Chapter 12 Local Area Networks

1
Chapter 12Local Area Networks

• Project 802
• Ethernet
• Token Ring
• FDDI

2
LAN

• LAN is a data comm system that allows a number of
  independent devices to communicate directly with
  each other in a limited geographic area.
• Dominated by four architecture ethernet, token
  bus, token ring and FDDI
• Data link control portion of the LAN protocols in
  use today are all based on HDLC.
• However, each protocol has adapted HDLC to fit
  the specific requirements of its own tech.

3
PROJECT 802

• 1985 IEEE started a project called project 802 to
  set standards to enable intercommunication
  between equipment from a variety of
  manufacturers.
• P802 is a way of specifying functions of the
  physical layer, data link layer and network layer
  for interconnectivity of major LAN protocols.
• Project 802 not intend to replace OSI.

4

• See fig. 12.1 about the relationship of IEEE
  P802 and OSI.
• The IEEE has subdivided the data link layer into
  two sublayers
• LLC Logical Link Control
• MAC Medium Access Control
• About the two sublayer, P802 contains a section
  governing internetworking. This section assures
  the compatibility of different LANs and MANs
  across protocols and allows data to be exchanged
  across otherwise incompatible networks.

5

• The strength of P802 is modularity.
• Subdivide the function and numbering it (pls
  refer fig 12.2)
• This benefit to LAN designer who were able to
  standardize functions that can be generalized and
  to isolate functions that must remain specific.

6
Figure 12-1
OSI Model and Project 802
7
Figure 12-2
Project 802
8
IEEE 802.1

• IEEE 802.1 is the section of Project 802 devoted
  to internetworking issue in LANs and MANs.
• Not yet complete.
• Although not yet complete, it seeks to resolve
  the incompatibilities between network
  architectures without requiring modifications in
  existing addressing, access, and error recovery
  mechanisms, among others.
• Used in networking/internetworking devices such
  as repeater, bridge, routers and gateway.

9
LLC Logical Link Control

• IEEE P802 model takes the structure of HDLC frame
  and divides it into 2 sets of functions.
• One set of function contains the end-user
  portions of the frame the logical addresses,
  control information and data which handled by the
  802.2 LLC protocol.

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MAC Medium Access Control

• The second set of functions, (MAC) sublayer
  resolves the contention for the shared media.
• It contains the synchronization, flag, flow, and
  error control specification necessary to move
  info from one place to another, as well as the
  physical address of the next station to receive
  and route a packet.
• MAC protocols are specific to the LAN using
  them(Ethernet, Token Ring, Token Bus, etc.)

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LLC Protocol Data Unit (PDU)

• PDU? Data unit in the 1st sublayer LLC
• Contain 4 field familiar from HDLC a destination
  service access point (DSAP), source service
  access point(SSAP), a control field, and an info
  field. (see fig 12.3)
• The DSAP and SSAP are addresses used by the LLC
  to identify the protocol stacks on the receiving
  and sending machines that are generating and
  using the data. See fig. 12.3 for description.
• The control field of the PDU is identical to the
  control field in HDLC. Refer fig 12.4 about
  I-frame, S-frames and U-frames.
• The PDU has no flag fields, no CRC, and no
  station address. These fields are added in the
  lower sublayer (the MAC layer)

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Figure 12-3
PDU Format
13
Figure 12-4
PDU Control Field
14
802.3 Ethernet

• Dev by Xerox, intel and digital equipment
• IEEE 802.3 defines 2 categories baseband and
  broadband. Base specifies a digital signal. Broad
  specifies analog signal.
• IEEE divides the baseband category into 5
  different standard see fig 12.5
• IEEE defines only one specification for the
  broadband. See fig 12.5
• Access method for ethernet is CSMA/CD

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Figure 12-5
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Ethernet access method CSMA/CD

• Whenever multiple users have unregulated access
  to a single line, there is a danger of signals
  overlapping and destroying each other.
• Such overlaps, which turn the signals into
  unusable noise are called collisions.
• A LAN therefore needs a mechanism to coordinate
  traffic, minimize the number of collision that
  occur, and maximize the number of frames that are
  delivered successfully.
• So access mechanism should be used is CSMA/CD

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• How CSMA/CD work?
• Any workstation wishing to transmit must first
  listen for existing traffic on the line. A device
  listens by checking for a voltage. If no voltage
  is detected, the line is considered idle and the
  transmission is begin.
• During the data transmission, the station checks
  the line for the extremely high voltages that
  indicate a collision.
• If a collision is detected, the station quits the
  current transmission and waits a predetermined
  amount of time for the line to clear, then sends
  its data again.

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Ethernet addressing and electrical specification

• Addressing each station got NIC which provide
  6-byte physical address
• Signal baseband system use manchester digital
  encoding digital/digital conversion
• For broadband (only one 10broad36) it uses
  digital/analog conversion differential PSK.
• Data rate support between 1 and 100Mbps

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Ethernet frame format

• Consist of 7 fields se fig 12.7
• Preamble this 7 bytes field alert the receiving
  system to the coming frame and enable it to
  synchronize its input timing.
• Start Field Delimeter tells the receiver that
  everything that follows is data, starting with
  the addresses.
• Destination Address this field contain the
  physical address of the packets next destination

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• Source Address contains the physical address of
  the last device to forward packet
• Length type of PDU This two bytes field indicate
  the number of bytes in the coming PDU.
• 802.2 frame PDU. This field of the 802.2 frame
  contains the entire 802.2 frame as a modular,
  removable unit. The PDU can be anywhere from 46
  to 1500bytes long.
• CRC. The last field in the 802.3 frame contains
  the error detection information.

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Figure 12-7
MAC Frame
22
Ethernet 802.3 implementation

• Although project 802 standard focus on the data
  link layer of the OSI model, the 802 model also
  defines some of the physical specifications for
  each of the protocol defined in the MAC layer.
• In the 802.3 standard, the IEEE defines the types
  of cable, connections, and signals that are to be
  used in each of five different Ethernet
  implementations.
• All Ethernet LANs are configured as logical
  buses, although they may be physically
  implemented in bus or star topologies. Each frame
  is transmitted to every station on the link but
  read only by the station to which it is addressed.

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Thick Ethernet 10BASE5

• The first of the physical standards defined in
  the IEEE 802.3 model. Also known as thicknet.
  (See fig 12.9)
• This is a bus topology LAN that uses baseband
  signaling and has a maximum segment of 500m.
• In 10BASE5, networking devices (bridges,
  repeaters can be used to overcome the size
  limitation of local area networks.
• In thicknet LAN can be divided into segments by
  connecting devices. In this case, the length of
  each segment is limited to 500meters.
• However, to reduce collision, the total length of
  the bus should not exceed 2500 meters (5
  segment).
• Also, the standard demands that each station be
  separated from each neighbor by 2.5meters (200
  station per segment and 1000 stations in total.)
  see fig 12.8

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Figure 12-8
Ethernet Segments
25
Figure 12-9
26
10Base5 topology

• Physical connectors and cables utilized by
  10Base5 include coaxial cable, NIC, transceiver,
  and attachment unit interface AUI cables. See fig
  12.9 for interaction of components
• RG-8 cable thick coaxial cable (RGRadio
  Government)
• Transceiver/MAU Each station is attached by an
  AUI cable to an intermediary device called
  transceiver (transmitter/receiver). This device
  performs the CSMA/CD function (check voltage and
  collison)
• AUI cables Each station is linked to its
  corresponding transceiver by an attachment unit
  interface (AUI), also called a transceiver cable.
  An AUI is a 15-wire cable with plugs that
  performs the physical layer interface functions
  between the station and the transceiver. 50m
  only. Each end of this cable is DB-15 connector
  see fig 12.10.
• Transceiver tap used to connect transceiver into
  the line. The tap is a thick cable-sized well
  with a metal spike in the centre. See fig 12.10.
  Also known as vampire tap because it bites the
  cable to allow electrical connection between
  trancseiver and the cable.

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Figure 12-9-continued
10BASE5
28
Figure 12-10
Transceiver
29
Figure 12-11
The 2nd ethernet implementation defined by IEEE
802 bus topology . A.K.A. thin
ethernet/thinnet/cheapnet/cheapernet Provides an
inexpensive alternative to 10Base5 Ethernet with
same Data rate. Better choice than
10base5. Advantage reduced cost and ease of
installation. Cable lighter weight Disadv
shorter range and smaller capacity of station
attach it. Refer fig 12.11 the layout of 10base2
topology
30
Figure 12-11-continued
10BASE2
31

• Compared to 10Base5, 10Base2 topology is much
  simpler. In this technology, the transceiver
  circuitry has moved into the NIC and the
  transceiver tap has been replaced by a connector
  that splices the station directly into the cable,
  eliminating the need for AUI cables.
• Devices used in this topology are
• NIC provide same function as NIC in thicknet
  with transceiver capability to check for voltages
  on the link in order to perform CSMA/CD.
• Thin coaxial cable these cable are relatively
  easy to install and move around (especially
  inside existing building where cabling must be
  pulled through the walls and ceilings.)
• BNC-T The BNC-T connector is a T-shaped device
  with 3 port one for the NIC and one each for the
  input and output ends of the cable.

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Figure 12-12

• The most popular standard defined in the IEEE
  802.3 series.
• Also called twisted pair Ethernet. It is
  star-topology LAN.
• Use twisted pair cable instead of coaxial cable.
• It support data rate of 10 Mbps
• It has a maximum length (hub to station) of 100
  meters.

33
Figure 12-12-continued
10BASE T
34
10Base-T twisted pair ethernet

• Instead of individual transceiver, 10Base-T
  Ethernet places all of its networking operations
  in an intelligent hub with a port for each
  station.
• Station are linked into the hub by 4 pair RJ-45
  cable (8 wire unshielded twisted-pair cable)
  terminating at each end in a male-type connector
  much like a telephone jack.
• The hub fans out any transmitted frame to all of
  its connected stations.
• Logic in the NIC assures that the only station to
  open and read a given frame is the station to
  which that frame is addressed.

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• As fig 12.12 shows, each station contains an NIC.
  
• A length of 4pair UTP of not more than 100 meters
  connects the NIC in the station to the
  appropriate port in the 10Base-T hub.
• The weight and flexibility of the cable and the
  convenience of the RJ-45 jack and plug make
  10Base-T the easiest of the 802.3 LANs to install
  and reinstall.
• When a station needs to be replaced, a new
  station can simply be plugged in.

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Figure 12-13

• a.k.a StarLan. It is an ATT product
• It is not popular due to its slowness data rate
  speed.
• Advantages over previous discussed 802.3 LAN
  standards
• Mechanism called daisy chaining
• Has some basic similarities with 10Base-T
• Also has differences.
• Starlan allows as many as 10 stations to be
  linked, each to the next,
• in a chain in which only the lead device
  connects to the hub.

37
Figure 12-13-continued
1BASE5
? The McGraw-Hill Companies, Inc., 1998
WCB/McGraw-Hill
38
Other Ethernet Networks

• During the last decade there has been an
  evolution in Ethernet networks.
• Several new schemes have been devised to improve
  the performance and the speed of Ethernet LANs.
  This new improve LAN are
• Switched Ethernet
• Fast Ethernet
• Gigabit Ethernet

39
Switched Ethernet

• Switched ethernet is an attempt to improve the
  performance of 10Base-T Ethernet.
• The 10Base-T Ethernet is a shared media network,
  which means that the entire media is involved in
  each tranmission.
• This is because the topology, though physically a
  star, is logically a bus.
• When a station sends a frame to a hub, the frame
  is sent out from all ports(interfaces) and every
  station will receive it.
• In this situation, only one station can send a
  frame at any given time. If more than one
  station try to send frame simultaneously, of
  course there is a collision.

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Figure 12.14 An Ethernet network using a hub
Only station E is the actual destination, but all
the station Receive the frame sent from A
C
A
E
B
D
Fig. 12.14 shows this situation. Station A is
sending a frame to station E. The frame is
received by the hub and is sent to every
station. All of the cabling in the system is
indirectly involved in this transmi. Another way
to think about this is that one transmission uses
the entire Capacity of 10Mbps if one station
uses it, no other station can.
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Figure 12.15 An Ethernet using a switch
Only station E receives the frame, so The rest of
the media is free for Another transmission
A
B
C
D
E

• However, if we replace the hub with switch, a
  device that can recognize
• the destination address and can route the frame
  to the port to which
• the destination station is connected, the rest
  of the media are not
• involved in the transmission process.
• This means that the switch can receive another
  frame from another
• station at the same time and can route this
  frame to its own final
• destination. In this way, theoritically, there
  is no collision.
• In fig 12.15, when station A is sending a frame
  to station E, station B can also send a frame to
  station D without any collision.

42
Fast Ethernet

• With new applications such as CAD, image
  processing, and real-time audio and video being
  implemented on LANs, there is a need for a LAN
  with a data rate higher than 10Mbps.
• Andfast ethernet operates at 100 Mbps.
• Fast ethernet is a version of ethernet with a 100
  Mbps data rate.
• There is no change in the access method.
• There is no change in the frame format.

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• The only two changes in the MAC layer are the
  data rate and the collision domain.
• The data rate is increased by a factor of 10
• The collision domain is decreased by a factor of
  10.
• This means that the collision domain must be
  decreased 10 times from 2500meters to 250meters.
  This decrease is not a problem because LANs today
  connect desktop computers that are not more than
  50 to 100 meters away from the central hub.
• This means the collision domain is between 100
  and 200 meters.

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• In the physical layer, the specification
  developed for fast ethernet is a star topology
  similar to 10base-T.
• However, to match the physical layer to different
  resources available, IEEE has designed two
  categories of Fast Ethernet
• 100Base-X (use 2 cable between station and hub)
• 100Base-T4 (use 4 cable between station and hub)
• See fig 12.16.
• 100Base-X itself is divided into two types TX and
  FX.

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Figure 12-14
46
100Base-X 100Base-TX

• Uses 2 category 5 (CAT5) UTP or 2 STP to connect
  a station to the hub/switch.
• One pair is used to carry frames from the station
  to the hub and the other to carry frames from the
  hub to the station.
• The signaling method (dig/dig) is NRZ-I.
• Encoding method is 4B/5B to handle the 100Mbps.
• The distance between the station and the
  hub/switch should be less than 100 meters.
• Refer fig 12.17

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Figure 12.17 100Base-TX implementation
100 meters
2 CAT5 or 2 STP
4B/5B encoding, NRZ-I signaling
48
100Base-X 100Base-FX

• The 100Base-FX design uses 2 optical fibers, one
  to carry frames from the station to the hub.
• And the other from the hub to the station.
• The encoding is 4B/5B.
• Signaling method is NRZ-I.
• The distance between the station and the
  hub/switch should be less than 2000 meters.
• Refer fig 12.18

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Figure 12.18 100Base-FX implementation
2000 meters
2 optical fibers
4B/5B encoding, NRZ-I signaling
50
100Base-T4

• The 100Base-T4 scheme was designed in an effort
  to avoid rewiring.
• It requires four pairs of CAT3 UTP (voice grade)
  that are already available for telephone service
  inside most building.
• Two of the four pairs are bidirectional the
  other two are unidirectional. This means that in
  each direction, three pairs are used at the same
  time to carry data.
• Because a 100-Mbps data rate cannot be handled by
  a voice-grade UTP, the specification splits the
  100Mbps flow of data into three 33.66 Mbps flows.
• Refer fig 12.19

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Figure 12.19 100Base-T4 implementation
100 meters
4 CAT3 UTP
Hub/switch
8B/6T encoding,
station
52
Figure ? e.g. overall simple 100 base T LAN
infrastructure in a workplace
53
Gigabit Ethernet

• The migration from 10 Mbps to 100 Mbps encouraged
  the IEEE 802.3 committee to design Gigabit
  Ethernet, which has a data rate of 1000Mbps or 1
  Gbps.
• MAC layer and the access method remain the same,
  but the collision domain is reduced.
• The physical layer- the transmission media and
  the encoding system however changes.
• To cater 1000mbps, optical fiber cable is used
  instead of twisted pair cable although the
  protocol doesnt eliminate the use of twisted
  pair cables.
• Gigabit Ethernet usually serves as a backbone to
  connect Fast Ethernet networks.

54

• Contd
• Four implementations have been designed for
  gigabit ethernet
• 1000Base-LX
• 1000Base-SX
• 1000Base-CX
• 1000Base-T

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Table 12.1 comparison between the gigabit
ethernet implementation
56
10Broad36

• 10BROAD36 - 10BROAD36 is a seldom used Ethernet
  specification which
• uses a physical medium similar to cable
  television, with CATV-type cables, taps,
  connectors, and amplifiers.