بحث Media For Local Area Network

[/LIST]
Ethernet :
Ethernet is the most common LAN medium. In its traditional form, it is 10 Megabits per second (Mbps), shared-access network that maybe run on either coaxial cable or unshielded twisted pairs of copper wire.
Its primary advantages are that it is widely available, inexpensive, and very flexible - to add a new connection, you need only connect a cable between the workstation and the coaxial cable or twisted pair hub.
Its chief disadvantages are that two fast machines can monopolise the network while communicating with each other, and it is difficult to provide security because any single machine can monitor all traffic on a segment.

What are the advantages of Ethernet?

A direct, Ethernet connection provides substantial advantages over dial-up service. Here's why :

@ The Internet connection is continuously active (provided that your computer is turned on).

@ You do not have to dial-up.

@ One jack per occupant ensures that you have a personal connection to the Internet (no waiting or competition for a phone line).

@ The phone line is always available for talking.

@ There are no dropped lines so you will not be bumped off.

@ An Ethernet connection is much faster than a modem connection.

@ Students have access to academic fileservers to store and retrieve files.


Disadvantages :

@ Ethernet requires physical cables to connect devices, negating any benefits of a wireless solution.

@ Cabling costs can quickly outweigh any cost savings over a wireless solution.

@ The packet switching technology of Ethernet can cause considerable delays when more than 40% of users attempt to transmit at the same time.



Fast Ethernet :

Fast Ethernet is simply the traditional 10 Mbps Ethernet running at 100 Mbps. All the frame formats and media access protocols are the same as for 10 Mbps Ethernet, and like 10 Mbps Ethernet, Fast Ethernet maybe shared or switched, it may be seamlessly joined with 10 Mbps Ethernet.

The changes in Fast Ethernet occur in the network adapter card, the cabling, and the hub or switch. All of these components must be capable of supporting the faster communications speeds of Fast Ethernet.

Fast Ethernet is popular in part because because its 10 Mbps counterpart is well understood, and there isn't much difference between it and Fast Ethernet. However, where the differences do occur is important. Because Fast Ethernet runs at a higher speed, it is much less tolerant of violations of its cabling standards than Ethernet is. Individual copper cable runs must be limited to 100 metres total, as is specified for Ethernet, but additionally there may be only two class II repeaters between devices in a Fast Ethernet network, and those repeaters maybe seperated by a distance no greater than 5 metres. This means that the maximum length between any pair of devices in a Fast Ethernet network is only 205 metres. 10 Mbps Ethernet where each cable may be 100 metres, and up to four repeaters are permitted for a total length of 500 metres between any pair of stations.


Advantages of The Fast Ethernet :

@ One advantage is translation in moving 100-Mbps packets onto your 10-Mbps Ethernet network. In some topologies--like FDDI--packets must be translated from FDDI to Ethernet--an expensive intermediate step. Because 802.3u uses the same CSMA/CD protocol, translation is quick, easy and inexpensive.

@ Another advantage is choice of media. The 802.3u standard allows for several types, including 100BASE-TX (a two-pair Category 5 UTP connection), 100BASE-T4 (a four-pair Category 3 UTP connection) and 100BASE-FX (a fiber optic standard). All cards we tested only support the 100BASE-TX standard, but several will support the 100BASE-T4 standard soon. Intel officially is working on such a release.

@ A final advantage is cost and availability. Fast Ethernet and 802.3u are here now, and you can buy Fast Ethernet cards today for under $300 per interface.
What are the disadvantages?

@ Fast Ethernet is much slower than using the interconnect in a traditional supercomputer...

o 100Tx -- 100 microsecond latency, 100Mbits/s
o Gigabit Ethernet -- 100 microsecond latency, 1000 Mbits/s
o Myrinet -- 20 microsecond latency, 1250 Mbits/s

@ More traditional supercomputers

o SP switch -- single digit microsecond latency, 1200 Mbytes/s
o E10000 crossbar -- 300 nanosecond latency, 12.8 Gbytes/s
o Origin2000 CrayLink -- 1.6 Gbytes/s


Token Ring :

While called a ring, in practice the stations in a Token Ring network network are connected at one end of a spoke, with the other end connected to a device called a MAU. Then, the MAUs are connected to each other to form a pair of rings. The primary ring is used for communications, and the secondary ring is there for redundancy. This dualing structure is one of Token Rings greatest strengths - if the primary ring should fail, perhaps because of a cable brea, the second ring can be used as a built-in redundant path automatically.

The second major strength of Token Ring is its resistance to being monopolised by a pair of fast machines. Because of the token-passing design, every machine on the Token Ring that wishes to transmit is guaranteed a fair share of the available network bandwidth, traditionally 4 Mbps, but 16 Mbps is becoming increasingly common.

Its chief disadvantage is its limited availability. While many machines come with an Ethernet interface, or one that can be readily purchased, Token Ring interfaces are not as widely available, and when they are, they are often more expensive than an @@@@@alent Ethernet interface.


Advantages & Disadvantages :

Every technology has advantages and disadvantages, and Token Ring is not an exception. These advantages and disadvantages need to be weighed against the needs of an organization when the decision of which technology is made. Some of the advantages and disadvantages of Token Ring are listed below :

Advantages :

@ "Deterministic" Nature Token Ring networks are deterministic in nature. In English, this means that the nodes may only transmit at certain, well defined times. This eliminates incidents of two or more nodes transmitting at the same time and corrupting each other's transmission (collisions). The elimination of collisions results in a network where data almost always gets to its destination on the first attempt, greatly reducing retransmissions which waste bandwidth. The end result is a higher effective bandwidth at the same wire speed.

Disadvantages :

@ Network Planning Required Token Ring networks, especially those using passive MSAUs, must be properly preplanned before installation. This is due to limitations in the maximum length of the ring which can be handled. If a Token Ring network is simply "thrown together" without following the rules on our Design Rules page, it may not work.

@ Difficult to Physically Install Token Ring networks using STP cable can be very difficult to physically install due to the size, weight, and stiffness of the STP cable. Networks using UTP cable avoid these difficulties, but are very limited in distance and number of nodes which can be put on one ring.

@ Difficult Troubleshooting All ring networks share a somewhat difficult troubleshooting procedure. If any node or cable run fails, there is not usually an easy way to determine where the fault has occurred. This results in a long and tedious process of elimination where the network must be broken into several small pieces and each piece troubleshot individually until the problem is found and corrected. If an intermittant problem is encountered, it may be nearly impossible to find and correct it.

@ Cost Token Ring network equipment costs several times as much as Ethernet hardware. This is due to the complex token passing protocol which increases the difficulty of manufacturing such equipment. Networks built around STP cable require the use of a specialized, expensive cable which is not very useful for other applications, and networks built around UTP are so limited in size and maximum number of nodes that they generally require many small rings to be interconnected with costly bridges and routers.

@ Token Ring networks give the benefit of a very fast. deterministic network, but require a major commitment to properly plan and maintain. Before choosing Token Ring as the technology for a particular project, it is very important to ensure that the organization has the ability to commit the time and resources necessary. Not doing so is basically a recipe for disaster.