Transcript Chapter 1

Computer Networks
Chapter 1 Introduction
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1
Introduction
Chapter 1
• Uses of Computer Networks
• Network Hardware
• Network Software
• Reference Models
• Example Networks
• Network Standardization
• Metric Units
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Uses of Computer Networks
Computer networks are collections of autonomous computers,
e.g., the Internet
They have many uses:
• Business Applications »
• Home Applications »
• Mobile Users »
These uses raise:
• Social Issues »
This text covers networks for all of these uses
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Business Applications
• Companies use networks and computers for resource sharing with the clientserver model:
request
response
• Other popular uses are communication, e.g., email, VoIP, and e-commerce
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Home Applications
• Homes contain many networked devices, e.g., computers, TVs,
connected to the Internet by cable, DSL, wireless, etc.
• Home users communicate, e.g., social networks, consume content, e.g.,
video, and transact, e.g., auctions
• Some application use the peer-to-peer model in which there are no fixed
clients and servers:
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Network Uses
• 1. Interactions between persons and remote databases
• WWW, digital libraries, peer-to-peer file sharing
• 2. Person to person communications or Social networks
• Email, instant messaging, Twitter, Facebook, wikis, blogs, etc.
• 3. Electronic Commerce
• Bill paying, online auctions (ebay), online stores, etc.
• 4. Entertainment
• Music, radio, film, TV (IPTV), games
• 5. Ubiquitous computing ( embedded into daily life)
• Smart homes, security systems, RFID
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Mobile Users
• Tablets, laptops, and smart phones are popular devices; WiFi hotspots
and 3G cellular provide wireless connectivity.
• Mobile users communicate, e.g., voice and texts, consume content, e.g.,
video and Web, and use sensors, e.g., GPS.
• Wireless and mobile are related but different:
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Mobile Uses
• Sales of mobile devices are greater than desktops
• Connectivity to the Internet
• Cellular networks – operated by telephone companies, mobile and smart
phones for texting
• WI-Fi Hotspots (based on 802.11 standard) – used for “hand held” and
tracking devices, important to military, ebooks
• GPS ( global positioning systems) Google maps, etc…
• M-commerce ( mobile commerce)- text messages used in payment, bitcoins
• Senor networks ( cars, animals)
• Wearable computers ( google glass, motorola…)
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Social Issues
• Computer networks allow users to distribute and view content in ways
that were not possible until recently. This has resulted in many
unsolved social, political and ethical issues:
• Social networks, message boards, content sharing sites, allow people
to share views, etc….
• Some network operators block content, charge different rates to some clients,
provide better service to others, etc. These practices are opposed by network
neutrality.
• Music, films are distributed in violation of copyright; now automated systems
search for violators under the Digital Millennium Copyright Act.
• It has become easy to “snoop” ( employer’s accessing employee email, etc.)
• Government vs. citizens’ rights.
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Social Issues
• Location privacy- tracking through mobile devices
• Anonymous messages – threats, bullying
• Ability to find information but some is misleading or wrong
• Electronic junk mail or spam
• Botnets, viruses and other malware can be used for identity theft
(spoofing and phishing), to collect bank account number, passwords
• Botnets or zombies which distribute spam and viruses to computers
• CAPTCHAS are used to prevent computers form impersonating
people
• Computer gambling
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Social Issues Summarized
• Network neutrality – no network restrictions- treat
all data equally
• Content ownership, e.g., DMCA takedowns
• Anonymity and censorship
• Privacy, e.g., Web tracking and profiling
• Theft, e.g., botnets and phishing
http://www.wired.com/opinion/2013/11/so-the-internets-about-to-lose-its-net-neutrality/
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Network Hardware
• Technical issues in Network design:
• Two types of transmission technology: broadcast links and
point-to-point links
• Point-to-point links connect individual machines, sending packets. usually
unicast (one sender, one receiver)
• Broadcast shared and received among many machines (eg wireless)can be
addresses to all or a subset ( multicast)
• Network can also be classified by scale or rough physical size:
•
•
•
•
•
Personal area networks
Long range ( local, metropolitan, wide area)
Internets – connection of 2 or more networks
The Internet – global network of networks
Future – Interplanetary Internet – connects networks across space.
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Network Hardware
Networks can be classified by their scale:
Scale
Type
Examples
Vicinity
PAN (Personal Area Network) »
Bluetooth (e.g., headset)
Building
LAN (Local Area Network) »
WiFi, Ethernet
City
MAN (Metropolitan Area Network)
»
Cable, DSL
Country
WAN (Wide Area Network) »
Large ISP
Planet
The Internet (network of all
networks)
The Internet!
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Personal Area Network
Connect devices over the range of a person
Example of a Bluetooth (wireless) PAN: advantage- no wires needed
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Local Area Networks
• Connect devices in a home or office building –privately owned
• Called enterprise network in a company
Wired LAN with
switched Ethernet
Wireless LAN
with 802.11
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Local Area Networks
• WiFi or IEEE 802.11 is the standard for wireless LANs
• Wireless LANs
• Access point (AP), wireless routers, or base stations relay packets
between computers and the Internet.
• Wired LANs use different technologies, and media (copper wire,
fiber, etc.)
• Ethernet or IEEE 802.3 is the most common
• Usually connected to a switch through a port
• Switches can be connected together to form larger networks
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LANs in the Home
• Many devices are capable of being networked: computers, DVDs,
phones and other consumer electronics, such as cameras, appliances
such as clocks and radios and infrastructures such as utility meters
and thermostats.
• Some unique properties of home networks:
1. Networked devices must be easy to install
2. Network and devices must be foolproof
3. Devices must be economical
4. Must allow for expansion
5. Must be secure and reliable
Google is moving toward the “Internet of Things” with the purchase of NEST
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Metropolitan Area Networks
Connect devices over a metropolitan area
High speed wireless MANs based on IEEE 802.16 – called WIMAX
Example MAN based on cable TV:
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Wide Area Networks (1)
• Connect devices over a country- span large geographic areas
• Example WAN connecting three branch offices:
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Wide Area Networks (2)
• An ISP (Internet Service Provider) network is also a WAN.
• Customers buy connectivity from the ISP to use it.
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Wide Area Networks (3)
• A VPN (Virtual Private Network) is a WAN built from virtual links
that run on top of the Internet.
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Wide Area Networks
• WANs connect devices, called hosts, through subnets
• Subnets consist of:
• Transmission lines that move bits between machines
• Switches or routers – that connect two or more transmission lines
• Some differences from a LAN:
• Usually the components in a WAN are owned by different entities ( such as a
phone company owning a leased line) and are not private
• Routers usually connect different kinds of networking technologies
(heterogeneous) instead of usually homogeneous equipment as in a LAN
• Subnets can be made up of computers as in a LAN, but often connect larger
LANs, forming internetworks.
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Network Software
• Protocol layers »
• Design issues for the layers »
• Connection-oriented vs. connectionless
service »
• Service primitives »
• Relationship of services to protocols »
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Layering of Airline Functionality Example
ticket (purchase)
ticket (complain)
ticket
baggage (check)
baggage (claim
baggage
gates (load)
gates (unload)
runway (takeoff)
runway (land)
takeoff/landing
airplane routing
airplane routing
airplane routing
departure
airport
airplane routing
airplane routing
intermediate air-traffic
control centers
gate
arrival
airport
Layers: each layer implements a service
• via its own internal-layer actions
• relying on services provided by layer below
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Kurose Introduction
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Protocol Layers
• Most networks are organized as a stack of layers or levels, each one
built upon and communicating with the one below it.
• Each layer offers services to a higher layer, (like a virtual machine)
providing information hiding and encapsulation.
• A Protocol is an agreement between communicating parties on how
communication is to proceed.
• The entities which communicate are called peers ( software,
hardware or humans).
• There are 5 layer (Internet or TCP/IP) and 7 layer (OSI) models
• Between each pair of adjacent layers is an interface
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Protocol Layers (1)
Protocol layering is the main structuring method used to divide up
network functionality.
• Each protocol instance talks virtually to
its peer
• Each layer communicates only by using
the one below
• Lower layer services are accessed by an
interface
• At bottom, messages are carried by the
medium
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Protocol Layers (2)
• Example: the philosopher-translator-secretary architecture
• Each protocol at different layers serves a different purpose
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Protocol Layers (3)
• Each lower layer adds its own header (with control information)
to the message to transmit and removes it on receive
• Layers may also split and join messages, etc.
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Design Issues for the Layers
Each layer solves a particular problem but must include mechanisms
to address a set of recurring design issues
Issue
Example mechanisms at different layers
Reliability despite failures
Codes for error detection/correction (§3.2, 3.3)
Routing around failures (§5.2)
Network growth and evolution
Addressing (§5.6) and naming (§7.1)
Protocol layering (§1.3)
Allocation of resources like
bandwidth
Multiple access (§4.2)
Congestion control (§5.3, 6.3)
Security against various threats
Confidentiality of messages (§8.2, 8.6)
Authentication of communicating parties (§8.7)
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Some Definitions
• A packet is a message at the network layer
• Datagram service is unreliable or unacknowledged message delivery
service ( eg. Telegeam or email)
• Connection oriented –modeled after the phone system
• Connectionless – modeled after the postal system
• A Service is a set of primitives or operations that a layer provides to
the one above it. ( Like an object in OOP).
• A Protocol is a set of rules governing the format and meaning of the
packets or messages that are exchanged. Protocols implement the
services
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Connection-Oriented vs. Connectionless
• Service provided by a layer may be kinds of either:
• Connection-oriented, must be set up for ongoing use (and torn down after
use), e.g., phone call
• Connectionless, messages are handled separately, e.g., postal delivery
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Service Primitives (1)
• A service is provided to the layer above as primitives
• Hypothetical example of service primitives that may provide a
reliable byte stream (connection-oriented) service:
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Service Primitives (2)
• Hypothetical example of how these primitives may be used for a
client-server interaction
Server
Client
LISTEN (0)
Connect request
CONNECT (1)
Accept response
SEND
RECEIVE
(3)
ACCEPT
RECEIVE
(2)
Request for data
SEND (4)
Reply
DISCONNECT (5)
Disconnect
Disconnect
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DISCONNECT (6)
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Relationship of Services to Protocols
Recap:
• A layer provides a service to the one above
• A layer talks to its peer using a protocol
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[vertical]
[horizontal]
Reference Models
Reference models describe the layers in a network
architecture
• OSI reference model »
• TCP/IP reference model »
• Model used for this text »
• Critique of OSI and TCP/IP »
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OSI Reference Model
• A principled, international standard, called the
seven layer model to connect different systems:
– Provides functions needed by users
– Converts different representations
– Manages task dialogs
– Provides end-to-end delivery
– Sends packets over multiple links
– Sends frames of information
– Sends bits as signals
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OSI Architecture
The OSI 7-layer Model
OSI – Open Systems Interconnection
Description of Layers
•
•
Starting from the bottom up:
(Remember: “Please Do Not Throw Sausage Pizza Away”)
• Physical Layer
• Handles the transmission of raw bits over a communication link
• Data Link Layer
• Collects a stream of bits into a larger aggregate called a frame
• Network adaptor along with device driver in OS implement the protocol in this layer
• Frames are actually delivered to hosts
• Network Layer
• Handles routing among nodes within a packet-switched network
• Unit of data exchanged between nodes in this layer is called a packet
The lower three layers are implemented on all network nodes
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Description of Layers
• Transport Layer
• Implements a process-to-process channel
• Unit of data exchanges in this layer is called a message
• Session Layer
• Provides a name space that is used to tie together the potentially different transport streams that
are part of a single application
• Presentation Layer
• Concerned about the format of data exchanged between peers
• Application Layer
• Standardize common type of exchanges, includes protocols such as FTP, HTTP, etc.
The transport layer and the higher layers typically run only on end-hosts and not on the intermediate
switches and routers
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7 Layer Model
Layer
Functions
7
Application
How application uses network
6
Presentation
How to represent & display data
5
Session
How to establish communication
4
Transport
3
Network
How to provide reliable delivery (error checking, sequencing,
etc.)
How addresses are assigned and packets are forwarded
2
Data Link
How to organize data into frames & transmit
1
Physical
How to transmit “bits”
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OSI Headers
Multiple Nested Headers
Each layer (except the physical) places additional information in a header before
sending data to a lower layer. The header corresponding to the lowest level
protocol occurs first.
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OSI Model
http://computer.howstuffworks.com/osi1.htm
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OSI -7 Layer Model
• The OSI (Open System Interconnection) model has 7 layers and can
be summarized as follows:
1. A layer should be created where a different abstraction is needed.
2. Each layer should perform a well defined function
3. The function of each layer should be based on standard protocols.
4. The layer boundaries should minimize the information flow across
the interfaces.
5. The number of layers should be large enough to accommodate the
necessary functions and small enough not to be cumbersome.
This model is widely used and specifies what each layer should do.
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OSI -7 Layer Model
• Three concepts are central to this model:
1. Services
2. Interfaces
3. Protocols
The main contribution of this model is that it makes the distinction
among these 3 concepts explicit.
• Each layer performs a service for the layer above it.
• The layer’s interface tells the processes above it how to access it.
• Peer protocols can be used or changed by the layers.
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Internet Architecture
• Also called the TCP/IP Architecture
• Evolved from the packet switched ARPANET ( Advanced Research
Projects Agency) of the Department of Defense.
• Influenced the OSI model
• Usually shown as a 4 or 5 layer model
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Internet Architecture
Special Features
1. Does not imply strict layering
2. Has “hourglass shape” design philosophy- with IP as its central feature (
See protocol graph)
3. To propose a new protocol, both a protocol specification and at least one
representative implementations must be provided.
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Internet Architecture
Internet Protocol Graph
Alternative view of the Internet
architecture. The “Network” layer shown
here is sometimes referred to as the
“sub-network” or “link” layer.
Internet Architecture
1. At lowest or Network Layer there are many protocols, called NET1, NET2,
NET3, etc., implemented by a combination of hardware (Ethernet or
Fiberoptics)
2. Internet Layer - Internet Protocol (IP) supports connection of multiple
networks
3. Transport Layer –reliable Transmission Control Protocol (TCP) and
unreliable User Datagram Protocol (UDP) –alternative logical channels to
applications.
4. Application Layer – FTP, SMTP, Telnet, HTTP
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Internet Architecture
4 or 5 Layer Internet Model
Application Layer
Application
Transport Layer
TCP UDP
IP
Network
Internet Layer
Network Layer
Physical layer (implied)
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Protocols in TCP/IP Reference Model
• A four layer model derived from experimentation; omits some OSI
layers and uses the IP as the network layer.
IP is the “narrow
waist” of the Internet
Protocols are shown in their respective layers
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TCP/IP Reference Model
• This model was defined by Cerf and Kahn and defined as a
standard in the Internet community.
• The protocols came first and the model was a description of the
existing protocols
• The original TCP/IP model did not distinguish between services,
interfaces and protocols.
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Comparison of OSI and TCP/IP
OSI
7 Application
TCP/IP
Application
6 Presentation
5 Session
4 Transport
Transport
3 Network
Internet
2 Data Link
Link
1 Physical
(Physical ) implied
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Not
present
in model
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Model Used in this Book
It is based on the TCP/IP model but we include the physical layer and
look beyond Internet protocols.
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Critique of OSI & TCP/IP
OSI:
+Very influential model with clear concepts
• Models, protocols and adoption all bogged down by
politics and complexity
TCP/IP:
+Very successful protocols that worked well and thrived
• Weak model derived after the fact from protocols
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Example Networks
• The Internet »
• 3G mobile phone networks »
• Wireless LANs »
• RFID and sensor networks »
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Internet (1)
Before the Internet was the ARPANET, a decentralized, packetswitched network based on Baran’s ideas.
Nodes are IMPs, or early
routers, linked to hosts
56 kbps links
ARPANET topology in Sept 1972.
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Internet (2)
The early Internet used NSFNET (1985-1995) as its backbone;
universities connected to get on the Internet
T1 links (1.5
Mbps)
NSFNET topology in 1988
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Internet (3)
The modern Internet is more complex:
• ISP networks serve as the Internet backbone
• ISPs connect or peer to exchange traffic at IXPs
• Within each network routers switch packets
• Between networks, traffic exchange is set by business agreements
• Customers connect at the edge by many means
• Cable, DSL, Fiber-to-the-Home, 3G/4G wireless, dialup
• Data centers concentrate many servers (“the cloud”)
• Most traffic is content from data centers (esp. video)
• The architecture continues to evolve
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Internet (4)
Architecture of the Internet
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3G Mobile Phone Networks (1)
3G network is based on spatial cells; each cell provides wireless
service to mobiles within it via a base station
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3G Mobile Phone Networks (2)
• Base stations connect to the core network to find other mobiles and
send data to the phone network and Internet
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3G Mobile Phone Networks (3)
As mobiles move, base stations hand them off from one cell to the
next, and the network tracks their location
Handover
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Wireless LANs (1)
In 802.11, clients communicate via an AP (Access Point) that is wired
to the rest of the network.
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Wireless LANs (2)
Signals in the 2.4GHz ISM band vary in strength due to many effects,
such as multipath fading due to reflections
• requires complex transmission schemes, e.g., OFDM
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Wireless LANs (3)
Radio broadcasts interfere with each other, and radio ranges may
incompletely overlap
• CSMA (Carrier Sense Multiple Access) designs are used
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RFID and Sensor Networks (1)
Passive UHF RFID networks everyday objects:
• Tags (stickers with not even a battery) are placed on objects
• Readers send signals that the tags reflect to communicate
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RFID and Sensor Networks (2)
Sensor networks spread small devices over an area:
• Devices send sensed data to collector via wireless hops
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Network Standardization
Standards define what is needed for interoperability
Some of the many standards bodies:
Body
Area
Examples
ITU
Telecommunications
G.992, ADSL
H.264, MPEG4
IEEE
Communications
802.3, Ethernet
802.11, WiFi
IETF
Internet
RFC 2616, HTTP/1.1
RFC 1034/1035, DNS
W3C
Web
HTML5 standard
CSS standard
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice
Hall and D. Wetherall, 2011, modified by SJF
Metric Units
The main prefixes we use:
Prefix
Exp.
prefix
exp.
K(ilo)
103
m(illi)
10-3
M(ega)
106
μ(micro)
10-6
G(iga)
109
n(ano)
10-9
T(era)
1012
• Use powers of 10 for rates, powers of 2 for storage
• E.g., 1 Mbps = 1,000,000 bps, 1 KB = 1024 bytes
• “B” is for bytes, “b” is for bits
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice
Hall and D. Wetherall, 2011, modified by SJF
End
Chapter 1
CN5E by Tanenbaum & Wetherall, ©
Pearson Education-Prentice Hall and
D. Wetherall, 2011, modified by SJF
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