Setting up the Communication Network Problem

1
Setting up the Communication Network Problem

• Wade Trappe

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Lecture Overview

• What is a communication network?
• Core Questions
• Telephone Networks
• PSTN/GMSC/IGE/LE/PBX and all that stuff
• Circuits and Routing, aka. the phone number
• End Systems
• Transmission Systems
• Switching Overview

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Through the Looking GlassCommunication Networks

• This class is not about specific protocols, but
  rather about the fundamentals underlying
  networks.
• When you use the hypertext transfer protocol
  (http) or send an email on the Internet, there
  are many operations (the fundamentals) which
  are hidden from the protocol itself
• A web page might be slow, but what goes on
  underneath that makes it slow?
• Perhaps you are on a shared medium Ethernet and
  the slowness is due to backoffs and collision
  resolution
• Perhaps you are performing satellite
  communication and a sunflare has increased local
  radiation resulting in a higher bit error rate
  for the underlying signaling necessitating
  frequent retransmissions
• These are the underlying fundamentals of
  communication networks

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A Core Dump of Core Questions

• There are several questions that will arise in
  our study of communication network fundamentals
• How does one model the application? What are the
  salient properties of the application that affect
  operation of a communication network?
• e.g. Bit rate, traffic pattern, packet size,
  delay sensitivity, interarrival times,
  reliability requirements
• What models are the most appropriate for studying
  the network performance in different scenarios?
• e.g. Use random process models (good for basic
  understanding of the fundamentals involved in
  network protocols)
• Flow models (e.g. on/off) good for
  capturing and studying real-time application
  behaviors
• How does one manage simultaneous communications
  over a shared resource between different users or
  pairs of users?
• e.g. This arises in many different ways
  switching (to be circuit-oriented or not to be?),
  multiple access protocols (TDMA, FDMA, Aloha,
  etc)
• How does one build a network system from the
  ground up?
• e.g. The idea of modular construction, aka.
  layering. Shannon loved it and enjoyed it as the
  Law of Digital Communications. The Law is
  basically the same in networking. Interestingly,
  both Laws are changing now!!! (research hint!)

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Network Types

• There are two basic classes of communication
  networks circuit-switched and packet-switched
• For the most part, now days, we think of
  packet-switched networks
• This is because the concept behind packet
  switching (which we shall discuss later) has led
  to more engineering efficiency
• In particular, circuit-switching seeks to reserve
  dedicated resources for a communication,
  whereas packet-switching is more opportunistic
• We shall primarily discuss issues related to
  packet networks for the most of this class (some
  techniques will apply to circuit-switched
  networks)
• However, we will start with circuit-switched
  networks

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Grandfather of Networks Telephone Network

• Public switched telephone networks have been
  around for a long time
• Goal Provide voice service between two users,
  regardless of their (global) location
• The service is known as POTS (Plain Old Telephone
  Service)
• The term switching refers to the fact that we
  want to connect any users without requiring a
  separate wire for each possible pair
• Example In this class there are roughly 20
  students. If each one of you wanted to connect to
  every other person with a dedicated line, we
  would need 2019 190 total connections!!!
• The idea behind switching is to avoid this naïve
  approach to communication
• We have 1 connection line going into each house,
  and these lines will connect to a
  switching/signaling backbone that will route your
  call to the appropriate destination
• Let us look at a generic phone network

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Telephone Network Generica
Telephones
LE
LE
LE
LE
LE
National PSTN
PSTN
LE
IGE
GMSC
IGE
Cellular Network
LE
Digital Interconnection Circuits
LE
PBX
PSTN Public Switched Telephone
Network GMSC Gateway Mobile Switching
Center IGE International Gateway
Exchange LE Local Exchange PBX Private
Branch Exchange
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Telephone Network Explained

• Telephones at home (or a small office) connect
  directly to the nearest Local Exchange
• Phones located in a corporate office typically
  connect to a private switching office (Private
  Branch Exchange)
• Think of the PBX as administering a micro-phone
  universe where any two phones directly connected
  to the PBX can have an easy connection to each
  other via the PBX
• The PBX are connected to an LE so that calls may
  be routed outside of the PBX
• Cell Phone networks are a small universe and
  phone calls made within the cell network are
  administered by the MSC, while phone calls
  leaving the cellular universe pass through the
  Gateway Mobile Switching Center
• Finally, international calls are routed through
  International Gateway Exchanges, which are
  connected by digital connections

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The Life Cycle of a Phone Call
Backbone Network
A
D
E
B
C
Local Exchange _at_ Central Office

• End systems (phones) connect to the LEs, which
  connect to backbone switches
• LEs Backbone Switches
• The backbone network is nearly fully connected
  (dedicated lines between almost all switches)
  making a one-hop network

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Life Cycle of a Call, pt 2

• When an End User makes a call, it connects to its
  LE, which seeks to set up a circuit between two
  end systems
• To do so, if the call is not local, it connects
  to the nearest backbone switch, which connects to
  the switch nearest the target end users LE
• The target LE then connects its target End user
  to the circuit that has been set up
• Question So how does the system know which LEs
  and switches to connect to?
• Answer Its all in the phone number!

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867-5309 Whats that number?

• A call going from 732-445-0611 to 873-867-5309
  creates a circuit by
• Identifying the end systems area code, so the LE
  at 9732) notices that the area code (873) is
  different from its own, so it must connect out
• It establishes a connection with the nearest
  backbone swtich
• The backbone switch establishes a (short)
  connection to the switch servicing the (873) area
• The (873) switch establishes a connection with
  the -867- local exchange
• The final connection to the end system 5309 is
  made
• That is, the telephone number serves as a means
  to route through the electomechanical switches of
  the telephone network
• The telephone numbers form a natural hierarchy
  that is easily extendable to include new numbers
  some central agency simply creates new area code
  numbers
• Components End System, Transmission, Switching,
  Signaling

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Transmission System

• A transmission link is characterized by its
  information-capacity, the propagation delay, and
  its link attenuation
• Information capacity Bandwidth is the width of
  the data pipe, or more specifically, the average
  number of bits/second.
• Link Delay The time taken for a signal to
  propagate over the medium and is particularly
  important for long links with delay sensitive
  applications
• Example Speed of light in fiber is 70 speed of
  light in a vacuum. In fiber, light travels at
  8msecs/mile
• Voice application requirements non-frustrating conversations
• NewYork ? SanFrancisco is 20msec (2500 miles).
  Not as much of the delay is propagation, so
  switching and control architectures are important
• Satellite speed of light is higher, but the
  propagation delay is around 250msec (36000
  kilometers!)
• Link Attenuation As a signal travels, it
  attenuates and it is important to introduce
  regeneration/amplification on the links. Fiber
  optics are good as they have minimal attenuation

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Switching

• Switching governs how a user is connected with
  every other user
• Two components Switch Hardware (Data Plane), and
  the Switch Controller (Signaling/Control Plane)
• A switch transfers information from an input line
  to an output line.
• There are two basic ways to do switching Space
  division switching and time division switching
• Signaling Is the decision plane that controls
  the switches and which establishes how the
  switches will operate and forward their calls
  (setting up and tearing down the calls)

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Space Division Switching Example

• Cross-Bar
• Inputs arrive along rows and outputs are
  connected to columns
• To perform the connection, the switch establishes
  the circuit connection at the intersection
• To visualize, recall that this is
  electro-mechanical.

A
B
Input
C
D
E
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Time-Division Switching

• N inputs are stored in a temporary buffer
• The switch reads from the buffers N times faster
  according to a schedule
• Writes to the outputs before next input buffer is
  read

A
Read
1
B
2
Write
C
3
D
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Packet Switching A brief overview, pg. 1

• Circuit Switching provides a continuous, constant
  bit rate connection between two points
• By doing so, circuit switching implicitly
  provides quality of service guarantees (1) A
  guaranteed bandwidth (2) a bound on delay once a
  circuit is established
• Problem with circuit switching from a resource
  allocation point of view
• Once a circuit is formed, those resources are
  dedicated, regardless of whether they are being
  used!
• Example (Phone call) There are many instants
  during a conversation when silence occurs and no
  data is being created. In a circuit-switched
  network, where the connection is reserved,
  resources are wasted

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Packet Switching A brief overview, pg. 2

• Packet switching (i.e. store-and-forward
  switching) addresses these issues
• Note The difference between packet switching and
  message switching is where the packetization is
  done
• There are two types of packet switching
• Connection-oriented (Virtual-Circuit Based)
  Session causes the creation of a path (virtual
  circuit) much like circuit switching, but the
  capacity of each link is shared dynamically (e.g.
  with some scheduling policy) with other sessions
  that use the same link
• Connectionless (Datagram Based) Here, each
  packet contains its source and destination
  address, as well as payload. The packet and the
  network are responsible for finding the packets
  way to the destination. Here, intermediate nodes
  participate in dynamic routing, possibly taking
  advantage of local information to decide the best
  next step in the delivery
• We will look at each of these a little more.

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Connection-Oriented
PSE-2
VCI-3
1
3
VCI-2
2
1
2
B
PSE-3
2
VCI-1
A
1
PSE-1
VCI-4
3
3
2
1
3
PSE-4

• CO Connection oriented
• VCI Virtual Circuit Identifier
• PSE Packet Switching Exchange

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Connection-Oriented

• To set up a virtual circuit, the source sends a
  call request control (signal) packet to its PSE.
  Signal contains source and destination address as
  well as a label for this component of the virtual
  circuit (called a VCI)
• Each PSE contains a table that specifies the
  outgoing link that should be used to reach each
  network address
• The PSE uses this destination address to lookup
  which outgoing link should be used and assigns a
  new VCI for this link
• The routing table is updated
• The call request packet is then forwarded to the
  next PSE and the process continues

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A Connection-Oriented Routing Table

• In Out
• PSE-1 Routing VCI1 Link 1 VCI2 Link 2
• VCI2 Link 2 VIC1 Link 1
• PSE-2 Routing VCI2 Link 1 VCI3 Link 3
• VCI3 Link 3 VCI2 Link 1
• PSE-3 Routing VCI3 Link 1 VCI4 Link 2
• VCI4 Link 2 VCI3 Link 1
• Call clear packets are forwarded to tear down
  connection.

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Connectionless (datagram)

• Here, the establishment of an explicit connection
  is not required.
• Rather, a datagram is routed to an appropriate
  outgoing link based on the local routing table.

R2
B
A
R1
R3
R4
B
A
Payload
Packet
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Wrap-up

• Connection-oriented Examples
• X.25 Old style file transfer network
• ATM high bit rate backbone style network
• Connectionless the Internet
• Packet switching is the more popular style of
  network
• Regardless of which style of network, the process
  of communication involves protocols, which we
  will discuss next time.
• i.e. OSI and the PHY-layer