Chapter 9: Local Area Networks Principles of Computer Networks and Communications M. Barry Dumas and...

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

Principles of Computer Networks and Communications

2Chapter 9

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


3Chapter 9

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.”


4Chapter 9

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)


5Chapter 9

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.”


6Chapter 9


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


7Chapter 9


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


8Chapter 9


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


9Chapter 9


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


10Chapter 9

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!


11Chapter 9


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


12Chapter 9


Fig 9.1CSMA/CD

If a collisionis detected


13Chapter 9


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


14Chapter 9


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


15Chapter 9


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


16Chapter 9

Improving Traditional Ethernet

Bus and hub comparison

Fig 9.3

bus hub


17Chapter 9


Bus and star cabling comparison (8 nodes)

Fig 9.4

bus

star

Node


18Chapter 9


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)


19Chapter 9


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)


20Chapter 9


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


21Chapter 9


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


22Chapter 9


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


23Chapter 9


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


24Chapter 9


Fast Ethernet 100BASE-TX

Fig 9.5

LAN side device side


25Chapter 9


Fast Ethernet 100BASE-T4

Fig 9.6

LAN side device side

Designed to run

on cat 3 UTP


26Chapter 9


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


27Chapter 9


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


28Chapter 9


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)


29Chapter 9


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)


30Chapter 9

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


31Chapter 9

Token Ring

Fig 9.7


32Chapter 9

Token Ring

The token ring frame Three frame types

Token frame Data frame Control frame

Data and control frames have the same format


33Chapter 9

Token Ring

The token ring frame

Fig 9.8

data and control frames have the same format


34Chapter 9

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


35Chapter 9

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


36Chapter 9

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


37Chapter 9

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


38Chapter 9

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


39Chapter 9

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


40Chapter 9

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


41Chapter 9

LAN Segmentation

FDDI (Fiber Distributed Data Interface)

Fig 9.13


42Chapter 9

VLANs

Virtual LAN (VLAN) 802.3ac Grouped by

Station characteristics Switch characteristics Frame protocols

Physical LAN memberships or links are not changed


43Chapter 9

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


44Chapter 9

VLANs

Fig 9.14


45Chapter 9

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


46Chapter 9

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

Chapter 9: Local Area Networks Principles of Computer Networks and Communications M. Barry Dumas and...

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