1. LAN and MAN Data Link Protocols
Local Area Networks
Limited geographical area, usually within a building, often on a single floor
Typical speeds = Mbit/s
Backbone LAN
Usually within a building, connecting lower speed LANs, often between floors
Typical speeds = 100 Mbit/s – 10 Gbit/s
Metropolitan Area Networks
Within a city, or a campus
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2. LAN and MAN Data Link Protocols
LANs
Ethernet
Token Bus
Token Ring
Backbone LANs
Fast Ethernet
Gigabit Ethernet (also used in MANs and WANs)
Fibre Distributed Data Interface (FDDI)
MANs
Distributed Queue Dual Bus (DQDB)
Cable TV (CATV) Networks
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3. LAN and MAN Standards LANs are standardised by the IEEE 802 Committee
The main standards are:
IEEE Internetworking issues in LANs and MANs
IEEE Logical Link Control (LLC)
IEEE Ethernet (CSMA/CD)
IEEE Token Bus
IEEE Token Ring
IEEE Metropolitan Area Networks (DQDB)
IEEE Wireless Local Area Networks (WLANs)
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4. LAN Sublayers IEEE LANs operate two sublayers
Logical Link Control
Common to all LANs
Based on HDLC
Includes addressing, control information and data
Media Access Control
Specific to the media type
Resolves contention
Responsible for synchronisation, flow control and error control
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5. IEEE 802.2 (LLC) Frame Format
The LLC Headers are based on HDLC and are encapsulated in MAC frames
They therefore follow immediately after the MAC header
DSAP
SSAP
Control
Information
1 byte
1 or 2 bytes
Up to 1496 or 1497 bytes
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6. IEEE 802.2 (LLC) Frame Format
Destination Service Access Point (DSAP)
used to identify the network layer protocol at the receiver with one bit to signify whether it is a group or individual address
Source Service Access Point (SSAP)
used to identify the network layer protocol at the transmitter with one bit to signify whether it is a command or a response
Control Field
used to identify the type of LLC frames (similar to its use in HDLC)
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7. IEEE (Ethernet)
The Ethernet was developed in 1976 at Xerox’s Palo Alto Research Center (PARC)
It was marketed by a consortium of Digital, Intel and Xerox (known as the DIX Consortium) and soon became the de facto standard for LANs
There are two variations of Ethernet in use
Ethernet II (the original DIX protocol. It has a type field to identify the network layer protocol)
IEEE (replaces the type field with a length field and cannot identify network layer protocol so can only be used with one network layer or requires an additional LLC sublayer)
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8. IEEE 802.3 (MAC) Frame Format
© Tanenbaum, Prentice Hall International
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9. IEEE 802.3 (MAC) Header Fields
Preamble
7 bytes of to synchronise receivers
Start of Frame Delimiter ( )
Destination and Source Addresses
6 bytes each for MAC addresses
Length of Data Field (in bytes)
Data
0 to 1500 bytes
Padding
Up to 46 bytes to make sure that frame is at least 64 bytes long to ensure that collisions can always be detected by the transmitter
Checksum (CRC-32)
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10. IEEE 802.3 Specification Access Method Backoff Algorithm CSMA/CD
Binary exponential back-off
after n collisions a random number between 0 and 2n-1 is chosen and transmitter backs off for this number of timeslots (timeslot = worst case RTT = 51.2 microseconds at 10 Mbit/s)
After 10 collisions maximum backoff time frozen
After 16 collisions algorithm gives up
Hence delay is variable and there is no guarantee that the frame will ever be transmitted
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11. IEEE 802.3 Specification Electrical Data Rate Addressing
Baseband signaling with Manchester encoding
Data Rate
10 Mbit/s to 10 Gbit/s
Addressing
MAC addresses burnt into Network Interface Cards
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12. Ethernet Implementations
10Base5 (Thick Ethernet or Thicknet)
Leads to stations are tapped into up to 500 m of thick coaxial cable
10Base2 (Thin Ethernet or Thinnet, Cheapnet or Cheapernet)
Leads to stations are connected to a thin coaxial cable bus of up to 200m via a three-way connector
10Base-T (Twisted Pair Ethernet)
Each station is wired to a hub via twisted pair cable. The hub broadcasts frames to all stations apart from the one from which it received the frame
Topology is physically a star, but logically a bus
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13. Ethernet Implementations
10Base5
10Base2
10Base-T
© Tanenbaum, Prentice Hall International
Note: the 1st part of the name indicates the speed in Mbit/s, the 2nd
part whether it is baseband or broadband and the final part the maximum distance the signals will carry in 100s of meters or the type of media
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14. Switched Ethernets
In order to reduce the number of collisions, modern Ethernets are often connected to a switch rather than a hub
Whereas the hub will forward frames to all stations except the one from which the frame was received, a switch will examine the destination MAC address and forward the frame only to the one station it was intended for
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15. IEEE (Token Bus)
Developed by General Motors and used in factory automation, but is not very common
Topology is physically a bus or tree, but logically a ring
Overcomes problem with regarding the arbitrarily long time a station may have to wait to transmit
Overcomes the problem with ring networks being completely brought down by a cable break
Station can only transmit if it has the token which is passed around from station to station in turn
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16. IEEE 802.5 (Token Ring) Developed by IBM for office automation
Overcomes problem with regarding the arbitrarily long time a station may have to wait to transmit
Token is circulated around the ring
When a station wants to transmit, it seizes the token and transmits its data
The receiver marks the frame as having been read
When the frame returns to the sender, it checks that it has been successfully read an passes the token on to the next station
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17. Token Ring Protocol Access Method Electrical Data Rates Addressing
Token Passing
Electrical
Baseband with Differential Manchester Encoding
Data Rates
1, 4 or 16 Mbit/s
Addressing
MAC addresses burnt into Network Interface Cards
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18. Star-shaped Token Rings
Token Rings are often collapsed into MultiStation Access Units (MAUs) which solve the problems associated with cable breaks
© Tanenbaum, Prentice Hall International
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19. IEEE 802.11 (Wireless LANs) Also known as WiFi
Uses CSMA/CA access method (RTS, CTS, ACK)
Uses LLC and alternative MAC layers
IEEE a
Uses Orthogonal Frequency Division Multiplexing (OFDM) with 52 frequencies in the 5 GHz band and supports 6-54 Mbit/s
IEEE b
Uses Direct Sequence Spread Spectrum (DSSS) in the 2.4 GHz band and supports 1, 2, 5.5 and 11 Mbit/s
IEEE g
Uses OFDM in the 2.4 GHz band to support 54 Mbit/s
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20. IEEE Physical Layers
The IEEE Standard supports the following physical layers
(3 variants)
Infrared
Frequency Hopping Spread Spectrum
Direct Sequence Spread Spectrum
802.11a Orthogonal Frequency Division Multiplexing
801.11b High Rate DSSS
802.11g Orthogonal Frequency Division Multiplexing
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21. IEEE Services
Distribution Services provided by the Base station
Association - Connecting to Base Station
Disassociation – Disconnecting from Base Station
Reassociation – Changing Base Station
Distribution – routing frames to other stations or over wired network
Integration – Translating from frame format to other formats for non networks
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22. IEEE Services
Intracell Services used within a cell after association has taken place
Authentication – challenge to check if station has correct secret key
Deauthentication – used when a station wants to leave the network
Privacy – encryption using RC4 algorithm
Data delivery – connectionless (unreliable) delivery of data
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23. Backbone LAN Standards
IEEE 802.3u (Fast Ethernet – 100 Mbit/s)
IEEE 802.3z (Gigabit Ethernet –1000 Mbit/s)
IEE802.3ae (10G Ethernet – 10 Gbit/s)
ANSI X3TP.5 (Fibre Distributed Data Interface - FDDI)
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24. IEEE 802.3u (Fast Ethernet) Implementations
100Base-TX
Uses 2 cat 5 UTP or 2 STP cables up to 100m to hub or switch with 4B/5B encoding and NRZ-I signalling
100Base-FX
Uses 2 optical fibres up to 2000m to hub or switch with 4B/5B encoding and NRZ-I signalling
100Base-T4
Uses 4 cat 3 UTP cables up to 100m to hub or switch by splitting the 100 Mbit/s capacity into 3x33.66 Mbit/s channels
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25. IEEE 802.3z (Gigabit Ethernet) Implementations
1000Base-SX
Multi-mode optical fibre with short wave lasers up to 550m
1000Base-LX
Multi-mode or Single-mode optical fibre with long wave laser up to 550m (multi-mode) or 5000m (single-mode)
1000Base-CX
Shielded Twisted Pair up to 25m
1000Base-T
Unshielded Twisted Pair up to 25m
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26. Fibre Distributed Data Interface (FDDI)
FDDI is a high performance LAN protocol for use over optical fibres standardised by ANSI and ITU-T
The topology is base on two rings with data flowing in opposite directions
If there is a break in the cable, the stations either side of the break detect it and simply loop the cables to form one ring
As with Token Rings, the FDDI Ring can be collapsed into a wire centre to further protect against cable failures bringing the network down
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27. FDDI Dual Ring Back-up
Cable
Break
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28. FDDI Protocol Access Method Electrical Data Rates Addressing
Token Passing
Electrical
Baseband with 4B/5B (4 bits are represented by 5 bits containing no more than two consecutive zeros) encoding with NRZ-I signaling
Data Rates
100 Mbit/s
Addressing
MAC addresses burnt into Network Interface Cards
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29. MAN Implementions Distributed Queue Dual Bus (DQDB)
Used as Layer 2 protocol in Switched Multimegabit Data Service (SMDS)
Cable Television Networks
Community Access TeleVision or CATV
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30. IEEE 802.6 (Distributed Queue Dual Bus)
Topology is 2 parallel unidirectional buses
Each bus has a head end which generates a steady stream of 53 byte cells
Each cell carries a 44 byte payload and has two protocol bits (busy to indicate that a cell is occupied and request to indicate that a station wants to make a request)
Each station can see requests for one bus being made on the other bus
Stations can implemented a distributed First In First Out (FIFO) queue and know when they can use a cell
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31. Cable TV Networks
Cable TV networks are broadband networks that broadcast TV channels from the head end of the cable network to houses, but they can also support interactive data services using cable modems
A standard has been developed for cable modems called DOCSIS (Data Over Cable Service Interface Specification) or EuroDOCSIS
Users contend to request short slots on the upstream channel while the downstream channel is managed by the head end
Data is transferred downstream in 204 byte packets
The user interface is normally 10 Mbit/s Ethernet or USB
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