OSI Protocol Model
Local Area Networks
email@example.com 4. Transport
Department of Computer Science
University of Waikato 2. Link LANs are L2 networks
COMP312 - Local Area Networks – p. 1/58 COMP312 - Local Area Networks – p. 2/58
Lecture Outline Protocols and Standards
• Protocols and Standards.
• LAN Protocols.
• Hubs, Bridges and Switches
• LAN Standards.
• Modern Ethernet.
• Ethernet Frame Format.
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LAN Protocols LAN Framing
Network Header Network Data
LLC Header LLC Data
1. Physical MAC Header MAC Data
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IEEE 802.2 LAN Standards
• Most MAC Layers provide and unreliable Datagram service
• IEEE 802.2 provides a link layer service based on HDLC • Most LAN Standards are set by the IEEE
• There are three classes of service • Most LANs started outside the IEEE in industry or academia but
◦ Unacknowledged connectionless-mode service only. later got taken to the IEEE
◦ Connection-mode service plus service • The IEEE 802 committee is responsible for LANs.
◦ Acknowledged connectionless service
• 802.2 Also provides
◦ Service Access Points
• 802.2 Uses Sliding Window ﬂow control and Go-Back-N ARQ
Return to Section ToC
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IEEE 802 Standards IEEE 802 Standards
IEEE Number Name
802.1 Bridging and Management
802.2 Logical Link Control
802.2 Logical Link Control
802.3 CSMA/CD - Ethernet
802.4 Token Bus - ARCNet Link
802.1 Bridging Layer
802.5 Token Ring
802.6 MANs - DQDB 802.3 802.4 802.5 802.6 802.11 802.12 802.15 802.16
CSMA/CD Token Bus Token Ring DQDB W−LAN DPA W−PAN WB−MAN
802.11 Wireless LANs Physical
802.3 802.4 802.5 802.6 802.11 802.12 802.15 802.16 Layer
802.12 Demand Priority Access PHY PHY PHY PHY PHY PHY PHY PHY
802.15 PANs (Bluetooth)
Return to Section ToC
802.16 Broadband Wireless MANs
802.17 Resilient Packet Ring
802.20 Mobile Broadband Access
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Ethernet Frame Format Ethernet (802.2) Addresses
There are four different Ethernet frame formats • Addresses are six bytes long, Normally written as hyphenated hex
1. Ethernet Version II. This is from the original Ethernet speciﬁcation • The ﬁrst three bytes are an IEEE assigned Organizationally
released by Xerox, Intel and DEC.
Unique Identiﬁer (OUI)
2. Novell Proprietary ("802.3 Raw"). This format was used by Novell • The second three bytes are assigned by the manufacturer.
Netware and was based on an early version of the 802.3
speciﬁcation. • Properly assigned addresses are globally unique.
3. 802.3. The 802.3 standard speciﬁes a header that includes the • Some hardware allows manually assigned addresses.
802.2 LLC ﬁelds. • Destination address of all ones is the broadcast address.
4. 802.3 SNAP. This provides an extended header that allows • Some addresses are reserved for multicast applications (normally
backwards compatibility with the original Version II header. speciﬁc addresses are assigned for speciﬁc protocols).
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Ethernet Version II Novell Proprietary
Dst Src Etype Data FCS Dst Src Length Data FCS
• Dst: Destination Address - 6 bytes. • Dst: Destination Address - 6 bytes.
• Src: Source Address - 6 bytes. • Src: Source Address - 6 bytes.
• Ethertype: Speciﬁes the protocol being carried within the data section. Used for • Length: The length of the entire frame not including the preamble or CRC - 2 bytes.
multiplexing protocols. Ethertypes are all greater than 1536 and are assigned by • Data: Variable length payload. Netware Packets always start with 0xFFFF. Must be
Xerox- 2 bytes. padded if less than the minimum length - 46-1500 bytes.
• Data: Variable length payload. Must be padded if less than the minimum length - • FCS - Frame Check Sequence used for CRC - 4 bytes.
• FCS - Frame Check Sequence used for CRC - 4 bytes.
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802.3 802.3 SNAP
802.2 LLC Header 802.2 LLC Header
Dst Src Length DSAPSSAP Ctrl Data FCS Dst Src Length DSAPSSAP Ctrl SNAP Data FCS
• DSAP: Destination Service Access Point. References the process associated with • SNAP: SubNetwork Access Protocol - 5 bytes. The ﬁrst three bytes carry the
the protocol of data section of the packet at the receiving station - 1 byte. Organisation Unique Identiﬁer and are usually the same as the ﬁrst three bytes of
• SSAP: Source Service Access Point. References the process associated with the the source address. The last two bytes carry a protocol identiﬁer that is usually an
protocol of data section of the packet at the sending station - 1 byte. Ethertype.
• Ctrl: Speciﬁes the type of packet as used by the LLC protocol. May be
Informational, Control or Data.
04 - IBM SNA 06 - IP
80 - 3Com AA - SNAP
Common DSAP/SSAP values include:
BC - Banyan E0 - Novell
F4 - Lan Manager
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Protocols and Standards - Summary Hubs, Bridges and Switches
• The IEEE has separated Link Layer functions from Media Access
• This makes all IEEE standard LANs compatible in terms of the • Hubs.
services they offer. • Bridges.
• The Ethernet Frame format has evolved as the standard has • Switches.
Return to Section ToC Return to ToC
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Hubs Broadcast Bus
• 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.
A B C D
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• Bridges are intelligent repeaters. They forward packets without
• Bridges are Layer 2 devices so they are transparent to end
• Identical L2 protocols are required on both sides of a bridge (may
be LLC identical).
• They divide up collision domains so CSMA/CD runs either side of
a bridge, but not across it.
A B C D • Bridges can buffer packets to ensure they are not lost without the
original transmitter knowing.
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Bridge Advantages Bridge
• 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, but
compatible LLC layers (e.g. WLANs are normally bridged to
• Bridges may be used to connect remote networks using a wide
• Collision Detection does not work on ﬁbre optic links so they have
to be point to point links and bridged to the rest of the network.
A B C D
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Loops Spanning Tree
• Loops may be formed with multiple bridges on a LAN segments • A spanning tree is a subset of the bridge topology that:
◦ Deliberately for redundancy. ◦ Covers the entire network (spanning),
◦ Accidentally through misconﬁguration. ◦ Includes no loops (tree).
• This can cause signiﬁcant problems • Spanning Tree works by:
◦ Multiple copies of packets ◦ Bridges exchange topology information using speciﬁc bridge
◦ Bridges learning the wrong location of stations and not topology packets and a multicast address,
forwarding packets ◦ A root bridge is elected,
◦ Cascading multiplication of packets ◦ 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 of
the spanning tree.
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Spanning Tree 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 Switch Buffering
A B C D
• Switches and bridges can buffer packets that cannot be
immediately sent to a segment.
◦ The receiving segment is busy or experiencing collisions.
◦ The sending segment runs at a higher speed than the
◦ Multiple segments are sending packets to the same receiving
A B C D
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Switch Buffering -2 Autonegotiation
• It is possible that the buffering requirements may exceed the • Switches may be connected to devices of varying speeds.
memory of the switch. • UTP Ethernet uses the same connector for 10Mbps, 100Mbps,
• The switch can respond in various ways 1000Mbps
◦ Do Nothing; assume higher layer ﬂow/error control will • 10Mbps UTP Ethernet sends a half pulse every 16ms to verify the
respond. link status, called Normal Link Pulse - NLP. Reception of this
◦ Backpressure; Cause collisions on the sending segments to pulse causes the link status LED to light on a NIC and above a
slow down the sender. switch port .
◦ 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 - 2 Autonegotiation - 3
• 100Mbps and 1000Mbps send multiple clock and data pulses at 2ms burst
the same time intervals, called Fast Link Pulse - FLP. of 33 pulses
• The clock signals are used to determine the speed capability of
the communicating entities. FLP
• The data pulses contain information describing the device
capabilities (e.g. full duplex).
• The link speed defaults to the lowest capability level of the two
• Autoconﬁguration is useful to decrease the chance of user
data data data data data
i 2 3 4 16
clock clock clock clock clock
1 2 3 4 16
Return to Section ToC
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Hubs Bridges and Switches - Summary Modern Ethernet
• Hubs are compressed busses used to allow star wiring (UTP).
• Bridges break up collision domains and extend LANs. • Speeds.
• Bridges use spanning tree routing to break up loops. • VLANS.
• Switches are multiport bridges. • Other Features.
Return to Section ToC Return to ToC
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Speeds Modern Ethernet Timeline
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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Thick Ethernet (10base5) Thick Ethernet (10base5)
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Thin Ethernet (10base2) UTP Ethernet
COMP312 - Local Area Networks – p. 41/58 COMP312 - Local Area Networks – p. 42/58
Fibre Optic Ethernet Ethernet Physical Layers
Medium Signaling Topology Max Segment Nodes
10Base5 10mm 50 ohm Coax Manchester Bus 500m 100
10Base2 5mm 50 ohm Coax Mancheter Bus 185m 30
10BaseT UTP Manchester Star 100m 2
10BaseF Fiber Manchester Star 500m 2
100BaseTX (UTP) UTP 4B5B MLT−3 Star 100m 2
100BaseFX Fiber 4B5B NRZI Star 100m 2
1000BaseT UTP PAM5x5 Star 100m 2
1000BaseSX 50micron Fiber 8B10B Star 550m 2
1000BaseSX 62.5micron Fiber 8B10B Star 275m 2
1000BaseLX 50/72.5micron Fiber 8B10B Star 550m 2
1000BaseLX 9micron Fiber 8B10B Star 5000m 2
1000BaseLH ~9micron Fiber 8B10B Star 50~100km 2
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MLT3 Line Coding 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.
1 0 0 1 1 1 1 0 0 0 0 1 0 1 1 1 0 1 1 0 1 1 1 0
• 8B10B 8 bits coded as 10 -gives better DC balance.
• Three level code - transition on 1, not on 0.
• Lower bandwidth than NRZI - less crosstalk
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4B5B Block Coding 10 Gigabit Ethernet Physical Layers
4B5B Ten Gigabit Ethernet has many different physical layer options. Most
0000 11110 11111 idle are optical, but differences arise due to:
0001 01001 11000 delimiter
• The length of the link may be from 2 m to 40 km or more.
0010 10100 10001 delimiter
0011 10101 01101 delimiter
• The type of ﬁbre and its characteristics: multimode/ singlemode,
0100 01010 00111 delimiter dispersion shifted etc.
0101 01011 00100 transmit error • The wavelength of the laser used.
0110 01110 other invalid
• Whether a 10Gb/s LAN interface is required or a 9.9532Gb/s SDH
compatible WAN interface.
1001 10011 Copper interface was added to the 10GB speciﬁcation in Feb 2004. It
1010 10110 requires special shielded cable and connectors and has a maximum
1011 10111 distance of 15m.
1100 11010 Return to Section ToC
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Virtual LANs VLAN Concept
• The logical and physical structure of an organisation are not
always the same.
• Dividing a network according to the logical organisation may have
security and performance advantages through trafﬁc localisation.
• Virtual LANs (VLANs) allow a single physical network to be
subdivided 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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Virtual LAN Tags VLAN Tagging
• There is no support for VLANs in any standard Ethernet header VLAN Header
dest src proto = Tag Control type information data CRC
• A new extension header IEEE 802.1Q has been deﬁned that adds 0x8100 (optional)
VLAN information. 6 6 2 2 2 2−30 4 octets
• Normally this runs only between switches although newer
priority CFI reserved VLAN ID
interface cards may add VLAN support. 3 1 4 8 bits
• Packets may be assigned to a VLAN in three different ways: CFI indicated if routing data is present
◦ A switch port may be assigned to a VLAN. Return to Section ToC
◦ MAC addresses may be assigned to speciﬁc VLANs.
◦ Layer 3 protocols or IP addresses may be assigned to speciﬁc
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Other Features Link Aggregation
• Link aggregation is combining multiple switched links to appear as
• Link Aggregation a single high speed link.
• Packet Priority • Can be used to provide redundancy on a network connection.
• Management • 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 Management
• Standardised by the IEEE as 802.1p. • Many of the features of switches need some management control
• Uses three priority bits of VLAN header. to set up (e.g. VLANs).
• Indicates a relative priority. • Switches can count trafﬁc and provide usage statistics.
• Higher priority packets are transmitted ﬁrst. • Large networks may have hundreds of switches.
• Lower priority packets are dropped ﬁrst. • Most large equipment vendors provide some form switch
• At low loads there may be no packets in a switch buffer so it has
• There are some standards e.g. SNMP.
• Priorities may be assigned by switches the same way VLAN • Support for proprietry management systems is sometimes added,
membership is. e.g. Cisco.
• Priorities may be assigned by stations if they support 802.1Q • Often a simple telnet interface and a web based interface is
• The standard has no admission control so it provides relative
service classiﬁcation, but not strict service quality levels.
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Layer 3 Switches Modern Ethernet - Summary
• Although bridges and switches break up collision domains, they • Ethernet is now a switched network technology: for most links only
do not break up broadcast domains. the frame format is the same as the original 802.3 speciﬁcations.
• Traditional LAN protocols (e.g. Netware, Netbios) often use • Ethernet speeds have grown 1000x since the original
broadcast extensively. speciﬁcations.
• Every broadcast packet must be forwarded to every node on the • Ethernet links are limited in distance only by the choice of
LAN so the load grows as the square of the number of packets. transmission technology and can span hundreds of kilometers.
• Dividing Layer 2 broadcast domains requires Layer 3 devices; • Ethernet switches have sophisticated features to manage packet
routers in IP terminology. ﬂows, priorities and security.
• Traditional routers use general purpose CPUs running UNIX and
Return to Section ToC Return to ToC
are much slower than hardware based Ethernet switches.
• The solution is to implement some Layer 3 (IP) functions in switch
• Such devices are called Layer 3 switches.
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