香港浸会大学：《Data Communications and Networking》课程教学资源（PPT讲稿）Chapter 9 High Speed LANs and Wireless LANs

Data communications and Networking Chapter 9 High Speed LANs and Wireless LANs References Book Chapters 16 and 17 Data and Computer Communications, 8th edition ByW∥ lam Stallings

Data Communications and Networking Chapter 9 High Speed LANs and Wireless LANs References: Book Chapters 16 and 17 Data and Computer Communications, 8th edition By William Stallings

Outline Fundamentals of ethernet -aloHA, slotted aloHA, csma -CSMA/CD Ethernet Examples 10-Mbps Ethernet -Fast ethernet -Gigabit Ethernet 10-Gbps Ethernet 802. 11 Wireless Lans

2 Outline • Fundamentals of Ethernet —ALOHA, slotted ALOHA, CSMA —CSMA/CD • Ethernet Examples —10-Mbps Ethernet —Fast Ethernet —Gigabit Ethernet —10-Gbps Ethernet • 802.11 Wireless LANs

Ethernet Most widely used high-speed LANs Ethernet(10Mbps, 100Mbps, 1Gbps 10Gbps) Fibre channel High-speed wireless LANs Ethernet protocol is developed by iEee 802.3 standards committee, consisting of Medium Access Control ( Mac) layer(CSMA/CD protocol) the key part of this chapter Physical Layer Earlier mac schemes ALOHA Slotted aloha CSMA

3 Ethernet • Most widely used high-speed LANs — Ethernet (10Mbps, 100Mbps, 1Gbps, 10Gbps) — Fibre channel — High-speed wireless LANs • Ethernet protocol is developed by IEEE 802.3 standards committee, consisting of — Medium Access Control (MAC) Layer (CSMA/CD protocol) • the key part of this chapter — Physical Layer • Earlier MAC schemes: — ALOHA — Slotted ALOHA — CSMA

ALOHA ALOHA protocol is developed for packet radio networks, but applicable to any shared transmission medium a number of stations share the transmission medium two or more simultaneous transmissions will cause a collision Sender When station has frame it sends Station listens for an amount of time If its hears an aCK, fine. If not, it retransmits the frame after a random time If no ACK after several transmissions, it gives up Frame check sequence can be used for error detection Receiver If frame is ok and address matches receiver, sends acK Otherwise, ignores this frame and does nothing Frame may be damaged by noise or by another station transmitting at the same time(collision). Overlap of frames also causes collision ALOHA is simple, but very inefficient Assuming random traffic, the maximum channel utilization is only about 18%

4 ALOHA • ALOHA protocol is developed for packet radio networks, but applicable to any shared transmission medium. — A number of stations share the transmission medium. Two or more simultaneous transmissions will cause a collision. • Sender — When station has frame, it sends — Station listens for an amount of time — If its hears an ACK, fine. If not, it retransmits the frame after a random time — If no ACK after several transmissions, it gives up — Frame check sequence can be used for error detection • Receiver — If frame is OK and address matches receiver, sends ACK — Otherwise, ignores this frame and does nothing • Frame may be damaged by noise or by another station transmitting at the same time (collision). Overlap of frames also causes collision. • ALOHA is simple, but very inefficient — Assuming random traffic, the maximum channel utilization is only about 18%

ALOHA Node 1 Packet Waiting a random time Node 2 Packet Retransmission Retransmission [2:3 Ime Collision Node 3 Packet 5

5 ALOHA

Slotted AloHA To improve efficiency a modification of ALOHA known as slotted ALOHA, was developed Time is organized into uniform slots whose size equals the frame transmission time -Need a central clock ( or other sync mechanism) Transmission begins only at a slot boundary -Consequence: frames either miss or overlap totally Maximum channel utilization can be improved to about 37%

6 Slotted ALOHA • To improve efficiency, a modification of ALOHA, known as slotted ALOHA, was developed. • Time is organized into uniform slots whose size equals the frame transmission time —Need a central clock (or other sync mechanism) • Transmission begins only at a slot boundary —Consequence: frames either miss or overlap totally • Maximum channel utilization can be improved to about 37%

Slotted AloHA Node 1 packet Nodes 2& 3 packets Retransmission Retransmission 283 3 Time Slot Collision

7 Slotted ALOHA

CSMA Why aloha and slotted aloha are so inefficient? Stations don't check the channel status. They just send out frames without considering whether the channel is free or not, which creates too many collisions In fact it is not difficult for a station to sense the channel status(free or not) CSMA: Carrier Sense Multiple access Stations listen to the channel(carrier sense) Stations knowwhether the channel is free or not Stations transmit only if the channel is free -Collisions become rare Only if two or more stations attempt to transmit at about the same time, collisions could happen 8

8 CSMA • Why ALOHA and slotted ALOHA are so inefficient? — Stations don’t check the channel status. They just send out frames without considering whether the channel is free or not, which creates too many collisions. • In fact, it is not difficult for a station to “sense” the channel status (free or not). • CSMA: Carrier Sense Multiple Access — Stations listen to the channel (carrier sense) — Stations “know” whether the channel is free or not — Stations transmit only if the channel is free — Collisions become rare • Only if two or more stations attempt to transmit at about the same time, collisions could happen

CSMA (Cont) In traditional LANs, propagation time is much less than frame transmission time Remark: this may not be true for 1 Gbps and 10gbps ethernet All stations know that a transmission has started almost immediately by carrier sense Details of cSma Stations first listen for clear medium (carrier sense) -If medium is idle, transmit the frame -If two or more stations start at about the same instant there will be a collision To account for this a station waits for an acK If no ack after a reasonable time then retransmit What should a station do if the medium is found busy? Three different approaches: nonpersistent CSMA, 1-persistent CSMA, and p-persistent CSMA 9

9 CSMA (Cont.) • In traditional LANs, propagation time is much less than frame transmission time — Remark: this may not be true for 1Gbps and 10Gbps Ethernet • All stations know that a transmission has started almost immediately by “carrier sense” • Details of CSMA — Stations first listen for clear medium (carrier sense) — If medium is idle, transmit the frame — If two or more stations start at about the same instant, there will be a collision. • To account for this, a station waits for an ACK • If no ACK after a reasonable time, then retransmit • What should a station do if the medium is found busy? — Three different approaches: nonpersistent CSMA, 1-persistent CSMA, and p-persistent CSMA

Nonpersistent CSMA a station wishing to transmit listens to the medium and obeys the following rules 1. If medium is idle, transmit; otherwise, go to step 2 2. If medium is busy, wait an amount of time drawn from a probability distribution and repeat step 1 The use of random delays reduces probability of collisions Consider two stations become ready to transmit at about the same time while another transmission is in progress If both stations delay the same amount of time before retrying both will attempt to transmit at same time>collision Drawback: Capacity is wasted because medium will generally remain idle following the end of a transmission even if there are one or more stations waiting to transmit. 10

10 Nonpersistent CSMA • A station wishing to transmit listens to the medium and obeys the following rules: 1. If medium is idle, transmit; otherwise, go to step 2 2. If medium is busy, wait an amount of time drawn from a probability distribution and repeat step 1 • The use of random delays reduces probability of collisions — Consider two stations become ready to transmit at about the same time while another transmission is in progress — If both stations delay the same amount of time before retrying, both will attempt to transmit at same time → collision • Drawback: — Capacity is wasted because medium will generally remain idle following the end of a transmission, even if there are one or more stations waiting to transmit