Computer Network

Andrew S. Tanenbaum
• The mobile telephone system
• Cable television
• Wireless LANS
• Broadband wireless
• Bluetooth
• Data Link layer switching
• Quality of service
• The mobile telephone system
• Cable television
• Wireless LANS
• Broadband wireless
• Bluetooth
• Data Link layer switching
• Quality of service
Wireless telephones
• Cordless phones
-Never used for networking
• Mobile phones – through three
generations with different
-Analog voice
-Digital voice
-Digital voice and data
The mobile telephone
• First-generation mobile phones: analog voice
• Second-generation mobile phones: digital voice
• Third-generation mobile phones: digital voice and data
• 2.5G scheme
First-generation mobile
phones: analog voice
• 1946 push-to-talk system
-A single channel for both sending and
• 1960 IMTS (Improved Mobile Telephone
-High-powered transmitter
-Two frequencies (sending/receiving)
-23 channels spread out from 150 MHz to
450 MHz
IMTS drawbacks
• Due to the small number of channels,
users often had to wait a long time
before getting a dial tone.
• Due to the lager power of the hilltop
transmitter, adjacent systems had to
be several hundred kilometers apart
to avoid interference.
AMTS (Advanced Mobile
Phone System)
• In all mobile phone systems, a
geographic region is divided up into
• Key idea:
-increases the system capacity
(reuse of transmission frequencies)
-less power is needed.
The idea of frequency reuse
MTSO (Mobile Telephone
Switching Office)
• Handoff
- soft handoff
-no loss of continuity
-telephone needs to be able to
tune to two frequencies at the
same time
- hard handoff
AMTS Channels
• The AMPS system uses 832 full-duplex
channels, each consisting of a pair of
simplex channels
-832 simplex transmission channels
from 824 to 849 MHz
-832 simplex receive channels
from 869 to 894 MHz
• AMPS uses FDM to separate the channels
AMPS call management
• When a phone switch on
• When a caller makes a call
Second-generation mobile
phones: digital voice
• D-AMPS is fully digital
• D-AMPS is designed to co-exist with
• Upstream channels are in the
1850-1910 MHz
• Downstream channels are in the
1930-1990 MHz
• The voice signal is digitized and
• Users can share a single frequency
pair using TDM
A D-AMPS channel with
Difference between AMPS
and D-AMPS
• How handoff is handled
GSM (The Global System for
Mobile Communications)
• GSM versus D-AMPS:
-FDM is used with each mobile
transmitting on one frequency
receiving on a higher frequency
-A single frequency pair is split by
TDM into time slot shared by multiple
-GSM has a much higher data rate
per user than D-AMPS
GSM uses 124 frequency
channels, each of which uses
an eight-slot TDM system
A partition of the GSM
framing structure
CDMA (Code Division
Multiple Access)
• D-AMPS , GSM use both FDM and
• CDMA allows each station to transmit
over the entire frequency spectrum
all the time.
• Multiple simultaneous transmissions
are separated using coding theory.
CDMA coding theory
• Each bit time is subdivided into m short
intervals called chips (There are 64 or 128
chips per bit).
• Each station is assigned a unique
m-bit code called a chip sequence.
- To transmit 1 bit , a station sends
its chip sequence
- To transmit 0 bit , a station sends
the one’s complement of its chip
Example (1/2)
Example (2/2)
Third-generation mobile
phones: digital voice and data
• WCDMA (Wideband CDMA)
-uses direct sequence spread spectrum
-runs in a 5 MHz bandwidth
-has been designed to interwork with GSM
• CDMA2000
-not be designed to interwork with GSM
-has the differences between WCDMA :
chip rate, frame time, spectrum used, the
way to do time synchronization
2.5 G schema GPRS (General
Packet Radio Service)
• Is an overlay packet network on top
of D-AMPS or GSM.
• Allows mobile stations to send and
receive IP packets in a cell running a
voice system
• The mobile telephone system
• Cable television
• Wireless LANS
• Broadband wireless
• Bluetooth
• Data Link layer switching
• Quality of service
Cable television
• Community antenna television
• Internet over cable
• Spectrum allocation
• Cable modems
• ADSL versus cable
An early cable television
Part of a modern HFC
The fixed telephone system
Spectrum allocation
Cable modems
• Internet access requires a cable
• Cable modem is always on
• Cable operators do not charge for
connect time
What happens when a cable
modem is plugged in and
powered up? (1/2)
• The modem scans the downstream
channels looking for a special packet
periodically put out by the headend to
provide system parameters to modems.
• Modem announces its presence on one of
the upstream channels
• The headend responds by assigning the
modem to its upstream and downstream
What happens when a cable
modem is plugged in and
powered up? (2/2)
• The modem determines its distance
from the headend –ranging
• During initialization, the headend
also assigns each modem to a
minislot to use for requesting
upstream bandwidth
• What happens when a computer
wants to send a packet?
Typical details of the upstream
and downstream channels
ADSL versus cable
• Both use fiber in the backbone, but
they differ on the edge
• The increasing numbers have
different effects on existing users on
the two system
• Availability and security and reliability
are issues on which ADSL and cable
• The mobile telephone system
• Cable television
• Wireless LANS
• Broadband wireless
• Bluetooth
• Data Link layer switching
• Quality of service
Wireless LANS
• The 802.11
• The 802.11
• The 802.11
• The 802.11
• services
protocol stack
physical layer
MAC sublayer protocol
frame structure
The 802.11 protocol stack
• MAC sublayer determines how the channel
is allocated, that is, who gets to transmit
• LLC sublayer hides the differences
between the different 802 variants
• 802.11 specifies three transmission
techniques allowed in the physical layer
-infrared method
-short-range radio (FHSS/DSSS)
Part of the 802.11
protocol stack
The 802.11
physical layer(1/7)
• Infrared option
-uses diffused transmission at 0.85
or 0.95 microns
-two speeds are permitted: 1 Mbps,
-infrared signals can’t penetrate walls
The 802.11
physical layer(2/7)
• FHSS (Frequency Hopping Spread
-uses 79 channels, each 1 MHz wide,
starting at the low end of the
2.4GHz ISM band
-A pseudorandom number generator
is used to produce the sequence of
frequencies hopped to
The 802.11
physical layer(3/7)
-The amount of time spent at each
frequency—dwell time
1.a fair way to allocate spectrum
3.good resistance to multipath fading
4.relatively insensitive to radio
-disadvantage: low bandwidth
The 802.11
physical layer(4/7)
• DSSS (Direct Sequence Spread
-restricts to 1 or 2 Mbps
-has some similarities to the CDMA
-each bit is transmitted at 11 chips,
using Barker sequence
-uses phase shift modulation
The 802.11
physical layer(5/7)
• High-speed wireless LANs, 802.11a, uses
OFDM (Orthogonal Frequency Division
-deliver up to 54 Mbps in the wider
5GHz ISM band
1.good immunity to multipath fading
2.using noncontiguous bands (good
spectrum efficiency)
The 802.11
physical layer(6/7)
• 802.11b uses HR-DSSS (High Rate
Direct Sequence Spread Spectrum)
-uses 11 million chips/sec to achieve
11Mbps in the 2.4GHz band
-data rate 1,2 Mbps use phase shift
modulation (compatibility with DSSS)
-data rate 5.5,11Mbps use
Walsh/Hadamard codes
The 802.11
physical layer(7/7)
• Although 802.11b is slower than
802.11a, its range is about 7 times
• 802.11g uses OFDM modulation of
802.11a, but operates in the narrow
2.4GHz ISM band along with 802.11b
The 802.11 MAC
sublayer protocol
• The 802.11 MAC sublayer protocol is
quite different from that of Ethernet
due to the inherent complexity of the
wireless environment compared to
that of a wired system
Two Problems
802.11 supports two modes of
operation to deal with the problem
• DCF (Distributed Coordination
-uses a protocol CSMA-CA
1.physical channel sensing
2.virtual channel sensing (based on
• PCF (Point Coordination Function)
The use of virtual channel
sensing using CSMA/CA
Fragment frame
• The probability of a frame making it
through successfully decreases with
frame length
• To deal with the problem of noisy
channels, 802.11 allows frames to be
fragmented into smaller pieces, each
with its own checksum
A fragment burst
• The base station polls the other
stations, asking them if they have
any frames to send
• Transmission order is completely
controlled by the base station in PCF
mode, no collisions ever occur
• The basic mechanism is for the base
station to broadcast a beacon frame
Four different intervals are defined,
each for a specific purpose
The 802.11 frame structure
Services (1/2)
• Distribution services-manage cell
membership and interact with
stations outside the cell
Services (2/2)
• Station services- activity within a
single cell
3.privacy delivery
• The mobile telephone system
• Cable television
• Wireless LANS
• Broadband wireless
• Bluetooth
• Data Link layer switching
• Quality of service
Broadband wireless
• Comparison of 802.11 with 802.16
• The 802.16 protocol stack
• The 802.16 physical layer
• The 802.16 MAC sublayer protocol
• The 802.16 frame structure
Comparison of 802.11
with 802.16 (1/2)
• 802.11 and 802.16 are very different as
they try to solve different problems
-similar :they were designed to provide
high-bandwidth wireless communications
-differ:802.16 provides service to buildings
1.buildings are not mobile
2.buildings can have more than one
computer in them
Comparison of 802.11
with 802.16 (2/2)
802.16(wireless MAN) properties:
-Because of distances, the perceived power at the
base station vary widely from station to station
(affects the signal-to-noise ratio)
-802.16 operate in the much higher 10-to-66 GHz
frequency range
-These millimeter waves have different physical
properties than the longer waves in the ISM
bands (requires a completely different physical
-802.16 provide QoS.
The 802.16 protocol stack
The 802.16 physical layer
• Because signal strength in the millimeter
band falls off sharply with distance from
the base station, the signal-to-noise ratio
also drops with distance from the base
• 802.16 employs three different modulation
schemes, depending on how far the
subscriber station is from the base station
-the farther the subscriber is from the base
station, the low the data rate
802.16 transmission
802.16 provides a more flexible
way to allocate the bandwidth
• Two schemes:
-FDD (Frequency Division Duplexing)
-TDD (Time Division Duplexing)
The 802.16 MAC
sublayer protocol
• Downstream and upstream maps
-tell what is in which time slot and which
time slots are free
• Downstream channel
-base station simply decides what to put in
which subframe
• Upstream channel
-there are competing uncoordinated
subscribers that need access to it
-Its allocation is tied closely to the QoS
Four classes of service
• Constant bit rate service
• Real-time variable bit rate service
• Non-real-time variable bit rate
• Best-efforts service
The 802.16 frame structure
• The mobile telephone system
• Cable television
• Wireless LANS
• Broadband wireless
• Bluetooth
• Data Link layer switching
• Quality of service
• Bluetooth architecture
• Bluetooth application
• The Bluetooth protocol stack
• The Bluetooth radio layer
• The Bluetooth baseband layer
• The Bluetooth L2CAP layer
• The Bluetooth frame structure
• A wireless standard for interconnecting
computing and communication devices
and accessories using short-range, lowpower, inexpensive wireless radios
• Bluetooth specification is for a complete
system, from the physical layer to the
application layer
Bluetooth architecture
• The basic unit of a Bluetooth system
is a piconet, which consists of a
master node and up to seven active
slave nodes within a distance of 10
• An interconnected collection of
piconets is called a scatternet
Two piconets can be connected
to form a scatternet
Master/slave design
• The reason is that the designers intended
to facilitate the implementation of
complete Bluetooth chips for under $5
• Slaves are fairly dumb, doing whatever
the master tells them to do
• A piconet is a centralized TDM system,
with master controlling the clock and
determining which device gets to
communicate in which time slot
The Bluetooth application
The Bluetooth protocol stack
The Bluetooth
radio layer (1/2)
• The radio layer moves the bits from
master to slave, or vice versa
• Bluetooth is a low-power system with a
range of 10 meters operating in the 2.4GHz ISM band
• The band is divided into 79 channels of
1MHz each
• To allocate the channels fairly, frequency
hopping spread spectrum is used with
1600 hops/sec and a dwell time of
The Bluetooth
radio layer (2/2)
• All the nodes in a piconet hop
simultaneously, with the master
dictating the hop sequence
• 802.11 and Bluetooth operate in the
2.4GHz ISM band on the same 79
channels, they interfere with each
The Bluetooth
baseband layer (1/2)
• The baseband layer turns the raw bit
stream into frames and defines some
key formats
• Longer frames are much more
efficient then single-slot frames
• Each frame is transmitted over a
logical channel, called a link,
between the master and a slave
The Bluetooth
baseband layer (2/2)
• Two kinds of links :
-ACL (Asynchronous Connection-Less)
1.It is used for packet-switched
data available at irregular intervals
2.traffic is delivered on a bestefforts basis
-SCO (Synchronous Connection Oriented)
1.It is used for real-time data
2.the type of channel is allocated a fixed slot in
each direction
The Bluetooth L2CAP layer
• It accepts packets of up to 64KB
from the upper layers and breaks
them into frames for transmission
• It handles the multiplexing and
demultiplexing of multiple packet
• It handles the quality of service
The Bluetooth frame
• The mobile telephone system
• Cable television
• Wireless LANS
• Broadband wireless
• Bluetooth
• Data Link layer switching
• Quality of service
Data link layer switching
• Bridges from 802.x to 802.y
• Local internetworking
• Spanning tree bridges
• Remote bridges
• Repeaters, hubs, bridges, switches,
routers, gateways
• Virtual LANs
Why a single organization may
end up with multiple LANs?(1/2)
• The goal of the various of departments
differ, different departments choose
different LANs.
• The organization may be geographically
spread over several buildings separates by
considerable distances.
• It may be necessary to split what is
logically a single LAN into separate LANs
to accommodate the load.
Multiple LANs connected by
bridges are used
Why a single organization may
end up with multiple LANs?(2/2)
• A single LAN would be adequate in
terms of load, but the physical
distance between the most distant
machines is too great
• There is the matter of reliability
• Bridges can contribute to the
organization’s security
Operation of a LAN bridge
from 802.11 to 802.3
some difficulties when trying to
build a bridge between the various
802 LANs
• Each of the LANs uses a different
frame format
• Interconnected LANs don’t
necessarily run at the same data rate
• Different 802 LANs have different
maximum frame lengths
• Another problems are security and
quality of service
The IEEE 802 frame formats
Local internetworking
• The bridges should be completely
• When a frame arrives, a bridge must
decide whether to discard or forward it
• The decision is made by looking up the
destination address in a hash table inside
the bridge
• The algorithm used by the transparent
bridges is backward learning
The routing procedure for
an incoming frame
• If destination and source LANs are
the same, discard the frame
• If the destination and source LANs
are different, forward the frame
• If the destination LAN is unknown,
use flooding
A configuration with four
LANs and two bridges
Two parallel
transparent bridges
Spanning tree bridges
• To increase reliability, some sites use two
or more bridges in parallel between pairs
of LANs, however, introduce some
problems because it creates loops in the
• The solution to this difficulty is for the
bridges to communicate with each other
and overlay the actual topology with a
spanning tree that reached every LAN
Remote bridges can be used
to interconnect distant LANs
Devices operate in
different layers
• There are analog devices that are
connected to two cable segments
• A signal appearing on one of them is
amplified and put out on the other
• Repeaters understand volts
• Frames arriving on any of the lines
are sent out on all the others
• If two frames arrive at the same
time, they will collide
• A bridge connects two or more LANs
• When a frame arrives, software in
the bridge extracts the destination
address from the frame header and
looks it up in a table to see where to
send the frame
• Switches are similar to bridges in
that both route on frame addresses
• The main difference is that a switch
is most often to connect individual
• Since each port is its own collision
domain, switches never lose frames
to collisions
• When a packet comes into a router,
the frame header and trailer are
stripped off and the packet located in
the frame’s payload field is passed to
the routing software
• The software uses the packet header
to choose an output line
Transport gateways
• These connect two computers that
use different connection-oriented
transport protocols
Application gateways
• Application gateways understand the
format and contents of the data and
translate messages from one format
to another
A building with centralized
wiring using hubs and a switch
Virtual LANs
• With hubbed (switched) Ethernet, it
was often possible to configure LANS
logically rather than physically
Why use virtual LANs?
• Security
• Load
• Broadcasting
• In response to user requests for
more flexibility-rewire building
entirely in software
• VLANs are based on speciallydesigned VLAN-aware switches
• To make the VLANs function correctly,
configuration tables have to be set
up in the bridges or switches
• These tables tell which VLANs are
accessible via which ports (lines)
The IEEE 802.1Q
standard (1/3)
• What actually matters is the VLAN of
the frame itself, not the VLAN of the
sending machine
• If there were some way to identify
the VLAN in the frame header, them
the need to inspect the payload
would vanish
• How about 802.11, 802.16, Ethernet?
The IEEE 802.1Q
standard (2/3)
• The new format (change the
Ethernet header) was Published in
IEEE standard 802.1Q
• The key to the solution is to realize
that the VLAN fields actually used by
bridges and switches and not by the
user machines
The IEEE 802.1Q
standard (3/3)
• The first VLAN-aware bridge or
switch to touch a frame adds the
VLAN fields, and the last one down
the road removes them
• Switches have to know which VLANs
are reachable on each port
Transition from legacy Ethernet to
VLAN-aware Ethernet
The 802.3 and 802.1Q
Ethernet frame formats
• To use VLANs property, each frame
carries a new special identifier that is
used as an index into a table inside
the switch to look up where the
frame is supposed to be sent
• That is precisely what happens in
connection-oriented networks
• The mobile telephone system
• Cable television
• Wireless LANS
• Broadband wireless
• Bluetooth
• Data Link layer switching
• Quality of service
Quality of service
• Requirements
• Techniques for achieving good quality
of service
• Integrated services
• Differentiated services
• Label switching and MPLS
• The needs of each flow can be
characterized by four primary
parameters: reliability, delay, jitter,
and bandwidth.
• Together these determine the QoS
the flow requires.
How stringent the qualityof-service requirements are
Techniques for achieving
good quality of service
Traffic shaping
The leaky bucket algorithm
The token bucket algorithm
Resource reservation
Admission control
Proportional routing
Packet sheduling
• Theorem: An easy solution is to
provide so much router capacity,
buffer space, and bandwidth that the
packets just fly through easily.
• Disadvantage : It is expensive.
• Application: The telephone system is
• Flows can be buffered on the receiving
side before being delivered.
• Buffering them
-does not affect the reliability or bandwidth.
-increases the delay.
-smoothes out the jitter.
• The source outputs the packets with a
uniform spacing between them.
Smoothing the output
stream by buffering packets
Traffic shaping
• How about if the server is handling
many streams at once?
-No uniform output is common.
• Traffic shaping smoothes out the
traffic on the server side. (server
transmits at a uniform rate)
The leaky bucket algorithm
• It is a single server queueing system
with constant service time and finite
• The leaky bucket algorithm enforces
a rigid output pattern at the average
rate, no matter how bursty the traffic
A leaky bucket
The token bucket
• It is better to allow the output to
speed up somewhat when large
bursts arrive.
• The token bucket algorithm:
-For a packet to be transmitted, it
must capture and destroy one token.
-It provides a different kind of traffic
The token bucket
-The token bucket algorithm throws
away tokens when the bucket fills up
but never discards packets.
-A packet can only be transmitted if
enough tokens are available to cover
its length in bytes.
The token bucket algorithm
Resource reservation
• A virtual circuit has to be set up from
the source to the destination, and all
the packets that belong the flow
must follow this route.
• Three kinds of resources can be
potential be reserved:
-buffer space
-CPU cycles
Admission control
• A router has to decide whether to
admit or reject the flow based on its
capacity and how many
commitments it has already made for
other flows.
• The sender, receiver, and all the
routers along the path between them
may be involved in the flow
An example flow
Proportional routing
• To provide a higher quality of service
by splitting the traffic for each
destination over multiple paths.
Packet sheduling
• Fair queueing algorithm
-routers have separate queues for
each output line, one for each flow.
-the round robin is done in such a
way as to simulate a byte-by-byte
round robin.
• Weighted fair queueing algorithm
-the weight is equal to the number of
flows coming out of a machine.
Fair queueing algorithm
byte-by-byte round robin
Integrated services-RSVP (the
resource reservation protocol)
• RSVP (Flow-based algorithm)
-It offer good quality of service to
one or more flows by reserving
whatever resources are needed along
the path.
-It requires an advance setup to
establish each flow. (not scalable)
RSVP uses multicast routing
using spanning tree
An example
Differentiated services
• Differentiated services (Class based QoS)
-It defines a set of service classes with
corresponding forwarding rules.
-It requires no advance setup, no resource
reservation, and no time-consuming endto-end negotiation for each flow.
• Expedited forwarding
• Assured forwarding
Expedited forwarding
Assured forwarding
Label switching and MPLS
• This work focused on adding a label
in front of each packet and doing the
routing based on the label rather
than on the destination address.
Transmitting a TCP segment
using IP, MPLS, and PPP
Thank you for your attention~

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