Data and Computer Communications Ninth Edition by William Stallings Data and Computer Communications, Ninth Edition by William Stallings, (c) Pearson Education - Prentice Hall, 2011 Chapter 16 – High Speed LANs

Introduction rapid changes in technology designs broader use of LANs new schemes for high-speed LANs high-speed technologies: – Fast and Gigabit Ethernet – Fibre Channel – High Speed Wireless LANs

Characteristics of Some High-Speed LANs

Network Access Policy Topology determines a network’s wiring configuration. After the physical network is wired, how do we control access to the network. Who can place messages onto the network and when? What happens when 2 stations send at the same time? How do we organize this?

Random Access earliest was ALOHA developed for packet radio networks station may transmit a frame at any time if frame is determined invalid, it is ignored maximum utilization of channel about 18% next came slotted ALOHA organized slots equal to transmission time increased utilization to about 37%

CSMA/CD Precursors Carrier Sense Multiple Access (CSMA) – station listens to determine in there is another transmission in progress – if idle, station transmits – waits for acknowledgment – if no acknowledgment, collision is assumed and station retransmits – utilization far exceeds ALOHA

Nonpersistent CSMA If the medium is idle, transmit; otherwise, go to step 2 If the medium is busy, wait an amount of time drawn from a probability distribution and repeat step 1 Disadvantage: capacity is wasted because the medium will generally remain idle following the end of a transmission even if there are one or more stations waiting to transmit Step 1 Step 2

1-persistent CSMA 1-persistent CSMA avoids idle channel time 1-persistent CSMA rules: 1.if medium idle, transmit; 2.if medium busy, listen until idle; then transmit immediately 1-persistent stations are selfish if two or more stations waiting, a collision is guaranteed

Propagation time Copper wires, optical cable: 2E8 meters/sec Wireless: 3E8 meters/sec Speed of light in vacuum: 3E8 meters/sec Propagation time = Distance / propagation speed 100 meter between computer and switching node over a UTP copper cable propagation delay ≈ 100 m / ( m/s) = 0.5 μs It is the time after full message has left the sender, until it has reached the receiving node.

Transmission time Speed = 100 Mbit/s Ethernet Packet size of 1526 bytes Transmission time = Packet size / Bit rate Transmission time = 1526*8 bit / ( bit/s) ≈ 122 μs Transmission time is the time from the first bit until the last bit of a message has left the transmitting node.

CSMA/CD Operation A starts transmitting to D. Signal reaches B but not C. C starts transmit. C detects the collision and ceases transmission. A ceases transmission.

Collision Detection on twisted pair (star-topology ) activity on more than one port is collision use special collision presence signal on baseband bus collision produces higher signal voltage collision detected if cable signal greater than single station signal signal is attenuated over distance limit to 500m (10Base5) or 200m (10Base2)

IEEE MAC Frame Format