Semester 1Chapter6 + Chapter7 Layer 2 Data Link Layer Concepts And Technologies
Data Link Layer • Data link layer provides reliable transit of data across a physical link by using the Media Access Control (MAC) addresses. • Data link layer is concerned with physical (as opposed to network, or logical) addressing, network topology, line discipline (how end systems will use the network link), error notification, ordered delivery of frames, and flow control.
Layer 2 Importance • Layer 1 cannot communicate with the upper level layers. Layer 2 does that with Logical Link Control (LLC). • Layer 1 cannot name or identify computers. Layer 2 uses an addressing (or naming) process. • Layer 1 can only describe streams of bits. Layer 2 uses framing to organize or group the bits. • Layer 1 cannot decide which computer will transmit binary data from a group that are all trying to transmit at the same time. Layer 2 uses a system called Media Access Control (MAC).
LLC is defined according to IEEE standard 802.2 LLC is independent of the specific LAN technology used LLC serves to communicate upward to Layer 3 and downward to the technology-specific MAC sublayer. Logical Link Control Sublayer
LLC- Cont • LLC takes the network protocol data, an IP packet, and adds more control information to help deliver the IP packet to its destination. It adds two addressing components of the 802.2 specification, the Destination Service Access Point (DSAP) and the Source Service Access Point (SSAP). • LLC supports both connectionless and connection-oriented services used by higher layer protocols.
Media Access Control Sublayer • Media Access Control (MAC) refers to protocols that determine which computer on a shared-medium environment (collision domain) is allowed to transmit the data. • categories of Media Access Control, • deterministic (taking turns) • nondeterministic (first come, first served).
A 48-bit address burned onto the NIC. It is a unique way of identifying each computer on a network. Flat Address The first six comprise the Organizational Unique Identifier (OUI) which identifies the manufacturer (assigned by the IEEE). The last six represent a unique ID number for the NIC. The MAC Address
Framing • Framing is the Layer 2 encapsulation process. A frame is the Layer 2 protocol data unit (PDU). • Framing importance: • Which computers are communicating with one another • When communication between individual computers begins and when it terminates • A record of errors that occurred during the communication • Whose turn it is to "talk" in a computer "conversation
The Six Parts of a Generic Frame • 1. Frame start field – indicates the beginning of a frame • 2. Address field – has source and destination address information in it • 3. Length/type/control field – indicates the end (the frame is considered ended after the FCS field); also sometimes called an end-frame delimiter.
The Six Parts of a Generic Frame • 4. Data field – the information you’re sending including LLC bytes • 5. Frame check sequence (FCS) field – contains a number that is calculated by the source computer and is based on the data in the frame. The destination computer recalculates the FCS number and compares it with the source FCS number. It is an error-checking device. • 6. Stop frame field – indicates the end of a frame
Layer 2 Technologies • Token Ring - logical ring topology (in other words, information flow is controlled in a ring) and a physical star topology (in other words, it is wired as a star) • FDDI - logical ring topology (information flow is controlled in a ring) and physical dual ring topology (wired as a dual ring) • Ethernet - logical bus topology (information flow is on a linear bus) and physical star or extended star (wired as a star)
Token Ring • Developed by IBM – still used today • Two frames: • Token • Start Delimiter • Access Control Byte • Priority and Reservation Fields: Only stations with a priority equal to, or higher than, the priority value contained in a token can seize that token. • Token and Monitor bits • End Delimiter
Token Ring • Data/Command Frame
Token Ring • Token Passing • Station can only transmit if it has the token • Station passes token on if it has no data to transmit • Station can hold the token • for a maximum amount of • Time depending on technology used
Token Ring • Characteristic: • Deterministic = Taking Turns • Calculate the maximum time to transmit • Ideal for applications where predictability and dependability are paramount.
Token Ring • Management Mechanisms • Active Monitor • One station acts as centralized source of timing information for other stations • Can be any station • Removes keep circulating frames
Token Ring • Management Mechanisms • MSAU • Multi Station Access Units • Can see all information in a Token Ring Network • Check for problems • Selectively remove stations from the ring if needed
Token Ring • Management Mechanisms • Beaconing • Detects and repairs network faults • Sends a beacon frame, defining a failure domain • Initiates auto-reconfiguration • Nodes within the failure domain automatically perform diagnostics • Attempt to reconfigure around the failure • MSAUs use electrical reconfiguration to accomplish this
Token Ring Signaling • Uses Manchester Encoding • 0 is high-to-low transition • 1 is low-to-high transition
Token Ring Media and Physical Topologies • Logical ring topology • Physical star topology • Stations are directly connected to MSAUs • Patch cables connect MSAUs • Lobe cables connect MSAUs to stations
Overview of FDDI • Fiber distributed data interface • FDDI is particularly popular as a campus backbone technology or in Internet critical applications where faults cannot be tolerated.
Preamble • Prepares each station for the upcoming frame • Start delimiter • Frame Control • Indicates the size of the address fields • Indicates whether frame contains asynchronous or synchronous data • Other control information
Destination address • 6 bytes • Unicast: to one address • Multicast: to several addresses • Broadcast: to all addresses • Source address • Data • Frame Check Sequence • End Delimiter • Frame Status
FDDI MAC • Token passing strategy • Early token release • New token can be released when the frame transmission has finished • Deterministic • Dual ring • Ensures transmission, even if one ring is damaged or disabled • Very reliable • Real-time allocation of bandwidth • Defines two types of traffic • Synchronous • Asynchronous
FDDI Signaling • Uses an encoding scheme called 4B/5B • Every four bits of data are sent as a 5 bit code • Signal sources are LEDs or lasers
FDDI Media • Optical fiber is being installed at a rate of 4000 miles per day in the United States • Explosive growth worldwide
Advantages of Optical Fiber • Security • Fiber does not emit electrical signals that can be tapped • Reliability • Fiber is immune to electrical interference • Speed • Optical fiber has much higher throughput potential than copper cable
FDDI Rings • FDDI specifies dual rings for physical connections • Traffic on each ring travels in opposite directions • Rings consist of two or more point-to-point connections between adjacent stations • Primary ring is for data transmission • Secondary ring is for back up
Ethernet • Shortly after the 1980 IEEE 802.3 specification, Digital EquipmentCorporation (DEC), Intel Corporation, and Xerox Corporation jointly developed and released an Ethernet specification. Version 2.0, that was substantially compatible with IEEE 802.3. Together, Ethernet and IEEE 802.3 currently maintain the greatest market share of any LAN protocol.
Ethernet • Today, the term Ethernet is often used to refer to all carrier sense multiple access/collision detection (CSMA/CD) LAN’s that generally conform to Ethernet specifications, including IEEE 802.3.
Ethernet • Ethernet performs three functions: • Transmitting and receiving data packets • decoding data packets and checking them for valid addresses before passing them to the upper layers of the OSI model • detecting errors within data packets or on the network In the CSMA/CD access method, networking devices with data to transmit over the networking media work in a listen-before-transmit mode.
Common LAN Devices • NIC • Bridges • Switches
NICs • Provides ports for network connection • Communicate with network via serial connection • Communication with computer through parallel connection • Resources required: • IRQ, I/O address, upper memory addresses
Selection Factors for NICs • Type of network • Ethernet, Token Ring, FDDI • Type of media • Twisted pair, coax, fiber • Type of system bus • PCI, ISA
NIC Operations • Layer 1 & Layer 2 device • Primarily Layer 2 • Communicates with upper layers in the computer • Logical Link Control (LLC) • Has MAC address burned in • Encapsulates data into frames • Provides access to the media • Also Layer 1 • Creates signals and interfaces with the media • On-board transceiver
Bridges • Connects two network segments • Can connect different layer 2 protocols • Ethernet, Token Ring, FDDI • Makes intelligent decisions about traffic • Reduces unnecessary traffic • Minimizes collisions • Filters traffic based on MAC address • Maintains address tables • Rarely implemented today • Conceptually important
Bridge Operations • Bridging occurs at the data link layer: • Controls data flow • Handles transmission errors • Provides physical addressing • Manages access to the physical medium
Bridge Operations • Transparent to upper layers • Best used in low traffic areas • Can cause bottlenecks • Must examine every packet • Broadcasts • Messages sent to all devices • Destination MAC address unknown • Bridge will always forward • Can cause Broadcast Storm • Network time outs, traffic slowdowns, unacceptable performance
Switching Operation Microsegmentation • Each switch port acts as a micro bridge (Layer 2 device) • Multiple traffic paths within the switch • Virtual circuits • Temporarily exist - only when needed • Each data frame has a dedicated path • No collisions • Increases bandwidth availability • Each host gets full bandwidth
Advantages of Switches • Much faster than bridges • Hardware based, not software • Support new uses • e.g. virtual LANs • Reduce collision domains
Advantages of Switches • Allows many users to communicate in parallel • Creates virtual circuits • Creates dedicated segments • Collision free • Maximizes bandwidth • Cost effective • Can simply replace hubs in same cable infrastructure • Minimal disruption • Flexible network management • Software based configuration
Broadcast Domains • All hosts connected to the same switch are still in the same broadcast domain • A broadcast from one node will be seen by all other nodes connected through the LAN switch
LAN Segmentaion • Two primary reasons for segmenting a LAN: • Isolate traffic between segments • Achieve more bandwidth per user by creating smaller collision domains
Bridge Drawback: • Bridges increase the latency (delay) in a network by 10-30% • A bridge is considered a store-and-forward device slowing network transmissions, thus causing delay.