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DCF255 Test 1 Review
Week 1: Overview
Terminology
- A computer is a programmable machine that can perform various computations, store data, and create documents by following a set of prerecorded instructions called a program. The data generated or stored by the computer is a collection of zeros and ones.
- Data communications is the sending of signals that represent zeros and ones over a point-to- point circuit between two computers.
- Networking begins when point-to-point circuits are joined together into a collection of computers for the exchange information and the sharing of resources.
Why should a programmer understand data communications?
- Build better applications if you understand how network is organized and what happens in the network cloud as data is forwarded from host to host
- Build better application if you understand how hackers can take advantage of a mobile app for malicious purposes
- The “Network is now the computer”(John Gage). Programming started with data and code installed on the same machine. Using Internet technology, the cloud through virtualization can be used to host software (SaaS), platform (PaaS) and infrastructure (IaaS). The cloud is now a platform for developing mobile applications and will increase in useage over time
History of Computing:
- Host to mainframe – powerful centralizaed processing connected to dumb terminals
- Client Server- powerfull distributed processing connected to PCs. Lower cost than mainframe
- Internet – increased the usage of client server architecture and lead to n-Tier programming with separate user presentation frontend, a logic processing web services in the middle, linked to a database backend.
- Cloud computing – extension of client server using virtualization to provide software, infrastructure or platfrom as a service.- changin the way IT dept and business function.
Week 2: Standards
Standards
For applications to share resources, communication standards must be worked out in advance called standards. National, International, Professional and Independent standard making bodies.
- 1980’s vendor lock in made computer networking expensive – no body happy – users not getting applcations they wanted, businesses had to buy software and hardware from same vendor, vendors not happy had to share source code with programmers to write applications – danger of intellectual theft.
-led to the ISO (International Organization for Standardization) to create a task force called the OSI (Open System Interconnection) to develop a set of protocols, to make interoperability of software and hardware purchased from different vendors.
Layers & Protocol - Data Communication
- The components involved in data communication are: Source, Destination, Media, Protocol and the message exchanged between the source and the destination.
Protocols are the set of rules or the
procedure which is followed by
communicating devices while
exchanging the data
- Protocol- The rules that allow two entities (source, destination) to exchange some message (text, audio, video) using a physical quantity (medium-wired/wireless).
- Protocols provide the common standardization and flexible interconnection of communication among different platforms and different kinds of computing equipment (interoperability) around the globe and locally. - Layer - specific piece of software following some pre-defined rules of functioning in data communications. - Layers are implemented at the source and destination and any piece of hardware which provides interfacing between the two. - The task of the layer is to ensure that data is physically delivered to the destination without any errors.
The application layer defines the protocols to exchange data and specifies how applications can access the services of the other layers using sockets. A socket is an IP address and a port number, separated by a colon, such as 137.234.56.15:
Protocol Specification:
Protocol specification 4 parts: example http-- message type -message syntax -message sequence -type of connection
Message Type – GET – go get the file on the server and POST – here is the file to process Message Sequence – who speaks first – only client can begin communication Message Syntax – the organization of the message. – header added to front of data – trailer added to end of data
HTTP Request Message
The syntax of the HTTP request message is fairly simple. The screen shot shows the message sent to retrieve a web site called “danny.roy” on a server called people in a domain called “senecac.on.ca”.
- The first line specifies the GET method which requests a path to a file for retrieval and the version of HTTP used by the sender. Notice this line and all other lines end with a character return and a new line [CRLF]
- The second line specifies the host to which the request is sent people.senecac.on.ca
- The third line specifies the host browser and operating system used to send the request
- The fourth line specifies the language to be used in this case English US
- The fifth line specifies the type of compression to use, such as gzip or deflate
- The sixth line specifies the type of TCP connection in this case “keep-alive” means to keep the connection active over multiple request-response cycles. Without this, the TCP connection would end after each request-response cycle and would have to be re-established. Notice, the 2 blank lines at the end of the request; these lines are required and indicate the end of the header field. The data field is empty.
HTTP Response Message
The HTTP Response to the request contains the file requested in the data portion of the packet (not shown).
- The first line begins with HTTP/1.1 which indicates the server has a compatible version. The 200 is a success code that the desired file is returned. The browser actually ignores this code; it is designed for humans to indicate the request was successful.
- The next line indicates the server that returned the request
- The next line gives the date and time the request was returned.
- The next heading specifies the content type which was returned, in this case an image file.
- Again, the end of the header is marked with 2 blank lines followed by the content returned in the data field (not shown). There is no trailer.
Type of Connection – determines the protocol to use – connection oriented – TCP – connectionless UDP
Character Encoding
Application Layer responsible for digital encoding of the message.
-unicode (universal encoding) can represent all of the worlds languages
-UTF-8 – Universal Coded Character Set + Transformation Format – 8-bit. Most popular web encoding system because it is fully compatible with ASCII)
Week 3: Physcial Layer
Electromagnetic Field – a moving magnet generates electricity and electricity generates a magnetic field accoring to the right hand rule.
Signal
A signal is a form in which data is transmitted from source to destination. It describes the behavior of data. In other word signal is an electronic representation of data.
Data
Entities that convey some meaning or information. Data can have different forms like text, image, voice and video (stored or real time).
Characteristics of Sigals – all signals have –
- Amplitude – height of the waveform – measured in volts
- Frequency – number of completed cycles of the waveform in 1 sec.
- Phase – position of the waveform a specific point in time.
Types of Signals
- Analog – continuous rising and falling of voltage – sine wave
- Digital – discrete square waveform with threshold values
Analog – can carry more signals than digital\
Digital – less affected by noise
Digital Data Carried by Digital Signal
- NRZI – non return to zero inverted – very efficient – change in voltage at beginning of clock cycle is a 1 no change in voltage is a 0 – baud rate ½ bit rate on average
- 4B/5B – converts 4 bits of original data into a special 5 bit transmission code which has no more than 2 consecutive 0s. – the 5 bit code is encoded using NRZI. – used on GigE and fiber optic cables
Analog Data Carried by Analog Signal
- AM radio – example – music signal is added to a powerful carrier signal set to a licensed Amplitude. – Tune radio to the amplitude and get the music signal
Digital Data Carried by Analog Signal
- ASK – Amplitue Shift Keying – uses different heights of the waveform to represent 0 and 1
- FSK – Frequency Shift Keying – uses different frequencies of the waveform to represent 0 and 1- used extensively on cable TV systems
- PSK – Phase Shift Keying – uses different phases of the waveform to represent 0 and 1
Analog Data Carried by Digital Signal
- 3 step process
- Analog data waveform is “sampled” – measures the amplitude at fixed internals
- Sampled amplitude – converted to a 7 bit digital value
- To play back the digital value, a special chip on audio card called PAM (Pulse Code Modulation) reads the digital value and recreates the amplitude waveform
- Not perfect representation – CD quality means that waveform must be sampled 2X highest frequency (Nyquist’s theorem) – 700 Hz – wave – needs to be sampled 1400 times per second to get a good representation of the waveform
- Can loose sound in original or create noise not in the original
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P Peta 1,000,000,000,000,000 One thousand trillion T Tera 1,000,000,000,000 One trillion G Giga 1,000,000,000 One billion M Mega 1,000,000 One million K kilo 1,000 One thousand
- If youu are converting from a smaller unit to a larger unit (moving upward in the table shown above), move the decimal place to the left in the number you are converting (dividing by 1000).
- If you are converting from a larger unit to a smaller unit (moving down in the table), move the decimal to the right (multiple by 1000).
- The number of places you move the decimal corresponds to the number of rows you are crossing in the table. For example, let's say you want to convert 8,500,000 bps to Mbps. Mega is two rows up so the decimal should be moved six places to the left to create 8.5 Mbps.
- Proper notation should always have one to three digits before the decimal point. So 8.5Mbps is good (1 place), but 8,500.0 kbps is bad (4 places.).
- You place a space between the number and the metric prefix, but not between the metric prefix and the base unit. For example, writing 8.5 Mbps is good, but writing 8.5M bps or 8.5Mbps is improper
Types of Cable
1. UTP/STP
- UTP refers to “Unshielded Twisted Pair” cable.
- Four pairs of wires each twisted around the other, inside a PVC protective jacket. Two wires carry equal but opposite signals; with the cables twisted, the flow of electrons generates opposite electro-magnetic fields minimizing crosstalk.
- STP stands for “Shielded Twisted Pair” where the wires are wrapped in a shielding which protects the signal from external EMI and increases attenuation by having electrons bounce back to the center of the cable.
- The most popular network cables today are CAT5e and CAT6 cables
- UTPs’ popularity is a result of its ease of use and expandability using the RJ45 connector
2. Coaxial
Figure 1: UTP and STP cables (htt)
- Coaxial cable conducts electrical signal using a solid copper wire surrounded by an insulating layer and all enclosed by a shield of woven metallic braid which are soldered at the ends to the BNC connectors.
- The shield protects the signal from outside EMI and preventing electron leakage from the centre of the cable.
- This property makes coaxial cable a good choice for carrying weak signals Coaxial cable is commonly used in CATV and RF installations.
3. Fiber Optic
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- A single fiber optic cable can carry about 90,000 TV stations, or 3 million full duplex telephone conversationFiber optic cables are bundles of glass fibers, smaller than a human hair, which are combined into a single cable. - Core - Thin glass center of the fiber where the light travels - Cladding - Outer optical material surrounding the core that reflects the light back into the core - Buffer coating - Plastic coating that protects the fiber from damage and moisture
- Light travels in a straight line and only in one direction at a time.
- The inside of the cable is like a mirror; so that light can travel down the core.
- Advantage - the light wave can travel great distances and is impervious to EMI and wiretapping.
Types of Connections
1. DSL – Digital Subscriber Line – dedicated circuit to Telcom – always on and provides high speed
internet using UTP
- Speed decreases the fartheryou are from switching centre
- DLS is multiplexed using a DSLAM – DSL Access Multiplexer
- Modem converts digital signal to discrete analog signal
- Assymetric – fast download, slower upload
2. Cable Modem
- Used CATV cable to provide internet access – uses coaxial
- The broadband signal splits CATV channels from the Internet channel.
- Assymetric speed like DSL
- Unlike DSL cable bandwidth shared - as traffic increases overall throughput decreases.
- Each cable modem uses Ethernet to connect to the local network providing DHCP services to local hosts. The cable modem works with the service provider’s cable modem termination system (CMTS) at the head office.
- The CMTS is responsible for connecting a group of customers to an Internet Service Provider (ISP) for connection to the internet. Downloaded Internet content is demodulated using QAM converting the radio frequency into a unique binary value.
- The upstream content is modulated using Quadrapture Phase Shift-Keying (QPSK). This modulation technique moves 2 bits at a time. A zero is represented as a 90 degree shift change and a 1 is the same waveform
3. Bell Fibe
- An all-digital IPTV streaming service,
- and delivered on Bell's high-speed fiber optic network. On the customer end is the IPTV modem which connects PVR which contains 1 TB hard drive for recording programs. The PVR includes an integrated TV receiver which can be connected via coaxial cable, Category 5 cable or wirelessly using 5 GHz – 802.11n.
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Synchronization Problem
Three methods have been developed to keep remote computers in time - even if they are in different parts of the world.
1. The best and most expensive way to keep computers in synch is to run a separate clocking wire
to between them. Used on MANs and some WANs, expensive and not practical if great distance.
2. The second method is to develop an encoding system which is “self-clocking”. Manchester
encoding is self-clocking, but too inefficient for high speed. NRZI is more efficient
3. The third method is to transmit data asynchronously so that timing is less of an issue.
applications like real time streaming or large data bases. Effecitive but too slow for data base or processor intensive applications.
Random
St Gap in
S Time
t
ar o
t
p
Samplingtime
4. The better approach is to create a synchronous transmission which is a large data block (for
Ethernet the block is 1500 bytes – Payload is much larger than indicated in the diagram).
- The address field holds the MAC address of the sending and receiving devices.
- The Control field is one or more bytes and contains information about the type of frame
- The Payload of the frame is the data field sent from the higher layer and is completely transparent to the DLL.
- The CRC refers to the “Cyclical Redundancy Check” which is a two byte field. The value of these bytes is the result of polynomial arithmetic based on every bit of data between the flags. Detects 99.99% of all errors. . 01111110 Address^ Control^ Payload^ CRC^01111110
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mission Advantages Disadvantages Asynchronous Simple, uses less hardware and programming. Equipment less expensive.
Low throughput because of high overhead and slower speed Synchronous High throughput because of less overhead and larger frame size
Requires more hardware and programming. Equipment more expensive
Data Link Layer Programming
Synchronous transmissions are the norm of network traffic.
- Each synchronous transmission has a flag of 126 in decimal or 7E in hexadecimal at the beginning and at the end of the frame. The flags are essential because they tell the receiving computer when a transmission starts and ends.
- The problem is if this value appears in anywhere else in the frame, the receiving computer could mistakenly interpret the value as the end of the frame. To avoid this program, the DLL layer has a build in programming routine called “bit stuffing”.
- Data link layer counts the ones and if 5 adds a 0, the receiving host after removing the flags, counts the ones and if 5 ones followed by a 0, removes the 0.
Simplex Half Duplex and Full Duplex
- Simplex - the data can only travel in one direction receive or send, but not both
- Half-duplex- Send and receive but not at the same time
- Full duplex – send and receive simultaneously.
Common Transmission Errors:
- Noise is called “thermal noise. Or random “spikes” – EIA\TIA strick limits on cable lenth
- EMI causes errors when cabling is located too close to noise sources which generate their own electromagnetic field,
- Crosstalk is a special type of electromagnetic interference. Cables with electrons travelling in opposite directions can pull electrons off of one cable, and they travel on the other cable, in the opposite direction.
- Jitter occurs when two computers “drift” out of synch with each other. This can happen if the receiving computer begins to sample the voltage too near the end of a clock cycle, it can misinterpret the value.
Error Detection
- Parity -One extra bit is sent along with the original bits to make the number of 1s either even, in case of even parity, or odd, in case of odd parity. The sender counts the number of 1s and if the number of 1a is odd, the layer adds a bit to keep the parity even, if even parity is used(even parity is more common than odd parity)
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32 bits
Netwo rk
n-b its
32-n bits
Defines
node
Subnet Mask
Hierarchy of Ipv4 Addressing
- Hides, or "masks," the network part of a system's IP address and leaves only the host part as the machine identifier.
- used for identifying the network segment to which packet belongs to.
Class Binary Dotted Decimal Prefix/CI
A 1111111.00000000.00000000.00000000 255.0.0.0 /
B 11111111.11111111.00000000.00000000 255.255.0.0 /
C 11111111.11111111.11111111.00000000 255.255.255.0 /
IPv4 and Changes to Preserve the Address Space
Prefix Suffix
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- Onlly 4.3 billion address run out in 2015The IPv4 address space,
- To preserve space as long as possible IETF made the following changes: o Private address spaces o NAT, Network Address Translator, was created to act as a proxy gateway converting private host ddresses to public addresses o DHCP, Dynamic Host Configuration Protocols was created to act as a server to allocate addresses from a pool of available LAN addresses o CIDR, Classless Inter-Domain Routing was a fundamental change in how IP addresses were assigned. The old class based system of allocating IPv4 addresses was discontinued for a classless system which used the 32 bit address space more efficiently, avoiding wasted IP addresses.
All of these network technologies we have heard before and are currently implemented on our home and Seneca networks. In North America, IPv4 will continue be used for a long time. Networks, won’t convert to IPv6 until new hardware is required.
IPv6 addressing
IPv6 uses three types of addresses -- unicast, multicast, and anycast. Unicast and multicast addresses also existed in IPv4, but Anycast is a new type of address defined by IPv6.
- Unicast: A unicast address is a one-to-one address. Packets send to a unicast address travel between two hosts on a single interface.
- Multicast: A multicast address is a one-to-many address. Packets sent to a multicast address travel to all interfaces identified by the multicast group address.(this replaces the broadcast address used in IPv4)
- Anycast: A anycast address is a one-to-one address sent to the nearest host. Packets sent to a anycast address are sent to a single interface of the nearest host identified by the address.
- Both IPv4 and IPv6 are connectionless protocols. “best effort” delivery system
- IP works with ICMP, (Internet Control Message Protocol) which is responsible for generating error messages when an error occurs during data transmission. The Transport layer protocols, namely TCP and UDP, decide to retransmit data based on the error message that is received. The table below is a comparison of the IPv4 and IPv6 layers and the protocol specifications of each layer.
Dual stackA
IPv4 IPv^6 IPv4Tunnel
ISP IPv only
Dual stackB
IPv
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- 128 bit address divided allow 8 16 bit blocks – then converted to hexadecimal
- 0s can be compressed using a “::” – but can be used only once in an address
- In dual environment special format for IPv4 used in low order bits ::192.168.2.1/
- 127.0.0.1 represented as ::1/
Tunnelling
- The dual stack approach is an important feature to ensure compatibility between IPv4 and IPv hosts. To allow packets to travel across IPv4 only device IPv6 over IPv4 tunneling has been defined.
- IPv6 packet encapsulated in IPv4 packet to pass through IPv4 only device.
- DNS records are used to determine if device IPv4 or IPv6 only or IPv4/v - IPv6 addresses are trued first, if fail try IPv4.
IPv6: A Programmer’s Perspective
From a programmer’s perspective IPv6 is very different that IPv4 programming and will present some new challenges.
- User Interface Design: Unlike the IPv4 address space, the IPv6 address space is much larger and must be displayed in hexadecimal notation. An advantage of dotted decimal notation was that it was very predictable when displaying addresses. IPv6 on the other hand, with its truncated notation of using double colons to represent a series of zeros is less predictable. Lastly, GUI application Users will normally use DNS names to identify hosts not IP addresses. Thus, GUI applications which supply text boxes, such as TCP/IP properties, would be unnecessary and should be avoided given the complexity of IPv6 format over IPv4. However, applications used by administrators would need such text boxes. - Should number based or named base notation be used? - Should the truncated addresses be used in the interface? The double colon is an optional method of notation to simplify the address, not a specification.
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- Does the user need specific parts of the address, such as the subnet prefix, scope identifier or other subfields?
- IP Family Independent: With a dual stack environment it is important not to write code that is family specific, such as IPv4 or IPv6. Family specific code can’t be handled correctly in a dual stack environment. The best approach is to use data structures and functions that are family independent.
- Determine IP Family Before Creating Socket: When creating a socket, the common procedure in IPv4 was to create the socket, and bind the address family information to the socket. This procedure will not work in IPv6. The address family must be determined first, then create the socket and bind the family to it. In IPv4, a node normally has a single IPv4 address associated with it. In IPv6, it is normal to have multiple IP addresses onto a single node. More specifically, IPv6 addresses are assigned to interfaces, not to nodes. An interface can have multiple IPv addresses
- if the user is to enter an IPv6 address as part of the URL, the address must be enclosed in square brackets to avoid ambiguity with the port number which is also separated by a colon. For example: http://[F380:DC28:ffff::1]:80/ ..
