Transcript Cellular Security Overview

Setting up the Communication
Network Problem
Wade Trappe
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
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Transmission Systems
Switching: Overview
Through the Looking Glass:
Communication Networks
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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
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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
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These are the “underlying fundamentals” of communication
networks
A Core Dump of Core Questions
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1.
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
2.
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
3.
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)
4.
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!)
Network Types
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There are two basic classes of communication networks: circuitswitched and packet-switched
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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 circuitswitched networks)
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However, we will start with circuit-switched networks
Grandfather of Networks: Telephone Network
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Public switched telephone networks have been around for a long time

Goal: Provide voice service between two users, regardless of their (global)
location
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The service is known as POTS (Plain Old Telephone Service)
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The term “switching” refers to the fact that we want to connect any users
without requiring a separate wire for each possible pair
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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
20*19= 190 total connections!!!
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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
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Let us look at a generic “phone network”
Telephone Network Generica
Telephones
LE
Cellular GMSC
Network
LE
LE
National
IGE
PSTN
LE
PBX
LE
LE
IGE
Digital
Interconnection
Circuits
PSTN
LE
LE
PSTN = Public Switched
Telephone Network
GMSC= Gateway Mobile Switching
Center
IGE= International Gateway
Exchange
LE= Local Exchange
PBX= Private Branch Exchange
Telephone Network Explained
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Telephones at home (or a small office) connect directly to the nearest Local
Exchange
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Phones located in a corporate office typically connect to a private switching
office (Private Branch Exchange)
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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
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The PBX are connected to an LE so that calls may be routed outside of the
PBX
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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
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Finally, international calls are routed through International Gateway
Exchanges, which are connected by digital connections
The Life Cycle of a Phone Call
Backbone Network
D
E
A
B
C
Local Exchange
@ Central Office
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End systems (phones) connect to the LEs, which connect to
backbone switches
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#LEs >> # Backbone Switches
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The backbone network is nearly fully connected (dedicated lines
between almost all switches)… making a one-hop network
Life Cycle of a Call, pt 2
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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 user’s
LE
– The target LE then connects its target End user to the circuit that
has been set up
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Question: So how does the system know which LEs and
switches to connect to?
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Answer: Its all in the phone number!
867-5309: What’s that number?
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A call going from 732-445-0611 to 873-867-5309 creates a circuit by:
1.
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
2.
It establishes a connection with the nearest backbone swtich
3.
The backbone switch establishes a (short) connection to the switch servicing
the (873) area
4.
The (873) switch establishes a connection with the -867- local exchange
5.
The final connection to the end system 5309 is made
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That is, the telephone number serves as a means to route through the
electomechanical switches of the telephone network
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The telephone numbers form a natural hierarchy that is easily extendable to
include new numbers: some central agency simply creates new area code
numbers
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Components: End System, Transmission, Switching, Signaling
Transmission System
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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 < 100ms for 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
Switching
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Switching governs how a user is connected with every other
user
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Two components: Switch Hardware (Data Plane), and the
Switch Controller (Signaling/Control Plane)
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A switch transfers information from an input line to an output
line.
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There are two basic ways to do switching: Space division
switching and time division switching
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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)
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.
Input
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A
B
C
D
E
Time-Division Switching
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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
C
Write
2
3
D
Packet Switching: A brief overview, pg. 1
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Circuit Switching provides a continuous, constant bit rate
connection between two points
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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
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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
Packet Switching: A brief overview, pg. 2
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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
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There are two types of packet switching:
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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
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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 packet’s 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
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We will look at each of these a little more.
Connection-Oriented
VCI-2
A
VCI-1
PSE-2
1
3
2
1
PSE-3 2
3
2
1 PSE-1
3
2
1
3
PSE-4
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CO = Connection oriented
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VCI = Virtual Circuit Identifier
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PSE = Packet Switching Exchange
VCI-3
VCI-4
B
Connection-Oriented
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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)
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Each PSE contains a table that specifies the outgoing link that
should be used to reach each network address
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The PSE uses this destination address to lookup which outgoing
link should be used and assigns a new VCI for this link
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The routing table is updated
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The call request packet is then forwarded to the next PSE and
the process continues
A Connection-Oriented Routing Table
In
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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.
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
A
R1
R3
R4
Packet:
B
A
Payload
B
Wrap-up
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Connection-oriented Examples:
– X.25: Old style file transfer network
– ATM: high bit rate “backbone” style network
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Connectionless: the Internet
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Packet switching is the more popular style of network
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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