).
• Bridges may be used to connect remote networks using a widearea link.
• Collision Detection does not work on fibre optic links so they haveto be point to point links and bridged to the rest of the network.
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Bridge
A B C D
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Loops
• Loops may be formed with multiple bridges on a LAN segments◦ Deliberately for redundancy.◦ Accidentally through misconfiguration.
• This can cause significant problems◦ Multiple copies of packets◦ Bridges learning the wrong location of stations and not
forwarding packets◦ Cascading multiplication of packets
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Spanning Tree
• A spanning tree is a subset of the bridge topology that:◦ Covers the entire network (spanning),◦ Includes no loops (tree).
• Spanning Tree works by:◦ Bridges exchange topology information using specific bridge
topology packets and a multicast address,◦ A root bridge is elected,◦ Bridges then caculate their path cost to the root bridge,◦ A designated bridge is elected to each lan segment,◦ Redundant bridge interfaces are set to not forward packets.
• Changes in link costs or link availability result in re-calculation ofthe spanning tree.
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Spanning Tree
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Switches
• Switches are multiport bridges.◦ Each port is regarded as a separate LAN segment.◦ They perform MAC learning◦ They participate in spanning tree calculations
• Forwarding speed is not limited by the segment speed.• Different ports may run at different speeds• Ports that have only one device attached may send and receive
simultaneously, i.e. full duplex.• The main disadvantage of switches over hubs is traditionally cost
but this is now much less significant than it used to be.
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Switch
A B C D
A B C D
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Switch Buffering
• Switches and bridges can buffer packets that cannot beimmediately sent to a segment.◦ The receiving segment is busy or experiencing collisions.◦ The sending segment runs at a higher speed than the
receiving segment.◦ Multiple segments are sending packets to the same receiving
segment.
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Switch Buffering -2
• It is possible that the buffering requirements may exceed thememory of the switch.
• The switch can respond in various ways◦ Do Nothing; assume higher layer flow/error control will
respond.◦ Backpressure; Cause collisions on the sending segments to
slow down the sender.◦ Flow control; uses special Pause 802.3x Mac Control Frame
to tell senders to stop sending for a short period of time.
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Autonegotiation
• Switches may be connected to devices of varying speeds.• UTP Ethernet uses the same connector for 10Mbps, 100Mbps,
1000Mbps• 10Mbps UTP Ethernet sends a half pulse every 16ms to verify the
link status, called Normal Link Pulse - NLP. Reception of thispulse causes the link status LED to light on a NIC and above aswitch port .
NLP16ms
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Autonegotiation - 2
• 100Mbps and 1000Mbps send multiple clock and data pulses atthe same time intervals, called Fast Link Pulse - FLP.
• The clock signals are used to determine the speed capability ofthe communicating entities.
• The data pulses contain information describing the devicecapabilities (e.g. full duplex).
• The link speed defaults to the lowest capability level of the twostations.
• Autoconfiguration is useful to decrease the chance of usermis-configuration.
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Autonegotiation - 3
16ms
clock
datai
1
data data data data
clock clock clock clock2 3 4
16432
16
2ms burstof 33 pulses
FLP
Return to Section ToC
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Hubs Bridges and Switches - Summary
• Hubs are compressed busses used to allow star wiring (UTP).• Bridges break up collision domains and extend LANs.• Bridges use spanning tree routing to break up loops.• Switches are multiport bridges.
Return to Section ToC Return to ToC
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Modern Ethernet
• Speeds.• VLANS.• Other Features.
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Speeds
Ethernet has rapidly increased in speeds• 1973 - Original experimental Ethernet at Xerox PARC - 3Mbps• 1980 - DEC, Intel, Xerox (DIX) Ethernet - 10Mbps• 1982 - Ethernet II (DIX v2.0) - 10 Mbps• 1985 - IEEE 802.3 CSMA/CD - 10Mbps• 1995 - IEEE 802.3u Fast Ethernet - 100Mbps• 1998 - IEEE 802.3z - Gigabit Ethernet• 2002 - IEEE 802.3ae - 10 Gigabit Ethernet
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Modern Ethernet Timeline
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Thick Ethernet (10base5)
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Thick Ethernet (10base5)
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Thin Ethernet (10base2)
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UTP Ethernet
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Fibre Optic Ethernet
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Ethernet Physical Layers
1000BaseSX
1000BaseT
100BaseFX
100BaseTX (UTP)
10BaseF
10BaseT
10Base2
10Base5
UTP
Fiber
Fiber
UTP
5mm 50 ohm Coax
10mm 50 ohm Coax
Medium Signaling
Manchester
Mancheter
Manchester
Manchester
4B5B MLT−3
4B5B NRZI
PAM5x5
8B10B 2
NodesMax SegmentTopology
100
30
2
2
2
2
2UTP
1000BaseSX
Bus
Bus
Star
Star
Star
Star
Star
Star
185m
100m
100m
550m
100m
100m
500m
500m
50micron Fiber
8B10B Star 2
8B10B
8B10B
8B10B
Star
Star
Star
550m 2
2
2
62.5micron Fiber 275m
1000BaseLX 50/72.5micron Fiber
1000BaseLX 9micron Fiber 5000m
1000BaseLH ~9micron Fiber 50~100km
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MLT3 Line Coding
MLT−3
0110 110 11 0 0 1 1 1 1 0 0 0 0 1 0 1 1 1
• Three level code - transition on 1, not on 0.• Lower bandwidth than NRZI - less crosstalk
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Block Coding
• Used with NRZI or MLT3 coding• Ensures that there are always several 1s in a block.• Ensure transitions for synchronisation.• 4B5B 4bits coded as five.• 8B10B 8 bits coded as 10 -gives better DC balance.
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4B5B Block Coding
11111
11000
10001
01101
00111
00100
other invalid
idle
delimiter
delimiter
delimiter
delimiter
transmit error
1111
1110
1101
1100
4B5B
111100000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
01001
10100
10101
01010
01011
01110
01111
10010
10011
10110
10111
11010
11011
11100
11101
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10 Gigabit Ethernet Physical Layers
Ten Gigabit Ethernet has many different physical layer options. Mostare optical, but differences arise due to:
• The length of the link may be from 2 m to 40 km or more.• The type of fibre and its characteristics: multimode/ singlemode,
dispersion shifted etc.• The wavelength of the laser used.• Whether a 10Gb/s LAN interface is required or a 9.9532Gb/s SDH
compatible WAN interface.
Copper interface was added to the 10GB specification in Feb 2004. Itrequires special shielded cable and connectors and has a maximumdistance of 15m.Return to Section ToC
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Virtual LANs
• The logical and physical structure of an organisation are notalways the same.
• Dividing a network according to the logical organisation may havesecurity and performance advantages through traffic localisation.
• Virtual LANs (VLANs) allow a single physical network to besubdivided arbitrarily into multiple virtual networks.
• Packets are tagged according to which VLAN they belong to.• Switches maintain separate forwarding tables for separate VLANs
and will not forward packets from one VLAN to another
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VLAN Concept
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Virtual LAN Tags
• There is no support for VLANs in any standard Ethernet headertype.
• A new extension header IEEE 802.1Q has been defined that addsVLAN information.
• Normally this runs only between switches although newerinterface cards may add VLAN support.
• Packets may be assigned to a VLAN in three different ways:◦ A switch port may be assigned to a VLAN.◦ MAC addresses may be assigned to specific VLANs.◦ Layer 3 protocols or IP addresses may be assigned to specific
VLANs.
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VLAN Tagging
priority CFI reserved VLAN ID
CFI indicated if routing data is present
3 1 4 8 bits
proto =0x8100
informationrouting
(optional)dest src Tag Control type CRCdata
2266
VLAN Header
2−302 octets4
Return to Section ToC
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Other Features
• Link Aggregation• Packet Priority• Management
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Link Aggregation
• Link aggregation is combining multiple switched links to appear asa single high speed link.
• Can be used to provide redundancy on a network connection.• Proprietry solutions offered for several years, then standardised
asIEEE 802.3ad in 1999.• Used for switch to switch links and also server-switch links.• Tends to become redundant as higher speed Ethernet becomes
available at reasonable prices.
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Packet Priority
• Standardised by the IEEE as 802.1p.• Uses three priority bits of VLAN header.• Indicates a relative priority.• Higher priority packets are transmitted first.• Lower priority packets are dropped first.• At low loads there may be no packets in a switch buffer so it has
no effect.• Priorities may be assigned by switches the same way VLAN
membership is.• Priorities may be assigned by stations if they support 802.1Q
headers.• The standard has no admission control so it provides relative
service classification, but not strict service quality levels.
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Management
• Many of the features of switches need some management controlto set up (e.g. VLANs).
• Switches can count traffic and provide usage statistics.• Large networks may have hundreds of switches.• Most large equipment vendors provide some form switch
management.• There are some standards e.g. SNMP.• Support for proprietry management systems is sometimes added,
e.g. Cisco.• Often a simple telnet interface and a web based interface is
provided.
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Layer 3 Switches
• Although bridges and switches break up collision domains, theydo not break up broadcast domains.
• Traditional LAN protocols (e.g. Netware, Netbios) often usebroadcast extensively.
• Every broadcast packet must be forwarded to every node on theLAN so the load grows as the square of the number of packets.
• Dividing Layer 2 broadcast domains requires Layer 3 devices;routers in IP terminology.
• Traditional routers use general purpose CPUs running UNIX andare much slower than hardware based Ethernet switches.
• The solution is to implement some Layer 3 (IP) functions in switchcircuits.
• Such devices are called Layer 3 switches.
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Modern Ethernet - Summary
• Ethernet is now a switched network technology: for most links onlythe frame format is the same as the original 802.3 specifications.
• Ethernet speeds have grown 1000x since the originalspecifications.
• Ethernet links are limited in distance only by the choice oftransmission technology and can span hundreds of kilometers.
• Ethernet switches have sophisticated features to manage packetflows, priorities and security.
Return to Section ToC Return to ToC
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