Reasonable rate of return(what it is, good & bad, how it affected telephone company)
- For every dollar they spent, they got reimbursed & made a profit off it (ATT)
- Guaranteed not to lose money
- It allowed R&D to improve which lead to our great network now
Under the Bell System monopoly (post Communications Act of 1934), the Bell System owned the phones and did not allow interconnection, either of separate phones (or other terminal equipment) or of other networks
Monopoly, natural monopoly
ATT justified their monopoly saying they were a "natural monopoly" the more people on their network, the more valuable it is & more beneficial so it prevented them from law suits until late 70's
Monopoly of MCI decision and Carterphone decision (happened at the same time did did the decisions affect the growth of the telephone industry)
This victory for Hush-A-Phone was widely considered a watershed moment in the development of a secondary market for terminal equipment, in addition to contributing to the breakup of the Bell System. It and the related Carterfone decision were seen as precursors to the entry of MCI Communications and the development of more pervasive telecom competition.
- In 1978 government sued Att for being a monopoly, then in 1984 divestiture occured and Att had to break up into smaller regional bell operating companies
- Gave Att long distance
- Main influence is that it seperated Att from homes
Telecommunications Act of 1934
The Act replaced the Federal Radio Commission with theFederal Communications Commission(FCC). It also transferred regulation of interstate telephone services from theInterstate Commerce Commission to the FCC.
OSI Model (layers)
- data bits (zeros or ones) over a communication circuit, layer defines the rules by which ones and zeros are transmitted, such as voltages of electricity, number of bits sent per second, and the physical format of the cables and connectors used.
- 2)Data Link: performs error detection and correction. It also decides when a device can
- transmit so that two computers do not try to transmit at the same time, it must mark where a message starts and where
- it ends
- 3)Network:r performs routing. It determines the next
- computer the message should be sent to so it can follow the best route through the
- network and finds the full address for that computer if needed.
- 4) Transport: deals with end-to-end issues, such
- as procedures for entering and departing from the network, It is responsible for breaking a large data
- transmission into smaller packets (if needed), Although error control
- is performed by the data link layer, the transport layer can also perform error checking
- 5)Session: must arrange for all the desired and required services
- between session participants, such as logging onto circuit equipment, transferring files,
- and performing security checks
- 6) Presentation: Its job is to accommodate different interfaces on different computers
- so the application program need not worry about them. It is concerned with displaying,
- formatting, and editing user inputs and outputs.
- 7) Application: the end user’s access to the
- network. The primary purpose is to provide a set of utilities for application programs.
- contention computers wait until the circuit is free and then transmit whenever they have data to send. Contention is commonly
- used in Ethernet LANs. contention approaches work better than controlled approaches for small
- networks that have low usage.
- Most wireless LANS use controlled access. In this case, the wireless access point controls
- the circuit and determines which clients can transmit at what time. There are two
- commonly used controlled access techniques: access requests and polling
- The reverse is true for large networks with high usage: controlled access works
- better. In high-volume networks, many computers want to transmit, and the probability
- of a collision using contention is high
- With the access request technique, client computers that want to transmit send a
- request to transmit to the device that is controlling the circuit
- Polling is the process of sending a signal to a client computer that gives it permission
- to transmit. Periodically, the controlling device (e.g., a wireless access point) polls the client to see
- if it has data to send. If the client has data to send, it does so. If the client has no data to
- send, it responds negatively, and the controller asks another client if it has data to send.
Cyclical Redundancy Check
- It adds 8, 16, 24, or 32 bits to the message. The remainder, R, is appended to the message as the error-checking characters
- before transmission. The receiving hardware divides the received message by the same
- G, which generates an R. The receiving hardware checks to ascertain whether the received
- R agrees with the locally generated R. If it does not, the message is assumed to be in error.
Copper wire vs Fiber optic
How to convert digital to analog
- With amplitude modulation (AM) the amplitude or height of the wave is changed. One amplitude is the
- symbol defined to be 0, and another amplitude is the symbol defined to be a 1
- Frequency modulation (FM) is a
- modulation technique whereby each 0 or 1 is represented by a number of waves per second
- Phase modulation (PM) refers to the direction in which the wave begins. Until now,
- the waves we have shown start by moving up and to the right Waves can also start down and to the right.
Analog to digital
- This is done by sampling the amplitude of the sound
- wave at regular intervals and translating it into a binary number. A quick glance will show that the digitized signal is only a rough approximation
- of the original signal. The original signal had a smooth flow, but the digitized signal has
- jagged “steps.” The difference between the two signals is called quantizing error. The first method is to increase the number of
- amplitude levels.The second method is to sample more frequently.
Difference between digital and analog
- • Digital transmission produces fewer errors than analog transmission. Because the
- transmitted data is binary (only two distinct values), it is easier to detect and correct
- • Digital transmission permits higher maximum transmission rates. Fiber-optic cable,
- for example, is designed for digital transmission.
- • Digital transmission is more efficient. It is possible to send more data through a
- given circuit using digital rather than analog transmission.
- • Digital transmission is more secure because it is easier to encrypt.
- • Finally, and most importantly, integrating voice, video, and data on the same circuit
- is far simpler with digital transmission.
- The bandwidth is the difference between the highest and the lowest
- frequencies in a band or set of frequencies
- Multiplexing means to break one high-speed physical communication circuit into several
- lower-speed logical circuits so that many different devices can simultaneously use it but
- still “think” that they have their own separate circuits
- Frequency division multiplexing (FDM) can be
- described as dividing the circuit “horizontally” so that many signals can travel a single
- communication circuit simultaneously. The circuit is divided into a series of separate
- channels, each transmitting on a different frequency, much like series of different radio
- or TV stations. All signals exist in the media at the same time, but because they are on
- different frequencies, they do not interfere with each other. FDM was commonly used in older telephone systems,
- Time division multiplexing (TDM) shares a communication
- circuit among two or more computers by having them take turns, dividing the
- circuit vertically, so to speak TDM generally is more efficient
- than FDM because it does not need guardbands.
- Statistical time division multiplexing
- (STDM) is the exception to the rule that the capacity of the multiplexed circuit must
- equal the sum of the circuits it combines. STDM allows more terminals or computers to be connected to a circuit than does FDM or TDM STDM is called statistical because selection of transmission speed for the multiplexed
- circuit is based on a statistical analysis of the usage requirements of the circuits to be
- multiplexed. When traffic is particularly heavy, you may have a 1- to 30-second delay. The
- second problem is that because the logical circuits are not permanently assigned to
- specific devices as they are in FDM and TDM
- Wavelength division multiplexing (WDM) is a
- version of FDM used in fiber-optic cables each
- logical circuit is assigned a different frequency, and the devices attached to the circuit
- don’t “know” they are multiplexed over the same physical circuit.
- e. Inverse multiplexing
- (IMUX) combines several low-speed circuits to make them appear as one high-speed
- circuit to the userOne of the most common uses of IMUX is to provide T1 circuits for WANs. T1 circuits
- provide data transmission rates of 1.544 Mbps by combining 24 slower-speed
- circuits (64 Kbps). As far as the users are concerned, they have access to one highspeed
- circuit, even though their data actually travel across a set of slower circuits.
- In these networks, the client is responsible for the presentation logic,
- whereas the server is responsible for the data access logic and data storage. The application
- logic may either reside on the client, reside on the server, or be split between both
- a Web browser to get pages from a Web server, you used a client-server architecture.
- Likewise, if you’ve ever written a program that uses SQL to talk to a database on a
- server, you’ve used a client-server architecture.
- it can be difficult to get software from different vendors to work
- together. One solution to this problem is middleware, software that sits between the application
- software on the client and the application software on the server
connects multiple LANs