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Chapter 9:Local Area Networks
Principles of Computer Principles of Computer Networks and CommunicationsNetworks and Communications
M. Barry Dumas and Morris SchwartzM. Barry Dumas and Morris Schwartz
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Objectives
Describe how different forms of LANs originated and how they evolved
Differentiate LAN physical and logical topologies Identify LAN addressing issues and the role of MAC
addresses Describe the role of LAN segmentation and its impact on
performance Compare and contrast Ethernet, Token Ring, and FDDI
LAN models Describe the role of VLANs and LANE configurations in
networking schemes
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Overview
LAN decisions (configuration, speed, O/S, access, etc.) are made by businesses (LAN owners)
WAN links are owned by public carriers
“Despite the traditional classification of LANs by span, a more relevant classification is link ownership.”
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Overview
Two basic LAN classifications Dedicated-server (server-centric or client-server)
Servers function only as servers with specialized functions (printing, database, websites, etc.)
One server must be a file-server Used by vast majority of businesses!
Peer-to-peer Each station is a functional equal of every other station Any computer can access files from any other computer Any computer can be a server (i.e., take on special functions)
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LAN Hardware and Software
“LAN hardware and software are the concern of the two lowest layers of the
Open Systems Interconnection Model (OSI) and TCP/IP model architectures:
The two lower layers handle all the protocols and specifications needed to run the LAN
Higher layers get involved only when interconnecting LANs
Layer 1, the physical layer, and
Layer 2, the data link layer.”
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LAN Hardware and Software
Network Interface Card (NIC) Hardware/firmware combination containing almost all
of the LAN protocols Contains port(s) to accommodate medium
(e.g., CAT 6 copper, fiber, etc.) Can provide device LAN address Required for each node on the LAN; a node is a
device Directly connected to the LAN Directly addressable by the LAN
A device must have a NIC to be a LAN node
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LAN Hardware and Software
Medium Access Control (MAC) address Physical address—different for each NIC Defined (and assigned) by IEEE Hard-coded by manufacturer Flat addresses contain no location or sequencing 48 bits long
First 24 bits—IEEE Organizationally Unique Identifier (OUI) Second 24 bits—manufacturer ID
[224 = 16,777,216 addresses] Stored in read-only memory (ROM) on the NIC
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LAN Hardware and Software
Network operating system (NOS)
Mediates between LAN workstations LAN resources LAN processes
Computer operating system (OS) Mediates individual workstation
resources
Full-blown NOS:MS Windows Server Novell Netware
Partial NOS:(newer) WindowsMacUNIXLinux
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LAN Hardware and Software
NOS functions Contains redirector that determines whether actions
are local (for workstation) or network Incorporates LAN protocols Enables LAN software to use LAN hardware Controls server operations Manages network storage, disk access, and memory Provides LAN management tools for administrators
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Ethernet: The Once and Future King
LAN protocols are designed for best effort delivery Data frames have a “good chance” of surviving
Receiver determines whether a frame has errors
Higher-layer protocols might provide more precise error detection and recovery
LANs do not guarantee error-free delivery!
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Ethernet: The Once and Future King
Ethernet—802.3 Was not the first [Arcnet was first in 1977] Is currently the most widely installed Is considered a contention protocol
(stations contend for access) Uses Carrier Sense Multiple Access with
Collision Detection (CSMA/CD) Station desiring access must listen If a transmission is detected—carrier sensed—station waits If no transmission is detected—bus is idle—station transmits Simultaneous transmissions cause collisions
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Ethernet: The Once and Future King
Fig 9.1CSMA/CD
If a collisionis detected
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Ethernet: The Once and Future King
The Ethernet frame Max frame size =1,518 bytes [data =1,500 bytes] Min frame size = 64 bytes [data = 46 bytes]
5 data fields Destination address Source address Network protocol or data length (if < 1,518) Data PDU (higher layer data) Frame check sequence (error detection)
2 synchronization fields Preamble for frame synchronization Start frame delimiter indicating frame start for receiver
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Ethernet: The Once and Future King
The Ethernet frame Max frame size =1,518 bytes (data =1,500 bytes) Min frame size = 64 bytes (data = 46 bytes)
DataSynchronization
Fig 9.2
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Ethernet: The Once and Future King
Ethernet collision window (“slot time”) Length of time for frame to travel
from one end of the LAN to the other Requires frame limits to work (64 byte min frame size)
For 10 Mbps
Key factors Bit rate—time for a station to transmit a complete frame Propagation speed—time for 1 bit to travel to the end of the bus
512 bit times = 512 bits/8 bits per byte = 64 bytes
Max length = 500 m
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Improving Traditional Ethernet
Bus and hub comparison
Fig 9.3
bus hub
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Improving Traditional Ethernet
Bus and star cabling comparison (8 nodes)
Fig 9.4
bus
star
Node
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Ethernet: The Once and Future King
Thicknet 10BASE5
10 Mbps data rate Baseband signaling over thick coaxial copper Max segment length: 500 m Up to 100 nodes Up to 4 repeaters Physical bus Connected by medium attachment unit (MAU)
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Improving Traditional Ethernet
Thinnet 10Base2
10 Mbps data rate Baseband signaling over pencil-thin coaxial copper Max segment length: 185 m Up to 30 nodes Up to 4 repeaters Physical bus Connected by NICs (MAU function moved to NICs)
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Improving Traditional Ethernet
Ethernet (with media type indicators) 10BASE-T
10 Mbps data rate Baseband signaling over twisted pair copper Max segment length: 185 m Node limits dictated by ports available on hubs Hubs could be repeaters (“active hubs”) Physical star operating as a logical bus Connected by hubs
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Improving Traditional Ethernet
Ethernet (with media type indicators) Advantages
Reliability improved—bus disruptions don’t take down LAN Management improved—simple network management
protocol (SNMP) installed on hub Maintenance improved—easier to add workstations
Disadvantages Physical stars require more cabling Hub becomes single point of failure
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Improving Traditional Ethernet
Replacing the hub with a switch
How it works Switch connects workstations in pairs Will not connect transmitting stations to a busy one LAN no longer operates as a bus—no contention!
Advantages No collisions—each station has own link to switch Compatibility with CSMA/CD is maintained Multiple workstations can transmit simultaneously Simple to upgrade—replace hub with switch
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Improving Traditional Ethernet
Fast Ethernet 100BASE-TX
100 Mbps data rate Baseband signaling, cat 5 UTP Max segment length: 100 m (span limit: 250m) Node limits dictated by ports available on hubs Hubs could be repeaters (“active hubs”) Physical star operating as a logical bus Connected by switches
100BASE-FX is multimode fiber-optic version
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Improving Traditional Ethernet
Fast Ethernet 100BASE-TX
Fig 9.5
LAN side device side
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Improving Traditional Ethernet
Fast Ethernet 100BASE-T4
Fig 9.6
LAN side device side
Designed to run
on cat 3 UTP
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Improving Traditional Ethernet
Fast Ethernet (100BASE-TX) Advantages
Speed boost (10 Mbps to 100 Mbps) Backward compatible—10/100 Mbps on same LAN Easy device upgrade
Upgrade switch With CAT 5 UTP or STP, swap NICs
Disadvantages Maximum segment length is 100 m [total span limit: 250 m] Switch is single point of failure
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Improving Traditional Ethernet
Gigabit Ethernet 1000BASE-T
1000 Mbps data rate Baseband signaling over cat 5 UTP Max segment length: 100 m (span limit: 100 m) Min frame size: 512 bytes (up from 64 byte) Connected by switches
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Improving Traditional Ethernet
Gigabit Ethernet (other classifications) 1000BASE-X
1000BASE-CX Copper over twinax or quad cabling Max span: 25 m
1000BASE-LX Fiber-optic (1,300 nm signals) Max span: 550 m (multimode) Max span:3,000 m (single-mode)
1000BASE-SX Fiber-optic (850 nm signals) Max span: 550 m (multimode) Max span3,000 m (single-mode)
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Improving Traditional Ethernet
10 Gigabit Ethernet 10GBASE-X
10 Gbps data rate Full duplex signaling over fiber-optic media 7 versions
10GBASE-SR (short-range) and –SW (short-wavelength) 10GBASE-LR (long-range) and –LW (long-wavelength) 10GBASE-ER (extended-range) and –EW (extra-long
wavelength) 10GBASE-LX4 (carries signals on 4 light wavelengths)
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Token Ring
Token ring – 802.5 Patented by Olof Söderblom in the late 60s—licensed to IBM Practically no new installations Speeds typically 4/16 Mbps (100 Mbps standard exists) A round-robin protocol (stations take turns in order) Most commonly configured as a physical star/logical ring
Each station is connected to a multistation access unit (MAU) Logically, each station is connected point to point to
a predecessor node and a successor node A small packet (token) controls medium access A station can transmit data only when it has the token Only one token is in circulation at any time
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Token Ring
Fig 9.7
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Token Ring
The token ring frame Three frame types
Token frame Data frame Control frame
Data and control frames have the same format
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Token Ring
The token ring frame
Fig 9.8
data and control frames have the same format
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LAN Segmentation
LAN segmentation Goal
Reduce congestion by grouping stations according to traffic
Approach Segments include workstations that often communicate with
On another Common data source Common resource
Each segment becomes a LAN in itself Segments can later be interconnected to share resources
40 stations : 10 Mbps LAN 10 Mbps/40 = 250 Kbps(2) 20 stations: 10 Mbps LAN `10 Mbps/20 = 500 Kbps
Segmentation in action
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LAN Segmentation Bridge operation and bridge types
What is a bridge? A traffic monitor between two LANs A filter to keep local traffic from crossing between LANs A segment device that keeps local traffic off other LANs
Bridge address tables Track device addresses on both sides How they are created distinguishes types of bridges
Types of bridges Manual bridge—addresses are manually loaded into a table Learning bridge—automatically creates its own tables
Bridge can flood both sides of a LAN to learn what devices respond
Bridge can learn device addresses when new source addresses appear in frames
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LAN Segmentation
Using backbones to interconnect LANs Instead of directly connecting LANs and bridges,
all interLAN links traverse the backbone Backbones may be
Linked to LANs by bridges Based on routers LANs themselves
LAN stations connect to the backbone via their LAN hubs or switches
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LAN Segmentation
Bridged backbone Each server has
one port connection to the backbone bus
One port connection to its LAN switch
Bridge forwards only to the bus frames from its LAN destined for a nonlocal LAN
Fig 9.11
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LAN Segmentation
Star-wired (collapsed) backbone
Each LAN switch is connected to a router that sends frames according to frame destination addresses
Backbone is considered to be shrunk (collapsed) into the router itself
Fig 9.11
If the router failsthe backbone fails
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LAN Segmentation
Backbone LAN
Same as star-wired backbone except a LAN takes the place of a router
Each connected LAN becomes a node on the backbone LAN
Fig 9.12
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LAN Segmentation
FDDI (Fiber Distributed Data Interface)
Token-passing protocol 100 Mbps Station separation up to 2 Km (1.25 mi)
on single-mode fiber Originally used as MAN backbone Superseded by higher speed Ethernet
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LAN Segmentation
FDDI (Fiber Distributed Data Interface)
Fig 9.13
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VLANs
Virtual LAN (VLAN) 802.3ac Grouped by
Station characteristics Switch characteristics Frame protocols
Physical LAN memberships or links are not changed
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VLANs Virtual LAN (VLAN)
Benefits Security Traffic reduction Flexibility Cost savings
Caveats Ease in setup does not presume well-designed Be wary of too many members on too many physical LANs Stations with occasional communications should not be members
Problems Congestion Network management difficulty
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VLANs
Fig 9.14
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VLANs
Attribute-based VLANs Configured by creating list mappings (access lists) Switches discern which ports belong to which VLANs Membership can be assigned
Mostly manual Partly manual Mostly automatic
Protocol-based VLANs Membership determined on a frame-by-frame basis Participation based on individual transmissions
instead of port assignment
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VLANs
Tagged Ethernet Enables workstations to belong to several VLANS at same time First 20 bytes are same as Ethernet frame Four tag bytes are inserted between the source address and the
type/length field
Fig 9.15