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Layer 2 LAN Technologies & Media Access Methods. Objectives. Compare and contrast media access methods used in network architectures Describe the operation of Ethernet Differentiate between Ethernet standards and speeds Explain the four Ethernet frame types and how they are used.
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Objectives • Compare and contrast media access methods used in network architectures • Describe the operation of Ethernet • Differentiate between Ethernet standards and speeds • Explain the four Ethernet frame types and how they are used
Objectives (continued) • Describe the token ring architecture and its components • Describe the AppleTalk network architecture • Explain the function of Fiber Distributed Data Interface • Describe other LAN and WAN architectures and their role in today’s networks
Putting Data on the Cable: Access Methods • Given that network architectures communicate in a number of different ways, some factors in network communications must be considered • How computers put data on the cable • How they ensure that the data reaches its destination undamaged
Function of Access Methods • The way in which computers attached to a network share the cable must be defined • A collision results from two or more devices sending a signal along the same channel at the same time • Splitting data in small chunks helps prevent collisions • Channel access methods specify when computers can access the cable or data channel • Ensure that data reaches destination by preventing computers from sending messages that might collide • Every computer on a network must use the same access method
Major Access Methods • Channel access is handled at the MAC sublayer of the Data Link layer in the OSI model • Five major types of channel access • Contention • Switching • Token passing • Demand priority • Polling
Contention • In early networks based on contention, computers sent data whenever they had data to send • As networks grow, outgoing messages collide more frequently, must be sent again, and then collide again • To organize contention-based networks, two carrier access methods were created • CSMA/CD • CSMA/CA
Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) • When the computer senses that no other computer is using the network, it signals its intent to transmit • Other computers with data to send must wait when they receive the “intent-to-transmit” signal and send their “intent-to-transmit” only when channel is free • The overhead created by intent-to-transmit packets reduces network speed significantly • Used in wireless LANs with an access point • Wireless NIC tells access point its intents to transmit • Access point hears transmissions from all devices, so it can determine whether it’s okay to transmit
Switching • Switching: nodes are interconnected through a switch, which controls access to the media • Contention occurs only when multiple senders ask to reach the same receiver simultaneously or when the simultaneous transmission requests exceed the switch’s capability to handle multiple connections • Advantages: centralized management (enables QoS), switch can have connection ports that operate at different speeds • Disadvantage: higher cost
Demand Priority • Demand priority: channel access method used solely by the 100VG-AnyLAN 100 Mbps Ethernet standard (IEEE 802.12) • 100VG-AnyLAN runs on a star bus topology • Intelligent hubs control access to the network • Hub searches all connections in a round-robin fashion • When an end node has data to send, it transmits a demand signal to the hub • The hub then sends an acknowledgement that the computer can start transmitting its data • The major disadvantage of demand priority is price
The Ethernet Architecture • 1960s and 1970s: many organizations worked on methods to connect computers and share data • E.g., the ALOHA network at the University of Hawaii • 1972: Robert Metcalf and David Boggs, from Xerox’s PARC, developed an early version of Ethernet • 1975: PARC released first commercial version (3 Mbps, up to 100 computers, max. 1 km of total cable) • DIX developed standard based on Xerox’s Ethernet (10 Mbps). Now referred to as Ethernet II, which is commonly used today. • 1990: IEEE defined the 802.3 specification • Defines how Ethernet networks operate at layers 1-2
Overview of Ethernet • Ethernet is the most popular network architecture • Advantages: easy to install, scalable, broad media support, and low cost • Supported transmission speeds: 10 Mbps to 10 Gbps • Uses the NIC’s MAC address to address frames • Ethernet variations are compatible with one another • Basic operation and frame formatting is the same • Cabling, speed of transmission, and method by which bits are encoded on the medium differ
Ethernet Operation • Ethernet is a best-effort delivery system • It works at the Data Link layer of the OSI model • Relies on the upper-layer protocols to ensure reliable delivery of data • Understanding the following concepts is important: • How Ethernet accesses network media • Collisions and collision domains • How Ethernet handles errors • Half-duplex and full-duplex communications
Accessing Network Media • Ethernet uses CSMA/CD in a shared-media environment (a logical bus) • Ethernet device listens for a signal or carrier (carrier sense) on the medium first • If no signal is present, no other device is using the medium, so a frame can be sent • Ethernet devices have circuitry that detects collisions and automatically resends the frame that was involved in the collision
CSMA/CD (CSMA with Collision Detection) • In CSMA, if 2 terminals begin sending packet at the same time, each will transmit its complete packet (although collision is taking place). • Wasting medium for an entire packet time. • CSMA/CD Step 1: If the medium is idle, transmit Step 2: If the medium is busy, continue to listen until the channel is idle then transmit Step 3: If a collision is detected during transmission, cease transmitting, and send jam signal Step 4: Wait a random amount of time and repeats the same algorithm
CSMA/CD (Cont’d) T0 A begins transmission A B T0+- B begins transmission A B T0+ B detects collision A B T0+2 - A detects collision just before end of transmission A B Time ( is the propagation time)
Ethernet Error Handling • Collisions are the only type of error for which Ethernet automatically attempts to resend the data • Errors can occur when data is altered in medium • Usually caused by noise or faulty media connections • When the destination computer receives a frame, the CRC is recalculated and compared against the CRC value in the FCS • If values match, the data is assumed to be okay • If values don’t match, the data was corrupted • Destination computer discards the frame • No notice is given to the sender
Half-Duplex Versus Full-Duplex Communications • When half-duplex communication is used with Ethernet, CSMA/CD must also be used • When using a switched topology, a computer can send and receive data simultaneously (full-duplex communication) • The collision detection circuitry is turned off because collisions aren’t possible • Results in a considerable performance advantage
Ethernet Standards • Each Ethernet variation is associated with an IEEE standard • The following sections discuss many of the standards, some of which are obsolete or had limited use • Keep in mind that Ethernet over UTP cabling has been the dominant technology since the early 1990s, and will likely to continue to be for the foreseeable future
Ethernet Framing • The 802.3 standard describes the operation of the MAC sub-layer in a bus LAN that uses carrier sense, multiple access with collision detection (CSMA/CD). • Beside carrier sensing, collision detection and the binary exponential back-off algorithm, the standard also describes the format of the frames and the type of encoding used for transmitting frames. • The minimum length of frames can be varied from network to network. This is important because, depending on the size of the network, the frames must be of a suitable minimum length. • The standard also makes some suggestions about the type of cabling that should be used for CSMA/CD bus LANs.
IEEE 802.3: MAC Addresses • Every network card in the world has a unique 46-bit serial number called a MAC address. The IEEE allocates these numbers to network card manufacturers who encode them into the firmware of their cards. • The destination and source address fields of the MAC frame have 48 bits set aside. • The most significant bit is set to 0 to indicate an ordinary address and 1 to indicate a group address (this is for multicasting, which means that frames are sent to several hosts). • If all 48 bits are set to 1 then frames are broadcast to all the hosts (FF:FF:FF:FF:FF:FF).
Notes IEEE 802.3: MAC Addresses The broadcast destination address is a special case of the multicast address in which all bits are 1s. The least significant bit of the first byte defines the type of address. If the bit is 0, the address is unicast; otherwise, it is multicast.
IEEE 802.3: MAC Addresses Define the type of the following destination addresses: a. 4A:30:10:21:10:1A b. 47:20:1B:2E:08:EE c. FF:FF:FF:FF:FF:FF Solution To find the type of the address, we need to look at the second hexadecimal digit from the left. If it is even, the address is unicast. If it is odd, the address is multicast. If all digits are F’s, the address is broadcast. Therefore, we have the following: a. This is a unicast address because A in binary is 1010 (even). b. This is a multicast address because 7 in binary is 0111 (odd). c. This is a broadcast address because all digits are F’s.
IEEE 802.3: MAC Addresses Show how the address 47:20:1B:2E:08:EE is sent out on line. Solution The address is sent left-to-right, byte by byte; for each byte, it is sent right-to-left, bit by bit, as shown below: ←11100010 00000100 11011000 01110100 00010000 01110111
