Transcript Physical Carrier Sense Method

Multiplexing
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Break one high-speed physical communication circuit into several lowerspeed logical circuits
Allows many different devices to use the circuit simultaneously while it
seems that each pair of devices has the physical circuit all to itself
Usually done in multiples of 4
Two multiplexers are needed for a circuit – one to combine the original
circuits into a multiplexed circuit and one to separate them back out into
separate circuits
Four types of multiplexing
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Frequency division multiplexing (FDM)
Time division multiplexing (TDM)
Statistical time division multiplexing (STDM)
Wavelength division multiplexing (WDM)
Frequency Division Multiplexing
• Divides the circuit “horizontally” by assigning different
frequencies (channels) to each logical circuit
• All signals exist in the media at the same time on noninterfering frequencies
• Available bandwidth is divided into channels and
“guardbands” which separate the channels
• Channels do not need to have identical capacities
Time Division Multiplexing
• Divides the circuit “vertically” by allowing different
devices to transmit on the circuit at different times
• Devices take turns using the circuit to transmit data
• Time is allocated to each device in turn, even if the
device is idle
• More efficient than Frequency Division Multiplexing, as
there is no need for “guardbands”
• The full capacity of the line is divided evenly among the
multiplexed circuits
Statistical Time Division Multiplexing
• Takes advantage of the fact that not all devices will be transmitting
all the time
• Multiplexed circuit bandwidth is typically smaller than the combined
bandwidths of the individual circuits
• Circuit capacity requirements are determined statistically by
analyzing the usage of the circuits to be multiplexed
– Given four 64kbps circuits, TDM would require a 256kbps multiplex
circuit
– If we find that, statistically, only two of the four circuits are typically
active simultaneously, we can provision a 128kbps multiplex circuit
using STDM
– Provides more efficient use of bandwidth
• Can cause time delay if all circuits become active simultaneously
• Increased complexity and overhead, since each transmission must
include an indication of the circuit it belongs to
Wavelength Division Multiplexing
• A version of FDM used in fibre-optic cabling
• Originally, fibre-optic transmission used a single “color”
(light wavelength) for transmitting 622Mbps (622 million
bits per second)
• By attaching devices that transmit and detect different
light wavelengths, multiple circuits can be multiplexed
over a single fibre-optic cable
• Dense Wave-Division Multiplexing (adding TDM to
WDM) has increased the capacity of a single fibre-optic
cable to 400 Billion bits per second
• DWDM Experimental results over 1.5Tbps
– 1,500,000,000,000 bits per second
– Approximately 200 Gigabytes/second
Inverse Multiplexing (IMUX)
• Opposite of multiplexing
• Combines two or more low-speed circuits making them
appear as a single high-speed circuit
• Commonly used to provide T1 circuits in Wide Area
Networks
– Combines 24 low-speed (64kbps) circuits to create a single
1.544 Mbps circuit
• IMUX equipment not standardized, so the same vendor
should be used for both ends of the circuit
• BONDING (Bandwidth on Demand Interoperability
Networking Group) standards have been adopted by
some vendors for using 6 separate ISDN links over six
telephone lines for room-to-room teleconferencing
Digital Subscriber Line (DSL)
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Provides high-speed data transmission over traditional telephone lines
Limitations on traditional phone lines is based in the telephone and
switching equipment in the end offices
Actual cabling (local loop) is capable of much higher transmission capacity
Conversion from POTS to DSL involves changing end equipment, not
rewiring the local loop, making it very cost effective
The customer premises equipment (CPE) includes a line splitter, which
separates the traditional voice traffic from the data transmissions
The line splitter directs data transmissions into the DSL modem (also called
a DSL router) which is both a modem and a Frequency Division Multiplexer
At the telephone company end office, the local loop enters into the Main
Distribution Frame (MDF), which works like the line splitter on the customer
premises
The MDF sends voice traffic to the voice telephone network, and sends DSL
traffic to the DSL Access Multiplexer (DSLAM)
The DSLAM demultiplexes the data streams and converts them to ATM data
which can be distributed to the Internet Service Provider (ISP)
The ISP’s Point of Presence (POP) can be co-located or located elsewhere
Asymmetric Digital Subscriber Line (ADSL)
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Uses Frequency Division Multiplexing to create three separate channels
over the one local loop circuit
– One traditional voice circuit
– One relatively high-speed simplex circuit from the end office to the customer
premises
– One slightly slower half-duplex circuit, primarily intended for upstream traffic from
the customer premises to the end office
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It is called asymmetric because each of the two data circuits have different
capacities
Each of the two data channels can be further subdivided using Time
Division Multiplexing
The size of the two digital channels depends on the distance from the CPE
to the end office
– Shorter distance to End Office results in less attenuation of the signal, allowing
higher frequencies to be used and yielding a faster connection
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Offering higher speed ADSL limits the number of potential customers, while
offering lower speed decreases product attractiveness
Cable modems
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Digital service offered by cable companies
Data over Cable Service Interface Specification (DOCSIS) is the dominant standard
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Architecture is very similar to DSL
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Main difference is that DSL is point-to-point, while cable modems share multi-point circuits
Each user must compete with other users for the available capacity
All messages on the circuit go to all computers on the circuit
Cable TV circuit enters the customer premises through a cable splitter
TV signals are sent to the TV network
Data signals are sent to the cable modem (both a modem and an FDM)
Standard coaxial cable circuit may be shared by from 300 to 1000 customers
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Standard most often used by cable companies running hybrid fibre coax (HFC) networks
Not all of these cable TV customers will subscribe to high-speed internet
The coax runs to a fibre node with an optical-electrical converter
Each fibre node may service up to half a dozen cable circiuts
The fibre nodes are connected to a distribution hub (also called a headend) through
two separate circuits
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Upstream circuit connects to a cable modem termination system (CMTS)
Downstream circuit carries both ordinary cable TV signal and downstream traffic destined for
the customer premises
Local Area Networks
• Introduction
– Why use a LAN? Dedicated servers vs. Peer-to-peer LANs
• LAN Components
– NICs, Cables, Hubs and Network Operating Systems
• Traditional Ethernet (IEEE 802.3)
– Topology, Media Access Control, Ethernet Types
• Switched Ethernet
– Topology, Media Access Control, Performance Benefits
• Wireless LANs (IEEE 802.11)
– Topology, Media Access Control, Wireless Ethernet Types
• Other Wireless Technologies
– Infrared Wireless, Bluetooth
• Improving LAN Performance
– Improving Server Performance, Improving Circuit Capacity,
Reducing Network Demand
Why use a LAN?
• There are two main benefits to using a local area
network: information sharing and resource sharing.
• Examples of information sharing include file sharing,
exchanging e-mail, and using the Internet.
• Examples of resource sharing include sharing hardware
and software, such as sharing an expensive printer.
• Another important resource sharing technique is to
purchase software on a per seat basis. For example,
only purchasing a 10-seat license for a software program
on a 20 client network instead of purchasing 20 copies of
the same program.
Dedicated Server Networks
• A basic LAN dichotomy exists between dedicated server
LANs and peer-to-peer LANs which don’t have servers.
Since 90% of all LANs have a dedicated server, this
chapter mostly focuses on server-based LANs.
• A dedicated server is a computer that is permanently
assigned a specific server task such as being a Web
server, e-mail server, file server or printer server.
• Servers also run a special operating system called a
server network operating system.
• When many servers are part of a network, it can be
referred to as a server farm.
Peer-to-Peer Networks
• Peer-to-peer networks do not use dedicated servers.
• Any computer on a peer-to-peer network can act as both
a client, accessing resources or information on other
computers on the network, or as a server, allowing
access to attached information or resources.
• Peer-to-peer networks tend to be small networks.
• The main advantage of peer-to-peer networking is lower
cost since there is no dedicated server, generally the
most expensive network component.
• The main disadvantage is that peer-to-peer networks are
generally slower than dedicated server networks, since
each computer is less powerful and may be in use as a
client and a server at the same time.
Basic LAN Components
• The six basic LAN components are:
1. Clients
2. Servers
3. Network Interface Cards
4. Network Cables
5. Hubs and Switches
6. Network Operating System
Basic LAN Components
Network Interface Cards
• Network interface cards, also called network
cards and network adapters include a cable
socket allowing computers to be connected to
the network.
• NICs are part of both the physical and data link
layer and include a unique data link layer
address (sometimes called a MAC address),
placed in them by their manufacturer.
• Before sending data onto the network, the
network card also organizes data into frames
and then sends them out on the network.
• Notebook computers often use NICs that are
plugged into the PCMCIA port.
Network Cables
• Each computer is physically connected to the
network using a cable.
• These cables are either untwisted wire pairs
(UTP, the most common choice), shielded
twisted pair (STP), coaxial cable, or optical
fiber.
• Wireless LANs use radio frequencies or
infrared light instead of cables.
• Sometimes two different types of cabling can
be linked using a special connector. A BALUN
(Balanced-Unbalanced) is one such device
that connects UTP and Coaxial Cable.
Hubs
• Hubs act as junction boxes, linking cables
from several computers on a network. Hubs
are usually sold with 4, 8, 16 or 24 ports.
• Some hubs allow connection of more than
one kind of cabling, such as UTP and coax.
• Hubs also repeat (reconstruct and
strengthen) incoming signals. This is
important since all signals become weaker
with distance.
• The maximum LAN segment distance for a
cable can therefore be extended using hubs.
Network Hub
Network Operating Systems
• The NOS is the software that runs the
LAN. It comes in two types: Server
NOSs & Client NOSs.
• Server NOSs enable server to execute
and respond to the requests sent to
them as web server, print servers, file
servers, etc.
• Client NOS functions are typically
included in most OS packages such as
Windows 98 and Windows 2000.
Network Profiles
• The network profile specifies what
resources on each server are available to the
network for use by other computers, including
data files, printers, etc.
• Devices that are not included in the network
profile can not be used over the network.
• User profiles describe what each user on a
LAN has access to.
• Most LANs also use auditing software which
keeps track of which user has accessed what
network resource.
Ethernet (IEEE 802.3)
• Almost all LANs today use Ethernet
• Originally, Ethernet was jointly developed
by a consortium of Digital Equipment
Corp., Intel and Xerox and was
standardized as IEEE 802.3.
• Ethernet LANs that use hubs are
sometimes called shared Ethernet.
Shared Ethernet Topology
• Ethernet’s logical topology is a bus topology.
• This means all computers on the network receive
messages from all other computers, whether the
message is intended for those computers or not.
• When a frame is received by a computer, the first task
is to read the frame’s destination address to see if the
message is meant for it or not.
• Although, a decade ago most Ethernet LANs used a
physical bus, almost all Ethernets today use a physical
star topology, with the network’s computers linked into
hubs.
• It is also common to link use multiple hubs to form more
complex physical topologies
Ethernet Topology
Multiple Hub Ethernet Design
Media Access Control
• Ethernet’s medium access control protocol,
called CSMA/CD, is contention-based
• With a contention-based protocol, frames can
be sent by two computers on the same
network at the same time, in which case they
will collide and become garbled.
• CSMA/CD, can thus be termed “ordered
chaos” because it tolerates, rather than
avoids, collisions caused by two computers
transmitting at the same time.
CSMA/CD
• Stands for: Carrier Sense Multiple Access
w/ Collision Detect
• Carrier Sense: computers listen to the network to
see if another computer is transmitting before
sending anything themselves.
• Multiple Access: all computers have access to
the network medium.
• Collision Detect: if they detect a collision (CD),
they then wait a random amount of time and
resend the frame (It has to be random in order to
avoid another collision).
Ethernet Physical Media Standards
• Ethernet Media are formatted as follows:
[Value1]Base/Broad[Value2]
• Value 1: Data Rate for Medium 10 = 10Mbps
• Base or Broad
– Base = Baseband Mode meaning only one
(digital) channel
– Broad = Broadband (analog) cable transmissions
use more than one channel (e.g., cable TV)
• Value2: (relates to maximum distance possible in
hundreds of meters or cable type T= twisted pair, F
=fiber)
Types of Ethernet
• Seven types of shared Ethernet have been in use:
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10Base5 = thick Ethernet, uses thick coax. This is the original Ethernet
specification. Now uncommon.
10Base2 = thin Ethernet, uses thin coax. Became popular in the early
1990s as a cheaper alternative to 10Base5. Now uncommon.
10BaseT = twisted pair Ethernet, most common type of Ethernet. Uses Cat
3 and Cat 5 UTP. Common but rapidly losing ground to 100BaseT.
100BaseT = also called Fast Ethernet, has replaced 10BaseT in sales
volume. Uses Cat 5 UTP (Sometimes combined 10/100 Ethernet is found in
which some segments run 10BaseT and some run 100BaseT is also used
by some organizations).
1000BaseT = Gigabit Ethernet. Maximum cable length is only 100 meters.
10GbE = 10 Gbps Ethernet. Uses fiber and is typically full duplex.
40GbE = 40 Gbps Ethernet. Uses fiber and is typically full duplex.
Name
Maximum Data
Rate
Cables
10Base5
10 Mbps
Coaxial
10Base2
10 Mbps
Coaxial
10BaseT
10 Mbps
UTP cat 3, UTP cat 5
100BaseT
100 Mbps
UTP cat 5, fiber
1000BaseX
1 Gbps
UTP cat 5, UTP cat 5e,
UTP cat 6, fiber
10 GbE
10 Gbps
UTP cat 5e, UTP cat 6, UTP
cat 7, fiber
40 GbE
40 Gbps
fiber
Types of Ethernet
Switched Ethernet Topology
• Switched Ethernet uses switches instead of
hubs.
• While a hub broadcasts frames to all ports,
the switch reads the destination address of
the frame and only sends it to the
corresponding port.
• The effect is to turn the network into a group
of point-to-point circuits and to change the
logical topology of the network from a bus to
a star.
Basic Switch Operation
• Switches make forwarding decisions based on forwarding tables
(similar to routing tables).
• When a frame is received, the switch reads its [data link layer]
destination address and sends the frame out the corresponding
port in its forwarding table.
• Switches making switching decisions based on data link layer
addresses are called layer-2 switches.
• When a switch is first turned on, its forwarding table is empty. It
then learns which ports correspond to which computers by reading
the source addresses of the incoming frames along with the port
number that the frame arrived on.
• If the switch’s forwarding table does not have the destination
address of the frame, it broadcasts the frame to all ports.
• Thus, a switch starts by working like a hub and then works more
and more as a switch as it fills its forwarding table.
Media Access Control
• Switched Ethernet still uses CSMA/CD media access
control, but collisions are less likely as each network
segment operates independently.
• The network’s modified topology also allows multiple
messages to be sent at one time.
• For example, computer A can send a message to
computer B at the same time that computer C sends
one to computer D.
• If two computers send frames to the same destination
at the same time, the switch stores the second frame
in memory until it has finished sending the first, then
forwards the second.
802.3 Ethernet versus switched Ethernet
Performance Benefits
• Switched Ethernet can dramatically improve
network performance.
• Shared Ethernet 10BaseT networks are only
capable of using about 50% of capacity
before collisions are a problem
• Switched Ethernet, however, runs at up to
95% capacity on 10BaseT.
• Another performance improvement can be
made by using a 10/100 switch that uses a
100BaseT connection for the server(s) and/or
routers, i.e., the network segments
experiencing the highest volume of LAN
traffic.
Wireless Ethernet (IEEE 802.11)
• Wireless LANs dispense with cables
and use radio or infrared frequencies to
transmit signals through the air.
• WLANs are growing in popularity
because they eliminate cabling and
facilitate network access from a variety
of locations and for mobile workers (as
in a hospital).
• The most common wireless networking
standard is IEEE 802.11, often called
Wireless Ethernet or Wireless LAN.
Wireless LAN Topology
• WLAN topologies are the same as on Ethernet: physical star,
logical bus
• Wireless LAN devices use the same radio frequencies, so they
must take turns using the network.
• Instead of hubs, WLANs use devices called access points
(AP). Maximum transmission range is about 100-500 feet.
Usually a set of APs are installed making wireless access
possible in several areas in a building or corporate campus.
• Each WLAN computer uses an NIC that transmits radio signals
to the AP.
• Because of the ease of access, security is a potential problem,
so IEEE 802.11 uses 40-bit data encryption to prevent
eavesdropping.
A wireless Ethernet access point
connected into an Ethernet Switch.
WLAN Media Access Control
• Wireless LANs use CSMA/CA where CA =
collision avoidance (CA). With CA, a station
waits until another station is finished
transmitting plus an additional random period
of time before sending anything.
• Two different WLAN MAC techniques are
now in use: the Physical Carrier Sense
Method and the Virtual Carrier Sense
Method.
Physical Carrier Sense Method
• In the physical carrier sense method, a node
that wants to send first listens to make sure that
the transmitting node has finished, then waits a
period of time longer.
• Each frame is sent using the Stop and Wait ARQ,
so by waiting, the listening node can detect that
the sending node has finished and can then
begin sending its transmission.
• With Wireless LANs, ACK/NAK signals are sent a
short time after a frame is received, while
stations wishing to send a frame wait a
somewhat longer time, ensuring that no collision
will occur.
Virtual Carrier Sense Method
• When a computer on a Wireless LAN is near the transmission
limits of the AP at one end and another computer is near the
transmission limits at the other end of the AP’s range, both
computers may be able to transmit to the AP, but can not detect
each other’s signals.
• This is known as the hidden node problem. When it occurs,
the physical carrier sense method will not work.
• The virtual carrier sense method solves this problem by
having a transmitting station first send a request to send (RTS)
signal to the AP. If the AP responds with a clear to send (CTS)
signal, the computer wishing to send a frame can then begin
transmitting.
Types of Wireless Ethernet
• Two forms of the IEEE 802.11b standard currently
exist, utilizing the 2.5 GHz band:
– Direct Sequence Spread Spectrum (DSSS) uses the entire
frequency band to transmit information. DSSS is capable of
data rates of up to 11 Mbps with fallback rates of 5.5, 2 and 1
Mbps. Lower rates are used when interference or congestion
occurs.
– Frequency Hopping Spread Spectrum (FHSS) divides the
frequency band into a series of channels and then changes
its frequency channel about every half a second, using a
pseudorandom sequence. FHSS is more secure, but is only
capable of data rates of 1 or 2 Mbps.
• IEEE 802.11a uses Orthogonal Frequency Division
Multiplexing (OFDM), operates in the 5 GHz band
with data rates of up to 54 Mbps.
• IEEE 802.11g uses OFDM in the 2.5 GHz band,
operates at up to 54 Mbps, and is compatible with
802.11b
Infrared Wireless LANs
• Infrared WLANs are less flexible than IEEE
802.11 WLANs because, as with TV remote
controls that are also infrared based, they require
line of sight to work.
• Infrared Hubs and NICs are usually mounted in
fixed positions to ensure they will hit their targets.
• The main advantage of infrared WLANs is
reduced wiring.
• A new version, called diffuse infrared, operates
without a direct line of sight by bouncing the
infrared signal off of walls, but is only able to
operate within a single room and at distances of
only about 50-75 feet.
Infrared Wireless LAN
Bluetooth
• Bluetooth is a 1 Mbps wireless standard developed
for piconets, small personal or home networks.
• It may soon be standardized as IEEE 802.15.
• Although Bluetooth uses the same 2.4 GHz band as
Wireless LANs it is not compatible with the IEEE
802.11 standard and so can not be used in locations
that use the Wireless LANs.
• Bluetooth’s controlled MAC technique uses a master
device that polls up to 8 “slave” devices.
• Examples of Bluetooth applications include; linking a
wireless mouse, a telephone headset, or a Palm
handheld computer to a home network.
Improving LAN Performance
• As networks become more and more intensively
used, LAN performance becomes a critical issue.
• The measure of LAN Performance is throughput,
i.e., the total amount of user data transmitted in a
given period of time.
• LAN performance can be improved by identifying and
eliminating bottlenecks, that is, points in the network
where congestion is occurring because the network
or device can’t handle all of the demand it is
experiencing.
Identifying Network Bottlenecks
• Two common network bottlenecks are related
to server access:
• If server performance is poor when server
utilization is high (>60%), then the bottleneck
is the server.
• If server performance is poor during periods
of low server utilization (<40%), then the
bottleneck is not the server but the network
circuit.
Improving Server Performance
• Two types of server performance
improvements are possible:
– Software improvements such as choosing
a faster Network Operating System, fine
tuning network and NOS parameters for
optimal performance.
– Hardware improvements such as adding a
second server, upgrading the server’s
CPU, increasing its memory space, adding
more hard drives or adding a second NIC
to the server.
Improving Server Performance: RAID
• Improving disk drive performance is
especially important, since disk reads are the
slowest task the server needs to do.
• Replacing one large drive with many small
ones can improve server performance.
• RAID or Redundant Array of Inexpensive
Disks, builds on this idea. RAID system can
be used to both improve performance and
increase reliability by building redundancy
into the hard drives, so that a hard drive
failure does not result in any loss of data.
Improving Circuit Capacity
• Improving circuit capacity can be done simply by
upgrading one or all segments of a network to a
faster protocol (which also means upgrading the
NICs), such as;
– Upgrading the network from 10BaseT to 100BaseT, or
– Upgrading the network segment to the server from 10BaseT
to 100BaseT
• Another approach to improving circuit capacity is by
increasing the number of network segments to the
server. Most servers can handle several network
segments simply by adding additional NIC cards,
thereby increasing access to the server
Network Segmentation: a. Before b. After
Reducing Network Demand
• Performance can also be improved by reducing
network demand. This can be done by:
– Moving more files, such as heavily used software packages
to client computers.
– Disk caching, software on client machines can also reduce
server demand.
– Moving user demands from peak times to off peak times, by
telling network users when peak usage times occur and
encouraging users to not use the network as heavily during
these times can also help improve performance.
– Delaying some network intensive jobs to off-peak times,
such as running heavy printing jobs at night, can also
improve performance.
Improving LAN performance
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Increase Server Performance
– Software: Fine-tune the NOS settings
– Hardware:
• Add more servers and spread the network applications across
the servers to balance the load
• Upgrade to a faster computer
• Increase the server's memory
• Increase the number and speed of the server's hard disk(s)
• Upgrade to a faster NIC
Increase Circuit Capacity
– Upgrade to a faster circuit
– Segment the network
Reduce Network Demand
– Move files from the server to the client computers
– Increase the use of disk caching on client computers
– Change user behavior