ECEN335-4-MAC Protocols.pdf

Computer Networks MAC Protocols Prof. Nashwa Abdelbaki NewGiza University  Medium Access Protocols The function of Medium Access Protocols is to control the usage of a common broadcast channel within a network – define the addressing that distinguishes each node...

Computer Networks MAC Protocols Prof. Nashwa Abdelbaki NewGiza University  Medium Access Protocols The function of Medium Access Protocols is to control the usage of a common broadcast channel within a network – define the addressing that distinguishes each node on a network The common broadcast channel is cabling in wired systems and air in wireless systems With the aid of access protocols, devices can share the common channel and communicate with the desired recipient NewGiza University Prof. Nashwa Abdelbaki 2  The addressing is implemented within the hardware of each Network Interface Controller (NIC) To move a packet to the proper node on a network, – a medium access control header is placed at the beginning of every packet – This header contains address fields for both the source and destination nodes NewGiza University Prof. Nashwa Abdelbaki 3  Access Methods There is a choice of access methods specified by committees like IEEE, ANSI or ATM The IEEE standards are: – ALOHA, the mother of medium access protocols – Slotted Aloha – Carrier Sense Multiple Access / Collision Detect (CSMA/CD) – Token Bus – Token Ring The ANSI standard is FDDI – Fiber Distributed Data Interface ATM Forum has produced the ATM (Asynchronous Transfer Mode) standard NewGiza University Prof. Nashwa Abdelbaki 4  ALOHA In the early 1970s, Norman Abramson and his colleagues at University of Hawaii devised a new technique, ALOHA, to solve the channel allocation problem It is the earliest method and is named in some literature as Pure Aloha It is a layer 2 (Data Link Layer of the OSI model) protocol for LAN networks with broadcast topology There is no central control Access to medium by nodes is random Any station sends data at any point in time – After transmission, the sender node listens to the channel for an amount of time – This time period is equal to twice of the time a packet needs to search the most widely separated station from the sender – It resends if no ACK has arrived after a certain time interval Every node uses the same frequency It is a predecessor to the Ethernet NewGiza University Prof. Nashwa Abdelbaki 5  Slotted ALOHA Pure ALOHA experienes throughput degradation when too many stations are attached to the network In slotted ALOHA, the time duration on the channel is organized into uniform time slots The duration of slots is equal to the packet transmission time Stations wait for the beginning of next slot to send – this eliminates overlapping of packets sent by different nodes ==> decreasing collision NewGiza University Prof. Nashwa Abdelbaki 6 Carrier Sense Multiple Access with Collision Detection CSMA/CD encapsulates the data in a frame wait for free medium to start transmission – medium must be idle for the Interframe Gap (IFG) time before sending If a collision is detected while transmitting – transmit a jam signal – wait for a random time and reattempt Collision detection by monitoring the voltage NewGiza University Prof. Nashwa Abdelbaki 7  CSMA/CD Host wants to transmit no Is carrier sensed? yes Assemble frame Start transmission yes collision detected? Broadband jam signal no attempts=attempts+1 Keep transmission no attempts>too many? no Algorithm calculates backoff Transmission done? yes yes Too many collisions; Wait for t seconds Transmission completed abort transmission NewGiza University Prof. Nashwa Abdelbaki 8  ACCESS PROBLEM IMPROVEMENT METHOD Pure Aloha Many collisions at any time Slotted Aloha Many collisions at the Bandwidth divided into beginning of the time slot equal time slots CSMA Collisions occur when The transmitting station various stations transmit waits until the medium is at the same time. available CSMA/CD Jam signal ensures that all stations will recognize the collision NewGiza University Prof. Nashwa Abdelbaki 9  IEEE LAN Standards IEEE 802.3: CSMA/CD – Carrier Sense, Multiple Access/Collision Detect IEEE802.4: Token Bus IEEE802.5: Token Ring IEEE802.11: Wireless LANs (Wi-Fi) IEEE802.15: Wireless PANs – Personal Area Network IEEE802.16: WiMAX – Broadband Wireless Access NewGiza University Prof. Nashwa Abdelbaki 10  IEEE 802 Standards 802.1 Overview, Architecture, Management 802.2 Logical Link Control (LLC) 802.1 Bridging MAC MAC MAC MAC MAC MAC MAC CSMA Token Token Wire- W-PAN WiMAX CD Bus Ring less LAN 802.3 802.4 802.5 802.11 802.15 802.16 others NewGiza University Prof. Nashwa Abdelbaki 11  First Ethernet Diagram Ethernet was developed by Dr. Robert Metcalfe, 1976, while he was a researcher at Xerox Palo Alto Research Center (PARC) to operate at 2.94Mbps NewGiza University Prof. Nashwa Abdelbaki 12  Ethernet Versions Ethernet II (DIX - consortium of the companies DEC, Intel and Xerox) – developed the standard for 10Mbps Ethernet, 1980 – thick coaxial multidrop cable Ethernet 802.3 (standard of IEEE) – PMD, PHY, and MAC layers are covered in the standard IEEE 802.3 – LLC - covered in the standard IEEE 802.2 (general standard, not only for Ethernet) NewGiza University Prof. Nashwa Abdelbaki 13  Ethernet II Principles Fixed 10Mbps Manchester coded signals CSMA/CD used for bandwidth sharing Frame size ranges from 64 to1518 bytes 64 bit preamble at front and 9.6μs delay between frames Bus topology Ethernet is a passive network – only transmitting stations can be detected NewGiza University Prof. Nashwa Abdelbaki 14  Ethernet 802.3 Frame Format Frame Header Destination Source Length Preamble SFD Address Address / Type Data FCS (7 Byte) (1B) (6 Byte) (6 Byte) (2 Byte) (46 - 1500 Byte) (4 Byte) DSAP SSAP Control Destination SAP Source SAP Field Data (1 Byte) (1 Byte) (1-2 Byte) (42 - 1497 Byte) 802.2 LLC Header Length/Type Length of data or Type SFD Start Frame Delimiter shows the start of a Frame LLC Header: SAP Service Access Point for the Layer 3 Protocol determination Control Field for the indication of supervisory, information, unnumbered frames NewGiza University Prof. Nashwa Abdelbaki 15  The 802.3 MAC Address I/G U/L OUI 24 Bits assigned 1 Bit 1 Bit 22 Bits by OUI Owner I/G Individual/Group U/L Universal/Local OUI Organizationally Unique Identifier specifies the manufacturer of the Ethernet card NewGiza University Prof. Nashwa Abdelbaki 16  Ethernet Evolution Fast Gigabit 10 Gigabit 100 Gigabit Ethernet Ethernet Ethernet Ethernet Ethernet 10Mb/s 100Mb/s 1000Mb/s 10000Mb/s 100000Mb/s 1973 1980 1995 1998 2002 2008 NewGiza University Prof. Nashwa Abdelbaki 17  Gigabit Ethernet Upper Layers Logical Link Control (LLC) GMII: Gigabit Media Independent Interface MAC PMA: Physical Media Attachment Reconciliation Reconciliation PMD: Physical Media Dependent MII GMII Reconciliation: Mapping between: PCS PCS Physical layer signaling primitives & PMA PMA Signals on GMII PMD PMD Medium Medium 100Mbps 1000Mbps NewGiza University Prof. Nashwa Abdelbaki 18  Frame Formats IEEE 802.3 frame format Gigabit Ethernet frame format 7 octets 7 octets Preamble Preamble 1 octet 1 octet Start-of-Frame Delimiter Start-of-Frame Delimiter Destination Destination 6 octets 6 octets Address Address Data Link Layer Encapsulation Source Source 6 octets Address 6 octets Address 2 octets Length/Type 2 octets Length/Type............ Data Data 46-1500 octets 46-1500 octets Frame Check Frame Check 4 octets 4 octets Sequence Sequence 1 octet.... 0-448 octets Extension NewGiza University Prof. Nashwa Abdelbaki 19  Typical Star/Tree Topology 10000Mb/s 10000Mb/s 1000Mb/s 1000Mb/s 1000Mb/s 1000Mb/s 100Mb/s 1000Mb/s 100Mb/s 10Mb/s 1000Mb/s NewGiza University Prof. Nashwa Abdelbaki 20