Department of Computer Science, University of Peshawar Multiple Access

Department of Computer Science, University of Peshawar Multiple-access protocols

Department of Computer Science, University of Peshawar Random Access MA CSMA CSMA/CD CSMA/CA

Department of Computer Science, University of Peshawar Evolution of random-access methods

Department of Computer Science, University of Peshawar ALOHA The earliest random-access method Developed at the University of Hawaii in early 1970’s Used on a wireless LAN with a data rate 9600 bits/sec A base station is the central controller The base station receives the frame and relays it to the intended destination The uploading transmission uses carrier frequency of 407 MHz The downloading uses carrier freq of 413 MHz

Department of Computer Science, University of Peshawar ALOHA network

Department of Computer Science, University of Peshawar Procedure for ALOHA protocol

Department of Computer Science, University of Peshawar Back off time The station waits an amount of time b/w 0 and 2 N *(maximum_propagation_time) For the first time between: 0 and 2 1 *(max_propg_time) For the second time between: 0 and 2 2 (max_propag_time), and so on N is the number of attempted transmissions

Department of Computer Science, University of Peshawar CSMA Carrier Sense Multiple Access To minimize the chance of collision Based on the principle “sense before transmit” Can reduce the possibility of collision, but can not eliminate it Collision because of the propagation delay

Department of Computer Science, University of Peshawar Collision in CSMA

Department of Computer Science, University of Peshawar Persistent Strategy The procedure for a station that senses a busy medium Two substrategies Nonpersistent Persistent

Department of Computer Science, University of Peshawar Persistence strategies

Department of Computer Science, University of Peshawar Persistence strategies cont… In 1-persistent, the station sends its frame immediately, when finds the line idle It increase the chance of collision due to same time sending In P-persistent, the station may or may not send, when finds the line idle The system generates a random number b/w 1 and 100 If the number is less than 20, then sends, otherwise not

Department of Computer Science, University of Peshawar CSMA/CD Carrier Sense Multiple Access with collision detection The station monitors the medium after sending a frame If successful, the station is finished If collision, the frame is resent Used in Ethernet

Department of Computer Science, University of Peshawar CSMA/CD procedure

Department of Computer Science, University of Peshawar CSMA/CA Carrier Sense Multiple Access with collision avoidance No chance of collision Used in Wireless LANs

Department of Computer Science, University of Peshawar CSMA/CA procedure

Department of Computer Science, University of Peshawar Control Access Reservation Polling Token Passing

Department of Computer Science, University of Peshawar Reservation access method

Department of Computer Science, University of Peshawar Polling One primary station, other devices are secondary stations All data transmission must be made through the primary device The secondary devices follow its instructions In case of receiving the data, the primary device does polling In case of sending the data, the primary device does selection

Department of Computer Science, University of Peshawar Select

Department of Computer Science, University of Peshawar Poll

Department of Computer Science, University of Peshawar Token passing A station is authorized to send data when it receives a special frame called token When no data are being sent, a token is circulating around the ring. If a station needs to send data, it waits for the token

Department of Computer Science, University of Peshawar Token-passing network

Department of Computer Science, University of Peshawar Token-passing procedure

Department of Computer Science, University of Peshawar Local Area Networks: Ethernet

Department of Computer Science, University of Peshawar History of Ethernet Developed by Xerox’s Palo Alto Research Center (PARC) in 1976 IEEE standard

Department of Computer Science, University of Peshawar Traditional Ethernet  Designed to operate at 10 Mbps  Access Method CSMA/CD  Ethernet frame contains 7 fields

Department of Computer Science, University of Peshawar MAC frame

Department of Computer Science, University of Peshawar MAC frame cont… Preamble 7 bytes (56 bits) alternating 0’s and 1’s alert the receiver to the coming frame SFD 1 byte ( ) Shows beginning of the frame Last 2 bits 11 alert the receiver that the next field is the DA DA 6 bytes Contains the physical address of the receiver SA 6 bytes Contains the physical address of the sender

Department of Computer Science, University of Peshawar MAC frame cont… Length/type Defines the length of the Data field Data Caries the data, min of 46 and max 1500 bytes CRC Contains error detection information

Department of Computer Science, University of Peshawar Minimum and maximum length

Department of Computer Science, University of Peshawar Ethernet addresses in hexadecimal notation

Department of Computer Science, University of Peshawar Categories of traditional Ethernet

Department of Computer Science, University of Peshawar Physical layer

Department of Computer Science, University of Peshawar PLS

Department of Computer Science, University of Peshawar Fast Ethernet Operates at 100 Mbps Physical Layer Implementation  RS (Reconciliation Sublayer)  responsible for passing of 4 bit data to MII  MII (Medium Independent Interface)  AUI was replaced with MII  PHY (Transceiver)  Responsible for encoding and decoding

Department of Computer Science, University of Peshawar Fast Ethernet implementations

Department of Computer Science, University of Peshawar Fast Ethernet physical layer

Department of Computer Science, University of Peshawar Gigabit Ethernet Operates at 1000 Mbps Physical Layer Implementation  RS  responsible for passing of 8 bit data to GMII  GMII  Connects RS to the PHY  PHY (Transceiver)  Encodes and decodes the data

Department of Computer Science, University of Peshawar Gigabit Ethernet implementations

Department of Computer Science, University of Peshawar Physical layer in Gigabit Ethernet

Department of Computer Science, University of Peshawar Thank you!