Semester 1Semester 1Chapter6 + Chapter7Chapter6 + Chapter7

Layer 2Layer 2

Data Link LayerData Link Layer

Concepts And TechnologiesConcepts And Technologies

The OSI LayersThe OSI Layers

Data Link LayerData Link Layer

Data link layer provides reliable transit of Data link layer provides reliable transit of data across a physical link by using the data across a physical link by using the Media Access Control (MAC) addresses. Media Access Control (MAC) addresses.

Data link layer is concerned with physical Data link layer is concerned with physical (as opposed to network, or logical) (as opposed to network, or logical) addressing, network topology, line addressing, network topology, line discipline (how end systems will use the discipline (how end systems will use the network link), error notification, ordered network link), error notification, ordered delivery of frames, and flow control. delivery of frames, and flow control.

Layer 2 ImportanceLayer 2 Importance

Layer 1 cannot communicate with the upper Layer 1 cannot communicate with the upper level layers. Layer 2 does that with level layers. Layer 2 does that with Logical Link Logical Link Control (LLC)Control (LLC). .

Layer 1 cannot name or identify computers. Layer 1 cannot name or identify computers. Layer 2 uses an Layer 2 uses an addressingaddressing (or naming) process. (or naming) process.

Layer 1 can only describe streams of bits. Layer Layer 1 can only describe streams of bits. Layer 2 uses 2 uses framingframing to organize or group the bits. to organize or group the bits.

Layer 1 cannot decide which computer will Layer 1 cannot decide which computer will transmit binary data from a group that are all transmit binary data from a group that are all trying to transmit at the same time. Layer 2 trying to transmit at the same time. Layer 2 uses a system called uses a system called Media Access Control Media Access Control (MAC)(MAC). .

Logical Link Control SublayerLogical Link Control Sublayer

LLC is defined according to IEEE standard LLC is defined according to IEEE standard 802.2 802.2

LLC is independent of the specific LAN LLC is independent of the specific LAN technology used technology used

LLC serves to communicate upward to LLC serves to communicate upward to Layer 3 and downward to the technology-Layer 3 and downward to the technology-specific MAC sublayer. specific MAC sublayer.

LLC- ContLLC- Cont

LLC takes the network protocol data, an IP LLC takes the network protocol data, an IP packet, and adds more control information packet, and adds more control information to help deliver the IP packet to its to help deliver the IP packet to its destination. It adds two addressing destination. It adds two addressing components of the 802.2 specification, the components of the 802.2 specification, the Destination Service Access Point (DSAP) and Destination Service Access Point (DSAP) and the Source Service Access Point (SSAP). the Source Service Access Point (SSAP).

LLC supports both connectionless and LLC supports both connectionless and connection-oriented services used by higher connection-oriented services used by higher layer protocols. layer protocols.

Media Access Control Media Access Control SublayerSublayer

Media Access Control (MAC) refers to Media Access Control (MAC) refers to protocols that determine which protocols that determine which computer on a shared-medium computer on a shared-medium environment (collision domain) is environment (collision domain) is allowed to transmit the data. allowed to transmit the data.

categories of Media Access Control, categories of Media Access Control, deterministicdeterministic (taking turns) (taking turns) nondeterministicnondeterministic (first come, first (first come, first

served). served).

The MAC AddressThe MAC Address

A 48-bit address A 48-bit address burned onto the burned onto the NIC. It is a unique NIC. It is a unique way of identifying way of identifying each computer on each computer on a network.a network.

Flat AddressFlat Address

The first six comprise The first six comprise the Organizational the Organizational Unique Identifier (OUI) Unique Identifier (OUI) which identifies the which identifies the manufacturer manufacturer (assigned by the IEEE).(assigned by the IEEE).

The last six represent The last six represent a unique ID number for a unique ID number for the NIC.the NIC.

Everything Has a Everything Has a FormatFormat

FramingFraming

Framing is the Layer 2 encapsulation process. Framing is the Layer 2 encapsulation process. A frame is the Layer 2 protocol data unit (PDU). A frame is the Layer 2 protocol data unit (PDU).

Framing importance:Framing importance: Which computers are communicating with one Which computers are communicating with one

another another When communication between individual computers When communication between individual computers

begins and when it terminates begins and when it terminates A record of errors that occurred during the A record of errors that occurred during the

communication communication Whose turn it is to "talk" in a computer Whose turn it is to "talk" in a computer

"conversation "conversation

The Six Parts of a Generic FrameThe Six Parts of a Generic Frame

1. Frame start field – indicates the 1. Frame start field – indicates the beginning of a framebeginning of a frame

2. Address field – has source and 2. Address field – has source and destination address information in itdestination address information in it

3. Length/type/control field – indicates the 3. Length/type/control field – indicates the end (the frame is considered ended after end (the frame is considered ended after the FCS field); also sometimes called an the FCS field); also sometimes called an end-frame delimiter.end-frame delimiter.

The Six Parts of a Generic FrameThe Six Parts of a Generic Frame

4. Data field – the information you’re sending Data field – the information you’re sending including LLC bytesincluding LLC bytes

5. Frame check sequence (FCS) field – 5. Frame check sequence (FCS) field – contains a number that is calculated by the contains a number that is calculated by the source computer and is based on the data in source computer and is based on the data in the frame. The destination computer the frame. The destination computer recalculates the FCS number and compares it recalculates the FCS number and compares it with the source FCS number. It is an error-with the source FCS number. It is an error-checking device.checking device.

6. Stop frame field – indicates the end of a 6. Stop frame field – indicates the end of a frameframe

Ethernet Frame FormatEthernet Frame Format

Layer 2 TechnologiesLayer 2 Technologies Token RingToken Ring - logical ring - logical ring

topology (in other words, topology (in other words, information flow is controlled information flow is controlled in a ring) and a physical star in a ring) and a physical star topology (in other words, it is topology (in other words, it is wired as a star) wired as a star)

FDDIFDDI - logical ring topology - logical ring topology (information flow is (information flow is controlled in a ring) and controlled in a ring) and physical dual ring topology physical dual ring topology (wired as a dual ring) (wired as a dual ring)

EthernetEthernet - logical bus - logical bus topology (information flow is topology (information flow is on a linear bus) and physical on a linear bus) and physical star or extended star (wired star or extended star (wired as a star) as a star)

Token RingToken Ring Developed by IBM – still used todayDeveloped by IBM – still used today

Two frames:Two frames: TokenToken

Start DelimiterStart Delimiter Access Control ByteAccess Control Byte

Priority and Reservation Fields:Priority and Reservation Fields:Only stations with a priority equal to, or Only stations with a priority equal to, or

higher than, the priority value contained in higher than, the priority value contained in a token can seize that token. a token can seize that token.

Token and Monitor bitsToken and Monitor bits End DelimiterEnd Delimiter

Token RingToken Ring Data/Command FrameData/Command Frame

Token RingToken Ring

Token PassingToken Passing Station can only transmit if it has the tokenStation can only transmit if it has the token Station passes token on if it has no data to Station passes token on if it has no data to

transmittransmit

– Station can hold the tokenStation can hold the token

for a maximum amount offor a maximum amount of

Time depending on Time depending on technology usedtechnology used

Token RingToken Ring

Characteristic:Characteristic: Deterministic = Taking TurnsDeterministic = Taking Turns

Calculate the maximum time to Calculate the maximum time to transmit transmit

Ideal for applications where Ideal for applications where predictability and dependability are predictability and dependability are paramountparamount..

Token RingToken Ring

Management MechanismsManagement Mechanisms Active MonitorActive Monitor

One station acts as centralized source One station acts as centralized source of timing information for other of timing information for other stationsstations

Can be any stationCan be any station Removes keep circulating framesRemoves keep circulating frames

Token RingToken Ring

Management MechanismsManagement Mechanisms MSAUMSAU

Multi Station Access UnitsMulti Station Access Units Can see all information in a Token Can see all information in a Token

Ring NetworkRing Network Check for problemsCheck for problems Selectively remove stations from the Selectively remove stations from the

ring if neededring if needed

Token RingToken Ring Management MechanismsManagement Mechanisms

BeaconingBeaconing Detects and repairs network faultsDetects and repairs network faults Sends a beacon frame, defining a failure Sends a beacon frame, defining a failure

domaindomain Initiates auto-reconfigurationInitiates auto-reconfiguration

Nodes within the failure domain Nodes within the failure domain automatically perform diagnosticsautomatically perform diagnostics

Attempt to reconfigure around the Attempt to reconfigure around the failurefailure

MSAUs use electrical reconfiguration MSAUs use electrical reconfiguration to accomplish thisto accomplish this

Token Ring SignalingToken Ring Signaling

Uses Manchester EncodingUses Manchester Encoding

0 is high-to-low transition0 is high-to-low transition 1 is low-to-high transition1 is low-to-high transition

Token RingToken Ring

Media and Physical Media and Physical TopologiesTopologies Logical ring topologyLogical ring topology Physical star topologyPhysical star topology

Stations are directly Stations are directly connected to MSAUsconnected to MSAUs

Patch cables connect Patch cables connect MSAUsMSAUs

Lobe cables connect Lobe cables connect MSAUs to stationsMSAUs to stations

Overview of FDDIOverview of FDDI Fiber distributed data interfaceFiber distributed data interface FDDI is particularly popular as a FDDI is particularly popular as a

campus backbone technology or campus backbone technology or in Internet critical applications in Internet critical applications where faults cannot be tolerated. where faults cannot be tolerated.

PreamblePreamble Prepares each station for the upcoming Prepares each station for the upcoming

frameframe Start delimiterStart delimiter

Frame ControlFrame Control Indicates the size of the address fieldsIndicates the size of the address fields Indicates whether frame contains Indicates whether frame contains

asynchronous or synchronous dataasynchronous or synchronous data Other control informationOther control information

Destination addressDestination address 6 bytes6 bytes Unicast: to one addressUnicast: to one address Multicast: to several addressesMulticast: to several addresses Broadcast: to all addressesBroadcast: to all addresses

Source addressSource address DataData Frame Check SequenceFrame Check Sequence End DelimiterEnd Delimiter Frame StatusFrame Status

FDDI TokenFDDI Token

FDDI MACFDDI MAC Token passing strategyToken passing strategy Early token releaseEarly token release

New token can be released when the frame New token can be released when the frame transmission has finishedtransmission has finished

DeterministicDeterministic Dual ringDual ring

Ensures transmission, even if one ring is damaged or Ensures transmission, even if one ring is damaged or disableddisabled

Very reliableVery reliable Real-time allocation of bandwidthReal-time allocation of bandwidth

Defines two types of trafficDefines two types of traffic SynchronousSynchronous AsynchronousAsynchronous

FDDI SignalingFDDI Signaling

Uses an encoding scheme called 4B/5BUses an encoding scheme called 4B/5B Every four bits of data are sent as a 5 bit Every four bits of data are sent as a 5 bit

codecode Signal sourcesSignal sourcesare LEDs or lasersare LEDs or lasers

FDDI MediaFDDI Media

Optical fiber is being installed at a Optical fiber is being installed at a rate of 4000 miles per day in the rate of 4000 miles per day in the United StatesUnited States

Explosive growth worldwideExplosive growth worldwide

Advantages of Optical FiberAdvantages of Optical Fiber

SecuritySecurity Fiber does not emit electrical signals Fiber does not emit electrical signals

that can be tappedthat can be tapped ReliabilityReliability

Fiber is immune to electrical Fiber is immune to electrical interferenceinterference

SpeedSpeed Optical fiber has much higher Optical fiber has much higher

throughput potential than copper cablethroughput potential than copper cable

FDDI RingsFDDI Rings

FDDI specifies dual rings for physical FDDI specifies dual rings for physical connectionsconnections

Traffic on each ring travels in opposite Traffic on each ring travels in opposite directionsdirections

Rings consist of two or more point-to-Rings consist of two or more point-to-point connections between adjacent point connections between adjacent stationsstations

Primary ring is for data transmissionPrimary ring is for data transmission Secondary ring is for back upSecondary ring is for back up

EthernetEthernet

Shortly after the 1980 IEEE 802.3 specification, Shortly after the 1980 IEEE 802.3 specification, Digital EquipmentDigital Equipment Corporation (Corporation (DECDEC), ), Intel Intel Corporation, and Corporation, and XeroxXerox Corporation jointly Corporation jointly developed and released an Ethernet developed and released an Ethernet specification. Version 2.0, that was substantially specification. Version 2.0, that was substantially compatible with IEEE 802.3. Together, Ethernet compatible with IEEE 802.3. Together, Ethernet and IEEE 802.3 currently maintain the greatest and IEEE 802.3 currently maintain the greatest market share of any LAN protocol. market share of any LAN protocol.

EthernetEthernet

Today, the term Ethernet is often Today, the term Ethernet is often used to refer to all carrier sense used to refer to all carrier sense multiple access/collision detection multiple access/collision detection ((CSMA/CDCSMA/CD) LAN’s that generally ) LAN’s that generally conform to Ethernet specifications, conform to Ethernet specifications, including IEEE 802.3. including IEEE 802.3.

EthernetEthernet

EthernetEthernet

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 decoding data packets and checking them for valid

addresses before passing them to the upper layers of addresses before passing them to the upper layers of the OSI model 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 In the CSMA/CD access method, networking devices with data to transmit over the networking media work in a listen-data to transmit over the networking media work in a listen-before-transmit mode. before-transmit mode.

Common LAN DevicesCommon LAN Devices

NICNIC BridgesBridges SwitchesSwitches

NICsNICs Provides ports for network connectionProvides ports for network connection Communicate with network via serial Communicate with network via serial

connectionconnection Communication with computer through Communication with computer through

parallel connectionparallel connection Resources required:Resources required:

IRQ, I/O address, upper memory addressesIRQ, I/O address, upper memory addresses

Selection Factors for NICsSelection Factors for NICs

Type of networkType of network Ethernet, Token Ring, FDDIEthernet, Token Ring, FDDI

Type of mediaType of media Twisted pair, coax, fiberTwisted pair, coax, fiber

Type of system busType of system bus PCI, ISAPCI, ISA

NIC OperationsNIC Operations Layer 1 & Layer 2 deviceLayer 1 & Layer 2 device Primarily Layer 2Primarily Layer 2

Communicates with upper layers in the Communicates with upper layers in the computercomputer Logical Link Control (LLC)Logical Link Control (LLC)

Has MAC address burned inHas MAC address burned in Encapsulates data into framesEncapsulates data into frames Provides access to the mediaProvides access to the media

Also Layer 1Also Layer 1 Creates signals and interfaces with the Creates signals and interfaces with the

mediamedia On-board transceiverOn-board transceiver

BridgesBridges Connects two network segmentsConnects two network segments

Can connect different layer 2 Can connect different layer 2 protocolsprotocols Ethernet, Token Ring, FDDI Ethernet, Token Ring, FDDI

Makes intelligent decisions about trafficMakes intelligent decisions about traffic Reduces unnecessary trafficReduces unnecessary traffic Minimizes collisionsMinimizes collisions Filters traffic based on Filters traffic based on MACMAC address address

Maintains address tablesMaintains address tables Rarely implemented todayRarely implemented today

Conceptually importantConceptually important

Bridge OperationsBridge Operations Bridging occurs at the data link layer:Bridging occurs at the data link layer:

Controls data flowControls data flow Handles transmission errors Handles transmission errors Provides physical addressingProvides physical addressing Manages access to the physical medium Manages access to the physical medium

Bridge OperationsBridge Operations Transparent to upper layersTransparent to upper layers Best used in low traffic areasBest used in low traffic areas

Can cause bottlenecksCan cause bottlenecks Must examine every packetMust examine every packet

BroadcastsBroadcasts Messages sent to all devicesMessages sent to all devices

Destination MAC address unknownDestination MAC address unknown Bridge will always forwardBridge will always forward Can cause Can cause Broadcast StormBroadcast Storm

Network time outs, traffic Network time outs, traffic slowdowns, unacceptable slowdowns, unacceptable performanceperformance

Switching OperationSwitching OperationMicrosegmentationMicrosegmentation

Each switch port acts as a micro Each switch port acts as a micro bridge (Layer 2 device)bridge (Layer 2 device)

Multiple traffic paths within the switchMultiple traffic paths within the switch Virtual circuitsVirtual circuits Temporarily exist - only when Temporarily exist - only when

neededneeded Each data frame has a dedicated pathEach data frame has a dedicated path

No collisionsNo collisions Increases bandwidth availabilityIncreases bandwidth availability

Each host gets full bandwidthEach host gets full bandwidth

Advantages of SwitchesAdvantages of Switches

Much faster than bridgesMuch faster than bridges Hardware based, not softwareHardware based, not software

Support new usesSupport new uses e.g. virtual LANse.g. virtual LANs

Reduce collision domainsReduce collision domains

Advantages of SwitchesAdvantages of Switches Allows many users to communicate in parallelAllows many users to communicate in parallel

Creates virtual circuitsCreates virtual circuits Creates dedicated segmentsCreates dedicated segments

Collision free Collision free Maximizes bandwidthMaximizes bandwidth

Cost effectiveCost effective Can simply replace hubs in same cable Can simply replace hubs in same cable

infrastructureinfrastructure Minimal disruptionMinimal disruption

Flexible network managementFlexible network management Software based configurationSoftware based configuration

Broadcast DomainsBroadcast Domains

All hosts connected to the same All hosts connected to the same switch are still in the same broadcast switch are still in the same broadcast domaindomain

A broadcast from one node will be A broadcast from one node will be seen by all other nodes connected seen by all other nodes connected through the LAN switchthrough the LAN switch

LAN SegmentaionLAN Segmentaion

Two primary reasons for segmenting Two primary reasons for segmenting a LAN:a LAN: Isolate traffic between segmentsIsolate traffic between segments Achieve more bandwidth per user by Achieve more bandwidth per user by

creating smaller collision domainscreating smaller collision domains

Bridge Drawback:Bridge Drawback:

Bridges increase the Bridges increase the latencylatency (delay) (delay) in a network by 10-30%in a network by 10-30%

A bridge is considered a store-and-A bridge is considered a store-and-forward device slowing network forward device slowing network transmissions, thus causing delay.transmissions, thus causing delay.

RoutersRouters

The Router is a layer 3 (Network) The Router is a layer 3 (Network) device, but operates at layers 1-device, but operates at layers 1-3.3. Routers create the highest level of Routers create the highest level of

segmentation because of their ability to make segmentation because of their ability to make exact determinations of where to send the exact determinations of where to send the data packet. data packet.

Because routers perform more functions than Because routers perform more functions than bridges, they operate with a higher rate of bridges, they operate with a higher rate of latency. latency.

Identify Broadcast Domains and Collision Domains:

ChallengeChallenge

EndEnd

Eng. Somoud Saqf El-HaitEng. Somoud Saqf El-Hait