Semester 1 Chapter6 + Chapter7 Layer 2 Data Link Layer Concepts And Technologies

The OSI Layers

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 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. 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 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 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 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). 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).

Logical Link Control Sublayer LLC is defined according to IEEE standard LLC is defined according to IEEE standard LLC is independent of the specific LAN technology used 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. LLC serves to communicate upward to Layer 3 and downward to the technology-specific MAC 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 specification, the Destination Service Access Point (DSAP) and the Source Service Access Point (SSAP). 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 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. 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. 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, categories of Media Access Control, deterministic (taking turns) deterministic (taking turns) nondeterministic (first come, first served). nondeterministic (first come, first served).

The MAC Address A 48-bit address burned onto the NIC. It is a unique way of identifying each computer on a network. A 48-bit address burned onto the NIC. It is a unique way of identifying each computer on a network. Flat Address Flat Address The first six comprise the Organizational Unique Identifier (OUI) which identifies the manufacturer (assigned by the IEEE). 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 last six represent a unique ID number for the NIC.

Everything Has a Format

Framing Framing is the Layer 2 encapsulation process. A frame is the Layer 2 protocol data unit (PDU). Framing is the Layer 2 encapsulation process. A frame is the Layer 2 protocol data unit (PDU). Framing importance: Framing importance: Which computers are communicating with one another Which computers are communicating with one another When communication between individual computers begins and when it terminates When communication between individual computers begins and when it terminates A record of errors that occurred during the communication A record of errors that occurred during the communication Whose turn it is to "talk" in a computer "conversation 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 1. Frame start field – indicates the beginning of a frame 2. Address field – has source and destination address information in it 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. 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 Data field – the information you’re sending including LLC bytes 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. 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 6. Stop frame field – indicates the end of a frame

Ethernet Frame Format

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) 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) 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) 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 Developed by IBM – still used today Two frames: Two frames: Token Token Start Delimiter Start Delimiter Access Control Byte Access Control Byte Priority and Reservation Fields: 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 Token and Monitor bits End Delimiter End Delimiter

Token Ring Data/Command Frame Data/Command Frame

Token Ring Token Passing Token Passing Station can only transmit if it has the token Station can only transmit if it has the token Station passes token on if it has no data to transmit Station passes token on if it has no data to transmit –Station can hold the token – Station can hold the token for a maximum amount of Time depending on technology used

Token Ring Characteristic: Characteristic: Deterministic = Taking Turns Deterministic = Taking Turns Calculate the maximum time to transmit Calculate the maximum time to transmit Ideal for applications where predictability and dependability are paramount. Ideal for applications where predictability and dependability are paramount.

Token Ring Management Mechanisms Management Mechanisms Active Monitor Active Monitor One station acts as centralized source of timing information for other stations One station acts as centralized source of timing information for other stations Can be any station Can be any station Removes keep circulating frames Removes keep circulating frames

Token Ring Management Mechanisms Management Mechanisms MSAU MSAU Multi Station Access Units Multi Station Access Units Can see all information in a Token Ring Network Can see all information in a Token Ring Network Check for problems Check for problems Selectively remove stations from the ring if needed Selectively remove stations from the ring if needed

Token Ring Management Mechanisms Management Mechanisms Beaconing Beaconing Detects and repairs network faults Detects and repairs network faults Sends a beacon frame, defining a failure domain Sends a beacon frame, defining a failure domain Initiates auto-reconfiguration Initiates auto-reconfiguration Nodes within the failure domain automatically perform diagnostics Nodes within the failure domain automatically perform diagnostics Attempt to reconfigure around the failure Attempt to reconfigure around the failure MSAUs use electrical reconfiguration to accomplish this MSAUs use electrical reconfiguration to accomplish this

Token Ring Signaling Token Ring Signaling Uses Manchester Encoding Uses Manchester Encoding 0 is high-to-low transition 0 is high-to-low transition 1 is low-to-high transition 1 is low-to-high transition

Token Ring Media and Physical Topologies Logical ring topology Logical ring topology Physical star topology Physical star topology Stations are directly connected to MSAUs Stations are directly connected to MSAUs Patch cables connect MSAUs Patch cables connect MSAUs Lobe cables connect MSAUs to stations Lobe cables connect MSAUs to stations

Overview of FDDI Fiber distributed data interface Fiber distributed data interface FDDI is particularly popular as a campus backbone technology or in Internet critical applications where faults cannot be tolerated. FDDI is particularly popular as a campus backbone technology or in Internet critical applications where faults cannot be tolerated.

Preamble Preamble Prepares each station for the upcoming frame Prepares each station for the upcoming frame Start delimiter Start delimiter Frame Control Frame Control Indicates the size of the address fields Indicates the size of the address fields Indicates whether frame contains asynchronous or synchronous data Indicates whether frame contains asynchronous or synchronous data Other control information Other control information

Destination address Destination address 6 bytes 6 bytes Unicast: to one address Unicast: to one address Multicast: to several addresses Multicast: to several addresses Broadcast: to all addresses Broadcast: to all addresses Source address Source address Data Data Frame Check Sequence Frame Check Sequence End Delimiter End Delimiter Frame Status Frame Status

FDDI Token

FDDI MAC Token passing strategy Token passing strategy Early token release Early token release New token can be released when the frame transmission has finished New token can be released when the frame transmission has finished Deterministic Deterministic Dual ring Dual ring Ensures transmission, even if one ring is damaged or disabled Ensures transmission, even if one ring is damaged or disabled Very reliable Very reliable Real-time allocation of bandwidth Real-time allocation of bandwidth Defines two types of traffic Defines two types of traffic Synchronous Synchronous Asynchronous Asynchronous

FDDI Signaling Uses an encoding scheme called 4B/5B Uses an encoding scheme called 4B/5B Every four bits of data are sent as a 5 bit code Every four bits of data are sent as a 5 bit code Signal sources 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 Optical fiber is being installed at a rate of 4000 miles per day in the United States Explosive growth worldwide Explosive growth worldwide

Advantages of Optical Fiber Security Security Fiber does not emit electrical signals that can be tapped Fiber does not emit electrical signals that can be tapped Reliability Reliability Fiber is immune to electrical interference Fiber is immune to electrical interference Speed Speed Optical fiber has much higher throughput potential than copper cable Optical fiber has much higher throughput potential than copper cable

FDDI Rings FDDI specifies dual rings for physical connections FDDI specifies dual rings for physical connections Traffic on each ring travels in opposite directions Traffic on each ring travels in opposite directions Rings consist of two or more point-to- point connections between adjacent stations Rings consist of two or more point-to- point connections between adjacent stations Primary ring is for data transmission Primary ring is for data transmission Secondary ring is for back up Secondary ring is for back up

Ethernet Shortly after the 1980 IEEE specification, Digital Equipment Corporation (DEC), Intel Corporation, and Xerox Corporation jointly developed and released an Ethernet specification. Version 2.0, that was substantially compatible with IEEE Together, Ethernet and IEEE currently maintain the greatest market share of any LAN protocol. Shortly after the 1980 IEEE specification, Digital Equipment Corporation (DEC), Intel Corporation, and Xerox Corporation jointly developed and released an Ethernet specification. Version 2.0, that was substantially compatible with IEEE Together, Ethernet and IEEE 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 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

Ethernet

Ethernet Ethernet performs three functions: Ethernet performs three functions: Transmitting and receiving data packets 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 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 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 NIC Bridges Bridges Switches Switches

NICs Provides ports for network connection Provides ports for network connection Communicate with network via serial connection Communicate with network via serial connection Communication with computer through parallel connection Communication with computer through parallel connection Resources required: Resources required: IRQ, I/O address, upper memory addresses IRQ, I/O address, upper memory addresses

Selection Factors for NICs Type of network Type of network Ethernet, Token Ring, FDDI Ethernet, Token Ring, FDDI Type of media Type of media Twisted pair, coax, fiber Twisted pair, coax, fiber Type of system bus Type of system bus PCI, ISA PCI, ISA

NIC Operations Layer 1 & Layer 2 device Layer 1 & Layer 2 device Primarily Layer 2 Primarily Layer 2 Communicates with upper layers in the computer Communicates with upper layers in the computer Logical Link Control (LLC) Logical Link Control (LLC) Has MAC address burned in Has MAC address burned in Encapsulates data into frames Encapsulates data into frames Provides access to the media Provides access to the media Also Layer 1 Also Layer 1 Creates signals and interfaces with the media Creates signals and interfaces with the media On-board transceiver On-board transceiver

Bridges Connects two network segments Connects two network segments Can connect different layer 2 protocols Can connect different layer 2 protocols Ethernet, Token Ring, FDDI Ethernet, Token Ring, FDDI Makes intelligent decisions about traffic Makes intelligent decisions about traffic Reduces unnecessary traffic Reduces unnecessary traffic Minimizes collisions Minimizes collisions Filters traffic based on MAC address Filters traffic based on MAC address Maintains address tables Maintains address tables Rarely implemented today Rarely implemented today Conceptually important Conceptually important

Bridge Operations Bridging occurs at the data link layer: Bridging occurs at the data link layer: Controls data flow Controls data flow Handles transmission errors Handles transmission errors Provides physical addressing Provides physical addressing Manages access to the physical medium Manages access to the physical medium

Bridge Operations Transparent to upper layers Transparent to upper layers Best used in low traffic areas Best used in low traffic areas Can cause bottlenecks Can cause bottlenecks Must examine every packet Must examine every packet Broadcasts Broadcasts Messages sent to all devices Messages sent to all devices Destination MAC address unknown Destination MAC address unknown Bridge will always forward Bridge will always forward Can cause Broadcast Storm Can cause Broadcast Storm Network time outs, traffic slowdowns, unacceptable performance Network time outs, traffic slowdowns, unacceptable performance

Switching Operation Microsegmentation Each switch port acts as a micro bridge (Layer 2 device) Each switch port acts as a micro bridge (Layer 2 device) Multiple traffic paths within the switch Multiple traffic paths within the switch Virtual circuits Virtual circuits Temporarily exist - only when needed Temporarily exist - only when needed Each data frame has a dedicated path Each data frame has a dedicated path No collisions No collisions Increases bandwidth availability Increases bandwidth availability Each host gets full bandwidth Each host gets full bandwidth

Advantages of Switches Much faster than bridges Much faster than bridges Hardware based, not software Hardware based, not software Support new uses Support new uses e.g. virtual LANs e.g. virtual LANs Reduce collision domains Reduce collision domains

Advantages of Switches Allows many users to communicate in parallel Allows many users to communicate in parallel Creates virtual circuits Creates virtual circuits Creates dedicated segments Creates dedicated segments Collision free Collision free Maximizes bandwidth Maximizes bandwidth Cost effective Cost effective Can simply replace hubs in same cable infrastructure Can simply replace hubs in same cable infrastructure Minimal disruption Minimal disruption Flexible network management Flexible network management Software based configuration Software based configuration

Broadcast Domains All hosts connected to the same switch are still in the same broadcast domain 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 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: Two primary reasons for segmenting a LAN: Isolate traffic between segments Isolate traffic between segments Achieve more bandwidth per user by creating smaller collision domains Achieve more bandwidth per user by creating smaller collision domains

Bridge Drawback: Bridges increase the latency (delay) in a network by 10-30% 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. A bridge is considered a store-and- forward device slowing network transmissions, thus causing delay.

Routers The Router is a layer 3 (Network) device, but operates at layers 1-3. The Router is a layer 3 (Network) device, but operates at layers 1-3. Routers create the highest level of segmentation because of their ability to make exact determinations of where to send the data packet. Routers create the highest level of segmentation because of their ability to make exact determinations of where to send the data packet. Because routers perform more functions than bridges, they operate with a higher rate of latency. Because routers perform more functions than bridges, they operate with a higher rate of latency.

Identify Broadcast Domains and Collision Domains: Challenge

End Eng. Somoud Saqf El-Hait