2. cnp3-ebook
3. principles/sharing
4. English
5. Zen

56 ../../principles/sharing.rst:369
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Medium Access Control algorithms 32/10000 · 32
57 ../../principles/sharing.rst:374
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The common problem among Local Area Networks is how to efficiently share the available bandwidth. If two devices send a frame at the same time, the two electrical, optical or radio signals that correspond to these frames will appear at the same time on the transmission medium and a receiver will not be able to decode either frame. Such simultaneous transmissions are called `collisions`. A `collision` may involve frames transmitted by two or more devices attached to the Local Area Network. Collisions are the main cause of errors in wired Local Area Networks. 563/10000 · 563
58 ../../principles/sharing.rst:376
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — All Local Area Network technologies rely on a `Medium Access Control` algorithm to regulate the transmissions to either minimize or avoid collisions. There are two broad families of `Medium Access Control` algorithms : 218/10000 · 218
59 ../../principles/sharing.rst:378
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — `Deterministic` or `pessimistic` MAC algorithms. These algorithms assume that collisions are a very severe problem and that they must be completely avoided. These algorithms ensure that at any time, at most one device is allowed to send a frame on the LAN. This is usually achieved by using a distributed protocol which elects one device that is allowed to transmit at each time. A deterministic MAC algorithm ensures that no collision will happen, but there is some overhead in regulating the transmission of all the devices attached to the LAN. 546/10000 · 546
60 ../../principles/sharing.rst:379
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — `Stochastic` or `optimistic` MAC algorithms. These algorithms assume that collisions are part of the normal operation of a Local Area Network. They aim to minimize the number of collisions, but they do not try to avoid all collisions. Stochastic algorithms are usually easier to implement than deterministic ones. 313/10000 · 313
61 ../../principles/sharing.rst:382
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — We first discuss a simple deterministic MAC algorithm and then we describe several important optimistic algorithms, before coming back to a distributed and deterministic MAC algorithm. 184/10000 · 184
62 ../../principles/sharing.rst:386
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Static allocation methods 25/10000 · 25
63 ../../principles/sharing.rst:388
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — A first solution to share the available resources among all the devices attached to one Local Area Network is to define, `a priori`, the distribution of the transmission resources among the different devices. If `N` devices need to share the transmission capacities of a LAN operating at `b` Mbps, each device could be allocated a bandwidth of :math:`\frac{b}{N}` Mbps. 369/10000 · 369
64 ../../principles/sharing.rst:392
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Limited resources need to be shared in other environments than Local Area Networks. Since the first radio transmissions by `Marconi <http://en.wikipedia.org/wiki/Guglielmo_Marconi>`_ more than one century ago, many applications that exchange information through radio signals have been developed. Each radio signal is an electromagnetic wave whose power is centered around a given frequency. The radio spectrum corresponds to frequencies ranging between roughly 3 KHz and 300 GHz. Frequency allocation plans negotiated among governments reserve most frequency ranges for specific applications such as broadcast radio, broadcast television, mobile communications, aeronautical radio navigation, amateur radio, satellite, etc. Each frequency range is then subdivided into channels and each channel can be reserved for a given application, e.g. a radio broadcaster in a given region. 880/10000 · 880
65 ../../principles/sharing.rst:397
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — `Frequency Division Multiplexing` (FDM) is a static allocation scheme in which a frequency is allocated to each device attached to the shared medium. As each device uses a different transmission frequency, collisions cannot occur. In optical networks, a variant of FDM called `Wavelength Division Multiplexing` (WDM) can be used. An optical fiber can transport light at different wavelengths without interference. With WDM, a different wavelength is allocated to each of the devices that share the same optical fiber. 517/10000 · 517
66 ../../principles/sharing.rst:402
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — `Time Division Multiplexing` (TDM) is a static bandwidth allocation method that was initially defined for the telephone network. In the fixed telephone network, a voice conversation is usually transmitted as a 64 Kbps signal. Thus, a telephone conservation generates 8 KBytes per second or one byte every 125 microseconds. Telephone conversations often need to be multiplexed together on a single line. For example, in Europe, thirty 64 Kbps voice signals are multiplexed over a single 2 Mbps (E1) line. This is done by using `Time Division Multiplexing` (TDM). TDM divides the transmission opportunities into slots. In the telephone network, a slot corresponds to 125 microseconds. A position inside each slot is reserved for each voice signal. The figure below illustrates TDM on a link that is used to carry four voice conversations. The vertical lines represent the slot boundaries and the letters the different voice conversations. One byte from each voice conversation is sent during each 125 microseconds slot. The byte corresponding to a given conversation is always sent at the same position in each slot. 1115/10000 · 1115
67 ../../principles/sharing.rst:440
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — TDM as shown above can be completely static, i.e. the same conversations always share the link, or dynamic. In the latter case, the two endpoints of the link must exchange messages specifying which conversation uses which byte inside each slot. Thanks to these control messages, it is possible to dynamically add and remove voice conversations from a given link. 362/10000 · 362
68 ../../principles/sharing.rst:442
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — TDM and FDM are widely used in telephone networks to support fixed bandwidth conversations. Using them in Local Area Networks that support computers would probably be inefficient. Computers usually do not send information at a fixed rate. Instead, they often have an on-off behavior. During the on period, the computer tries to send at the highest possible rate, e.g. to transfer a file. During the off period, which is often much longer than the on period, the computer does not transmit any packet. Using a static allocation scheme for computers attached to a LAN would lead to huge inefficiencies, as they would only be able to transmit at :math:`\frac{1}{N}` of the total bandwidth during their on period, despite the fact that the other computers are in their off period and thus do not need to transmit any information. The dynamic MAC algorithms discussed in the remainder of this chapter aim to solve this problem. 922/10000 · 922
69 ../../principles/sharing.rst:446
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — ALOHA 5/10000 · 5
70 ../../principles/sharing.rst:450
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — In the 1960s, computers were mainly mainframes with a few dozen terminals attached to them. These terminals were usually in the same building as the mainframe and were directly connected to it. In some cases, the terminals were installed in remote locations and connected through a :term:`modem` attached to a :term:`dial-up line`. The university of Hawaii chose a different organization. Instead of using telephone lines to connect the distant terminals, they developed the first `packet radio` technology [Abramson1970]_. Until then, computer networks were built on top of either the telephone network or physical cables. ALOHANet showed that it is possible to use radio signals to interconnect computers. 708/10000 · 708
71 ../../principles/sharing.rst:454
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The first version of ALOHANet, described in [Abramson1970]_, operated as follows. First, the terminals and the mainframe exchanged fixed-length frames composed of 704 bits. Each frame contained 80 8-bit characters, some control bits and parity information to detect transmission errors. Two channels in the 400 MHz range were reserved for the operation of ALOHANet. The first channel was used by the mainframe to send frames to all terminals. The second channel was shared among all terminals to send frames to the mainframe. As all terminals share the same transmission channel, there is a risk of collision. To deal with this problem as well as transmission errors, the mainframe verified the parity bits of the received frame and sent an acknowledgment on its channel for each correctly received frame. The terminals on the other hand had to retransmit the unacknowledged frames. As for TCP, retransmitting these frames immediately upon expiration of a fixed timeout is not a good approach as several terminals may retransmit their frames at the same time leading to a network collapse. A better approach, but still far from perfect, is for each terminal to wait a random amount of time after the expiration of its retransmission timeout. This avoids synchronization among multiple retransmitting terminals. 1310/10000 · 1310
72 ../../principles/sharing.rst:456
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The pseudo-code below shows the operation of an ALOHANet terminal. We use this python syntax for all Medium Access Control algorithms described in this chapter. The algorithm is applied to each new frame that needs to be transmitted. It attempts to transmit a frame at most `max` times (`while loop`). Each transmission attempt is performed as follows. First, the frame is sent. Each frame is protected by a timeout. Then, the terminal waits for either a valid acknowledgment frame or the expiration of its timeout. If the terminal receives an acknowledgment, the frame has been delivered correctly and the algorithm terminates. Otherwise, the terminal waits for a random time and attempts to retransmit the frame. 714/10000 · 714
73 ../../principles/sharing.rst:475
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — [Abramson1970]_ analyzed the performance of ALOHANet under particular assumptions and found that ALOHANet worked well when the channel was lightly loaded. In this case, the frames are rarely retransmitted and the `channel traffic`, i.e. the total number of (correct and retransmitted) frames transmitted per unit of time is close to the `channel utilization`, i.e. the number of correctly transmitted frames per unit of time. Unfortunately, the analysis also reveals that the `channel utilization` reaches its maximum at :math:`\frac{1}{2 \times e}=0.186` times the channel bandwidth. At higher utilization, ALOHANet becomes unstable and the network collapses due to collided retransmissions. 692/10000 · 692
74 ../../principles/sharing.rst:478
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Amateur packet radio 20/10000 · 20
75 ../../principles/sharing.rst:480
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Packet radio technologies have evolved in various directions since the first experiments performed at the University of Hawaii. The Amateur packet radio service developed by amateur radio operators is one of the descendants ALOHANet. Many amateur radio operators are very interested in new technologies and they often spend countless hours developing new antennas or transceivers. When the first personal computers appeared, several amateur radio operators designed radio modems and their own datalink layer protocols [KPD1985]_ [BNT1997]_. This network grew and it was possible to connect to servers in several European countries by only using packet radio relays. Some amateur radio operators also developed TCP/IP protocol stacks that were used over the packet radio service. Some parts of the `amateur packet radio network <http://www.ampr.org/>`_ are connected to the global Internet and use the `44.0.0.0/8` IPv4 prefix. 926/10000 · 926