Assignment 4

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Assignment 4

• A cell phone system using sockets

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Cell System
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A Single Cell
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Assignment

• Your task is to implement the following, using
  sockets
• 1. Control channel and voice channel.
• 2. Multiplexing CDMA or TDMA
• 3. Full duplex system.
• 4. Number dialing.
• 5. Traffic routing and switching.
• 6. Encryption of the voice channel.
• You may do this project in groups of one to
  three.
• Give your group a name for identification.
• You may use C or C for this assignment.

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Code Division Multiple Access (CDMA)

• Spread Spectrum Cellular
• IS-95 CDMA spread spectrum system
• Many users share a 1.25 MHz frequency channel
• Users distinguished by pseudo-random spreading
  sequences
• This spreading signal is formed from a
  pseudo-noise code sequence, which is then
  multiplied by a Walsh code for maximum
  orthogonality to (ie. to have low
  cross-correlation with) the other codes in use in
  that cell.
• Typically, CDMA pseudo-noise sequences are very
  long, thereby giving excellent cross-correlation
  characteristics. (IS-95 uses a 242-1 chip
  period, derived from a 42 bit mask.)

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Direct Sequencing
Binary Pseudo Random Noise Codes PN codes from
Shift Register Sequences CDMA 242-1 bit
sequences Repeat once per century

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5-bit PN Code

Lambda modulo(sumproduct(C4G4,C5G5),2)
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CDMA System Operation
Forward Channel (base to mobile)
Signal Coding
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Forward Channel Transmission Sequence
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CDMA System Operation
Forward Channel (base to mobile)
RF Modulation
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Forward Channel Transmission Sequence
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CDMA Spectral Shape

• I and Q Pilot codes modulate the I and Q channels
  independently at 1.2288 MHz
• QPSK process spreads out the spectral peaks left
  by the Walsh code.

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Walsh Coding
64 mutually orthogonal binary sequences 64
channels for improved interference
rejection Channel 0 pilot channel for timing
and reference data Channel 32 synchronization
channel
Walsh codes

M-sequences
Spectral Shape of PN sequences (before QPSK)
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Pilot and Sync Channels
Pilot channel corresponds to the all zeros Walsh
code (Walsh channel 0), and contains the
unmodulated quadrature PN spreading code. It
is transmitted at higher power than the user
channels, and is provided so that each subscriber
within the cell can determine and react to the
channel characteristics while employing coherent
detection. Walsh channel 32 is assigned to the
sync channel, which provides time and frame
synchronization to the mobile unit. Time of day
and station identification are continuously
broadcast on this channel.

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Pilot and Sync Channels
Several low numbered channels are assigned to
paging. As users are added to the system, they
are assigned user channels from the available
Walsh channels. When over 60 users are present,
the channels are assigned to multiple users, and
protection from mutual interference within the
same Walsh channel is provided by the private PN
sequences that encode each user link. The
number of users can therefore rise to large
values, while reasonable quality is maintained.

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CDMA
U.S. Cellular spectrum allocations (MHz)
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CDMA System Operation
Reverse Channel (mobile to base)
Signal Coding
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Reverse Channel Transmission Sequence
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Reverse Channel Transmission Sequence
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CDMA System Operation
Reverse Channel (mobile to base)
RF Modulation
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Reverse Channel Transmission Sequence
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Global System for Mobile Communications (GSM)

• The GSM system is based on 125 frequency
  allocations and eight timeslots per channel,
  giving a total of 1000 Channels. These may be
  message-carrying traffic channels, or control
  channels.
• The frequency allocations (ARFCN, or absolute
  radio frequency channel number), are each 200 kHz
  in bandwidth.
• The forward ARFCNs are between 935 and 960 MHz
  the reverse ARFCNs are between 890 and 915 MHz,
  so that a given mobile unit receives at a
  frequency exactly 45 MHz greater than the one
  that it transmits.

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Global System for Mobile Communications (GSM)
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Global System for Mobile Communications (GSM)
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Global System for Mobile Communications (GSM)
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Distributed Systems

• Chapter 15
• Section 15.5

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A Distributed System
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Network Topology
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Communication Protocol
A communication network is partitioned into the
following layers

• Physical layer handles the mechanical and
  electrical details of the physical transmission
  of a bit stream.
• Data-link layer handles the frames, or
  fixed-length parts of packets, including any
  error detection and recovery that occurred in the
  physical layer.
• Network layer provides connections and routes
  packets in the communication network, including
  handling the address of outgoing packets,
  decoding the address of incoming packets, and
  maintaining routing information for proper
  response to changing load levels.

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Communication Protocol

• Transport layer responsible for low-level
  network access and for message transfer between
  clients, including partitioning messages into
  packets, maintaining packet order, controlling
  flow, and generating physical addresses.
• Session layer implements sessions, or
  process-to-process communications protocols.
• Presentation layer resolves the differences in
  formats among the various sites in the network,
  including character conversions, and half
  duplex/full duplex (echoing).
• Application layer interacts directly with the
  users deals with file transfer, remote-login
  protocols and electronic mail, as well as schemas
  for distributed databases.

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Communication via ISO Network Model
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The ISO Network Message
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The ISO Protocol Layer
7
6
5
4
3
2
1
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The TCP/IP Protocol Layers
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UNIX Network Support

• Networking support is one of the most important
  features in 4.3BSD.
• The socket concept provides the programming
  mechanism to access other processes, even across
  a network.
• Sockets provide an interface to several sets of
  protocols.
• 4.3BSD supports the DARPA Internet protocols UDP,
  TCP, IP, and ICMP on a wide range of Ethernet,
  token-ring, and ARPANET interfaces.
• The 4.3BSD networking implementation, and to a
  certain extent the socket facility, is more
  oriented toward the ARPANET Reference Model (ARM).

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Network Reference models and Layering