2.  CSMA/CD  Token Passing  FDDI  DQDB  Wireless LANs

3.  Every LAN consist a collection of devices that requires to share the network’s transmission capacity.  A means of controlling access to the transmissions is required to provide an orderly and efficient use of that capacity.  Medium access control uses the where and how technique.

4.  Is control exercised in a centralized or distributed fashion.  In centralized scheme, a controller is designated that has the authority to grant access to the network, and a station wishing to transmit has to wait till permission is granted by the controller.  In decentralized network, the stations collectively perform a medium access control function to determine dynamically the order in which stations transmit.

5.  Greater control over access  Relatively simple access logic at each station  Avoids problems of distributed coordination among peer entities.

6.  It creates a single point of failure  Might act as a bottleneck, reducing performance. (in a decentralized scheme, pros and cons are vice versa)

7.  How, is constrained by the topology and is a tradeoff among competing factors, including cost, performance, and complexity.  In general access control techniques are categorized as being either synchronous or asynchronous.  Synchronous: A specific capacity is dedicated to a connection (not usually suitable for LAN as station needs are unpredictable)  It is preferred to be able to allocate capacity in an asynchronous manner, more or less in response to immediate demand.

8.  The IEEE 802.3 standard, know as Ethernet, now encompasses data rates of 10 Mbps, 100 Mbps, 1 Gbps and 10 Gbps.  For lower data rates, the CSMA/CD MAC protocol is used.  The most widely used high-speed LANs today are based on Ethernet and were developed by the IEEE 802.3 standard committee.

9.  CSMA/CD and its precursors could be referred to as random access techniques. Station transmission are ordered randomly.  With CSMA, a station wishing to transmit first listen to the medium to determine if another transmission is in progress (carrier sense)  With CSMA, an algorithm is needed to specify what a station should do if the medium is found busy. One such algorithm is nonpersistent CSMA

10. 1- If the medium is idle, transmit; otherwise go to step 2 2- If the medium is busy, wait an amount of time and repeat step 1  Usage of random delays reduces the probability of collisions.  Issue: capacity wasted because the medium would remain idle even if there are one or more stations ready for transmission.  To avoid idle channel time, the 1-persisten protocol can be used.

11.  A station wishing to transmit listen to the medium and obeys the following rules 1- If the medium is idle, transmit; otherwise go to step 2 2- if the medium is busy, continue to listen until the channel is sensed idle, then transmit immediately.  If two stations wait, collision is guaranteed!  A comparison that attempts to reduce collision and reduce idle time is p-persistent

12. 1- If the medium is idle transmit with probability p, and delay one time unit with probability (1 – p). The time unit is typically equal to the maximum propagation delay. 2- If the medium is busy, continue to listen until the channel is idle and repeat step 1. 3- If the transmission is delayed one time unit, repeat step 1.

13.  Under heavy load: if n stations have framed to send while a transmission is in progress, at the end of the transmission, the expected number of stations that will attempt to transmit is equal to the number of stations ready to transmit times the probability of transmitting (np)  It np is greater than 1, on average multiple stations will attempt to transmit and collision would occur.  Each station on realizing the collision occurred, they will be back again resulting in more collisions.  The retries with new transmissions from other stations increases the collision probability.  Eventually all stations will try to transmit, with a zero throughput.  To avoid this np must be less than 1 for the expected peaks of n; and if a heavy load is expected to occur p must be small.  As p is made smaller, stations must wait longer to attempt transmission.

14.  When two frames collide, the medium remains unsuitable for the duration of transmission of both damaged frames. For long frames, compared to propagation time, the amount of wasted capacity can be considerable. This waste can be reduced if a station continues to listen to the medium while transmitting.

15.  This leads to the following rules for CSMA/CD  1- if the medium is idle transmit; otherwise go to step 2  2- If the medium is busy, continue listening until the channel is idle, then transmit immediately.  3- if a collision is detected during transmission, transmit a brief jamming signal to assure that all stations know that there has been a collision and then cease transmission.  4- After transmitting the jamming signal, wait a random amount of time, referred to as the backoff, then attempt to transmit again (from step 1)

16.  An important rules followed in most CSMA/CD systems, including the IEEE standards, is that frames should be long enough to allow collision detection prior to the end of transmission.  If shorter frames are used, then collision detection does not occure.

17.  The IEEE 802.5 token ring standard is an outgrowth of IBM’s commercial token ring LAN product.

18.  A ring consist of a number of repeaters, each connected to two others by unidirectional transmission links to form a single close path.  Data are transferred from repeater to repeater sequentially. Each repeater regenerates and transmits each bit.  For a ring to operate as a communication network, data insertion, data reception and data removal functions are required.

19.  These functions are provided by the repeaters. Repeater act as a device attachment point.  Data insertion is accomplished by the repeater. Data are transferred in packets, each which contains a destination address field. As a packet passes past a repeater, the address field is copied. If the attached device recognizes the address, the remainder of the packet is copied.  Repeaters perform the data insertion and reception functions  A packet could be removed by the addressed repeater. Alternatively, each packet could be removed by the transmitting repeater after it has made one trip around the loop (auto acknowledgment and multicast addressing).  A variety of strategies can be used to determine how and when a packet is inserted onto the ring. The most common method is TOKEN RING

20.  The token ring technique is based on the use of a small frame, called a token, that circulates when all stations are idle. A station wishing to transmit must wait until it detects a token passing by.  It then seizes the token by changing one bit in the token, which transform it from a token to a start-of-frame sequence for a data frame. The station then appends and transmits the remainder of the fields needed to construct a data frame.  When a station seizes a token and begins to transmit a data frame, there is no token on the ring, so other stations wishing to transmit must wait. The frame on the ring will make a round trip and be absorbed by the transmitting station. In the default operation, the transmitting station will insert a new token on the ring when both the following conditions have been met: 1-The station has completed transmission of its frame 2- The leading edge of the transmitted frame has returned to the station.

21.  Under heavy loads, token rings are of advantage.  The principle advantage of token ring is the flexible control over access that it provides.  The principle disadvantage of token ring is the requirement for token maintenance.  Loss of a token prevents further utilization of the ring.  Duplication of the token can also disrupt ring operation.

22.  It is a fiber-optic token ring LAN running at a speed of 100 Mbps  FDDI standards cover the physical and data layers of the OSI model.  1000 nodes, 2Km with fiber length 200Km  Used as a backbone to connect LANs  Uses LED rather than laser.

23.  A network is a collection of interconnected access points with a software protocol structure that enables communication.  Networks allow many different types of data transfer, using software to implement the networking protocols and to provide flow control, error detection, and error recovery.  Networks typically manage transfers between end systems over local, metropolitan or wide area distances.

24.  Fiber channel is designed to combine the best features of both technologies; simplicity and speed of channel communications with the flexibility and interconnectivity that characterizes protocol-based network communications.

25.  The Fiber Channel Industry Association, which is the industry consortium promoting Fiber Channel, lists the following ambitious requirements that Fiber Channel industry is intended to satisfy:  1- Full-duplex links with two fibers per link  2- Performance from 100 Mbps to 800 Mbps on a single line  Support for distances up to 10 km  Small connectors  High-capacity utilization with distance insensitivity  Broad availability; standard components  Support for multiple cost/performances levels, from small systems to super-computers  Ability to carry multiple existing interfaces command sets for existing channel and network protocols.

26.  Data Over Cable Service Interface Distributed Queue Dual Bus (DQDB) is a Data-Link layer communication protocol for Metropolitan Area Networks, specified in the IEEE 802.6 standard, designed for use in MANs.  For a MAN to be effective it requires a system that can function across long, ‘city-wide’ distances of several miles, have a low susceptibility to error, adapt to the number of nodes attached and have variable bandwidth distribution.  Due to large distances inherent to MANs a different protocol is necessary compared to LANs

27. http://nislab.bu.edu/sc546/sc441Spring 2003/dqdb/index.html

28.  As the name suggests, a wireless LAN is one that makes use of a wireless transmission medium.  Until recently wireless LANs were little used due to high prices, low data rates, occupational safety concerns, etc.  With these problems solved usage of Wireless LANs has grown rapidly.

29.  Throughput (maximize capacity)  Number of nodes (100s across multiple cells)  Connection to backbone LAN (using control modules that connect to both type LANs)  Service Area (100-300 m)  Battery power consumption (Mobile workers user battery-powered workstations)

30.  Transmission robustness and security (reliable transmission in noisy environments and secure from eavesdropping)  Collocated network operations  License-free operation (no license for the frequency band)  Handoff/roaming (MAC protocols should enable mobility)  Dynamic configuration (dynamic/automated addition, deletion and relocation without disruption)

31. http://www.microsoft.com/windowsxp/ using/networking/setup/adhoc.mspx

32.  Infrared (IR) LANs (An individual cell of an IR LAN is limited to a single room, because infrared light does not penetrate opaque walls)  Spread spectrum LANs (This type of LAN makes use of spread spectrum transmission technology. In most cases, these LANs operate in the ISM (industrial, scientific, and medical) bands so that no Federal Communications Commission licensing is required for their use in the US  Narrowband microwave (These LANs operate at microwave frequencies but do not use spread spectrum. Some of these products operate at frequencies that require FCC licensing, while others use one of the unlicensed ISM bands)

33.  Advantages:  Diffusely reflected by light-colored objects, thus, using ceiling reflection an entire room could be covered.  Does not penetrate walls or other opaque objects (secure and separation)  Relatively inexpensive  Disadvantage: Background IR radiation acts as noise.

34.  Direct-beam IR : Range depends on emitted power and on the degree of focusing.  Can have a range of Kilometers  Used for cross-building interconnections within a line of sight.  By using IT transceviers, point-to-point IR links can be used to create a token ring LAN indoor.

35.  Omnidirectional Configuration : A single base station within line of sight of all other stations on the LAN  Diffused configuration: All IR transmitters are focused and aimed at a point on a diffusely reflecting ceiling.

36.  Usually multiple-cell arrangement, adjacent cells uses different center frequencies within the same band to avoid interference.  Cell topologies; hub or peer to peer  Hub topology uses a hub, peer to peer does not. (cordless telephones, wireless microphones, radios)

37.  The term narrowband microwave refers to the use of a microwave radio frequency band for signal transmission, with a relatively low bandwidth.  Licensed Narrowband RF: RF usable for voice, data and video transmission are licensed and coordinated within specific geographic areas to avoid potential interference between systems. To provide security from eavesdropping all transmissions are encrypted.  Licensed narrowband LANs guarantee interference-free communication.