1. The IEEE 802.3 committee has developed the following concise notation to distinguish the physical layer medium alternatives.
Accordingly, distinguish between 10BASE5, 10BASE2, 10BASE-T, and 10BASE-F.
10BASE5 – 10Mbps Baseband (a physical medium dedicated only to Ethernet) 500 meters 10BASE2 – 10Mbps Baseband 189/200 meters 10BASE-T – 10Mbps Baseband Ethernet/UTP 10BASE-F – 10Mbps Baseband Fiber
2. What are the physical layer medium alternatives used for Fast Ethernet?
100baseT4 Physical Layer :
This physical layer defines the specification for 100BaseT Ethernet over 4 pairs of either Category 3, 4 or 5 UTP wire. This allows 100BaseT to run over existing Category 3 wiring, which is the most popular wiring used today. 100BaseT4 is a half duplex signalling that uses three pairs of wire for 100Mbps signal travelling, and the fourth is used for collision detection. This method lowers the 100BaseT4 signaling to 33.3 Mbps per pair, which translates into a clock rate of 33 MHz. Unfortunately this violates the limit of 30 MHz set for UTP cabling. Therefore 100BaseT uses a three level ternary code known as 8B6T (8 binary, 6 ternary) instead of 2 level binary encoding. Using this ternary code reduces the clock rate to 25 Mhz, which is well within UTP limit. With 8B6T, before transmission each set of 8 binary bits (octet) is translated 6 ternary (0,1,2 or -1,0,1 instead of 0 and 1) symbols. The 3 signal levels used are -V, 0, V. The 6 ternary symbols means that there are 3^6 = 729 possible code combinations. Since only 2^8 = 256 are required to present the complete set of 8-bit combinations, the redundant codewords are used to achieve DC balance and to ensure that all codewords have at least 2 signals interval in between. This is done to maintain clock synchronization with the sender.
100BaseTX physical layer:
This physical layer defines the specification for 100BaseT Ethernet over 2 pairs of Category 5 UTP wire (Unshielded twisted pair of 22 and 24 gauge solid connector rated to support LAN speeds to 100 Mbps), or two pairs of Type 1 STP (two individually shielded twisted pairs of 22-gauge wire with an additional overall shield) wire. 100BaseTX uses one pair of wires for transmission, and the other pair - for collision detection and receive. (Transmission is done in 125-MHz frequency operating at 80% efficiency, that is to allow the 4B/5B - a coding scheme that uses 16 combinations of data, 4 control codes and the an idle code. The rest is invalid. Physically, it uses 5 bits). That's how FULL DUPLEX is guaranteed. Most of the products announced to date have been for 100BaseTX. 100BaseFX Physical layer : This physical layer defines the specifications for 10BaseT Ethernet for 2 standards of 62.5/125 micron fiber optic cables. 100BaseFX uses one fiber for transmission and the other fiber for collision detection and receive. The 100BaseFX, same as 100Base TX is used in FDDI physical layers as well. While 100BaseTX maintains the 100m limit from the wiring closet to the desktop for both half and full duplex modes (same as 10BaseT) 100BaseFX can be used up to 2km in the full duplex mode between DTE equipment such as bridges, routers or switches, Therefore 100BaseFX is used between wiring closets and campus buildings. The table below summarizes the difference between these three types of cabling
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Physical sublayer options Cable specifications Length(Meters)
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100BaseTX Category 5 UTP, 2 pairs 100 half/full duplex
Type 1 and 2 STP, two pairs 100 half/full duplex
100BaseT4 Categories 3,4 and 5 UTP 100 half/full duplex
4 pairs
100BaseFX 62.5/multimode fiber/two 400 half duplex
standards 2000 half duplex
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Website link: http://www2.rad.com/networks/1996/fasteth/technic1.htm
3. One of the strengths of the Fast Ethernet approach is that it readily supports a mixture of existing 10-Mbps LANs and newer 100-Mbps LANs. Elaborate on this.
One of the strengths of the Fast Ethernet approach is that it readily supports a mixture of existing 10 Mbps LANs and newer 100 Mbps LANs. For example, the 100 Mbps technology can used as a backbone LAN, with many of the stations attached to 10 Mbps hubs. These hubs are in turn connected to switching hubs that conform to 100BASE-T and that can support both 10 Mbps and 100 Mbps links. Additional high-capacity workstations and servers attach directly to these 10/100 switches. These mixed-capacity switches are in turn connected to 100 Mbps hubs using 100 Mbps links. The 100 Mbps hubs provide a building backbone and are also connected to a router that provides connection to an outside WAN. Reflective questions: 1. It requires finding an effective value for p in p-persistent CSMA. Elaborate on this.
For better channel utilization and shorter delays. If medium is idle, transmit with probability p, and delay one time unit with probability (1-p) Time unit typically maximum propagation delay. If medium is busy, listen until idle and repeat step 1. If transmission is delayed one time unit, repeat step 1.
What is the effective value of p? To avoid instability under heavy load. n stations waiting to send/ End of transmission, expected number of stations attempting to transmit is ‘number of stations ready’ x ‘probability of transmitting’ = np if np > 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more collisions. Retries compete with new transmissions. Eventually, all stations trying to send Continuous collisions, zero throughput. So np < 1 for expected peaks of n If heavy load expected, p is small. However, as p is made smaller, stations will have to wait longer. At low loads, this gives very long delays.
2. In IEEE 802.3 standard, the algorithm used for CSMA/CD LAN is 1-persistent. Discuss the possible reasons for this selection.
To avoid idle channel time, 1-persistent protocol is used. Station wishing to transmit listens and obeys following: 1) If medium is idle, it transmits, otherwise proceed to step 2 2) If medium is busy, listen until idle, then transmit immediately. 1-persistent stations are selfish. If two or more stations are waiting, collision chances will increase. Gets sorted out after the collision.