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Section 3 The OSI Data Link Layer
CSIS 479R Fall 1999
“Network +”
George D. Hickman, CNI, CNE
Objectives
Identify the basic purpose of the OSI Data Link layerIdentify the characteristics of the two logical topologiesIdentify the characteristics of the three media access methodsDescribe how addresses are defined and managed at the Data Link layer
Objectives (con’t)
Describe the transmission synchronization techniques used at the data link layerDescribe the connection services implemented at the Data Link layerDescribe the IEEE 802.x standardsDescribe the 802.3 standard and Ethernet
Objectives (con’t)
Describe the 802.3u Fast Ethernet standardDescribe the 802.5 and Token Ring standardsDescribe the Fiber Distributed Data Interface (FDDI) standardDescribe commonly used wide area networking protocols
Data Link Layer
Media Access Control Sublayer controls how transmitters share single
media
Logical Link Control Sublayer establishes and maintains device to
device link
Organize Physical layer’s bits into framesDetect and sometimes correct errorsControl Data Flow
Data Link Layer
Identify Computers on the network
Data Link layer header contains:Source and destination addressesFrame length information Indication of upper layer protocols involved
Data Link Devices
Network Connectivity DevicesBridgesSwitchesNICs
Data Link--MAC
Logical Topology ProcessBus and Ring methods
Media Access ProcessContention, Token Passing, & Polling
methods
Addressing ProcessPhysical device method
Data Link -- LLC
Transmission Synchronization ProcessAsynchronous, Synchronous, Isochronous
methods
Connection Services ProcessLLC-level flow controlError control
Logical Topology
The actual signal pathAs opposed to the Physical Topology,
which is the physical layout of wires
Logical and Physical paths do not have to be the sameToken Ring ExamplePhysical starLogical Ring
Media Access Control
Contention Devices transmit when ever they want Causes collisions
Carrier Sense Newer contention scheme Listens to media, transmits if no signal detected CSMA
Carrier Sense, Multiple Access
Carrier Sense
CSMA Collision detection and retransmission is the
responsibility of a “higher layer” protocol Waiting and overhead of going up and down OSI
models make CSMA less effective
CSMA/CD Adds collision detection at or below the DL layer
by sensing cable before and after transmitting After collisions, wait random time and retransmit Good for bursty traffic
Token Passing
The Token (a small frame) is passed to give media access control. Only devices with the token may transmit
Devices know where they get token from and where they pass it toIEEE 802.5 Token Ring Standard Token passing access control, physical or logical ring
topology
FDDIGood for time sensitive (voice, video) traffic or heavily populated networks
Polling Systems
One device (the controller, primary, or master) is media access administrator
Queries other devices (secondaries) in a predefined order to see if they need to transmit
Ideal for networking time-sensitive devices like automation equipment
Addresses–Defined and Managed
Data Link layer is concerned with the physical device address or MAC address
Most DL layer implementations place the source and destination addresses in the frame header The frame is sent to every device on network, which
reads header and reads or ignores the data as needed
Bridges use these addresses to let frames “through” or not
Switches use these addresses to know which port to send data frames to
Transmission Synchronization
Techniques—DL layer
The physical layer synchronization was bits
The Data Link layer synchronization is the coordination of frame transmission
Asynchronous
Each device has own clock, not synchronized with the otherStart and Stop bits usedGood for random interval transmissionsParity bit can be added to detect some errorsEven parity Parity bit is set to give an even number of 1 bits/byte
Odd parity Parity bit is set to give an odd number of 1 bits/byte
May not detect multiple bit errors
Synchronous
Devices responsible for a framing clockCan be separate channelOr use SYN or SYNC characters for “start”A CRC value can be put near end for error checkingBoth devices must use same algorithm to compute CRC
Isochronous
A clock signal is sent out to all network devices to create time slots
Other devices may fill “slots” with data
Clock signal is not provided with every frame (like asynchronous) or at start of a string of data (like synchronous)
DL layer implementations of Connection Services
Unacknowledged connectionless services Send and receive frames with no flow, error, or
packet sequence control
Connection-oriented services Flow, error, and packet sequence control provided
by use of acknowledgments
Acknowledged connectionless services Flow and error control provided by
acknowledgements between point to point transmissions
LLC level Flow ControlGuaranteed Rate Flow control A rate is agreed upon before transmission and is
maintained as long as the transmission lasts
Window flow control Static
An acceptable window or buffer size is determined. That number of frames is maximum sent without an acknowledgement
Dynamic Window or buffer size can be adjusted. A choke packet
is sent by receiver when the buffer exceeds a specified level, slowing down the sender. Transmission is slowly increased until another choke packet is sent.
Error Control
How lost or scrambled frames are handledSending device receives NAK (or nothing)Checksums do not matchPacket size is off (too small)
Can be caused by Noise, Interference, or DistortionCan be caused by a buffer overflow
IEEE 802.x standards
1980 IEEE defined LAN standards for Physical and Data Link layers
802.1Allows 802 compliant device to speak with
another 802 device on another LAN or WAN
802.2Defines LLC sublayer of Data Link layer
IEEE 802.x standards
802.3 Physical layer specifications
Baseband/broadband Media type Topologies Data rate
Three part naming convention Speed (megabits per second) BASE or BROAD Special designator or effective distance
10BASE2
IEEE 802.x standards
802.4 Factory and Industrial automation needs
Physical bus topology Token Passing media access Baseband or Broadband media 75 Ohm CATV-type cable or optical fiber
802.5 Based on IBM Token Ring
Token Passing media access 1, 4, or 16 Mbps No specific transmission media or physical topology
mandated (IBM Token Ring mandates both.)
IEEE 802.x standards
802.6Distributed Queue Dual Bus (DQDB)
802.7Standards for broadband communications
802.8Fiber Optic standards
802.9 Isochronous Ethernet (voice/data)
IEEE 802.x standards
802.10Used with encryption key information
802.11Used with wireless LAN implementations
802.12100 Mbps physical star, contention based
(100VG-AnyLAN)
Ethernet
Combination of 802.2 and 802.3Designed as simple, low access-overhead LAN architectureSee diagram on page 3-36Can use Thick or thin Co-axe BUS Topology
Can use Twisted Pair, or fiber optic cable, using either switches or hubs STAR Topology
10BASE5 Thick Coaxial cabling
NICs use external transceiver
50 Ohm terminator both ends, 1 grounded
500 Meter maximum segment length
100 devices per segment maximum (incl repeater)
3 populated segments maximum
2.5 M between taps / 5 M (max) tap to node
10BASE2 Thin Coaxial
NICs use internal transciever50 Ohm terminator both ends, 1 grounded185 M maximum segment length30 devices per segment max (incl repeaters)3 populated segments maximum.5 M minimum between T connectors
10BASE-T Twisted Pair
100 M maximum segment length
1,024 maximum workstations (theoretical)
4 repeaters maximum between communicating devices
5-4-3 Rule
Coaxial5 cable segments maximum4 repeaters maximum3 segments populated
UTP5 cable segments maximum4 hubs maximum
IEEE 802.3u (Fast Ethernet)
Physical and Logical Topologies Physical hierarchical star / Logical Star
Media Independent Interface (MII) 100BASE-TX, 2 pair Cat 5 UTP 100M/segment 100BASE-T4, 4 pair Cat 3+ UTP 100M/segment 100BASE-FX, 2 strand FO, 412-10,000M/segment
Auto Negotiation (AUTONEG) 10 or 100 Mbps auto negotiated
Media Access Control (MAC) CSMA/CD
IEEE 802.5 Token RingSpecifications for Physical Layer and MAC sublayer of Data LinkPhysical Star Logical Ring Topology802.5 does not specify cabling typeCable lengths differ with media used (page 3-46)
3 cable segments per series33 MSAUs maximum
802.5 specifies 250 nodes maximum IBM STP specifies 260 nodes IBM UTP specifies 72 nodes
All network devices must run at same speed (4 or 16 Mbps) unless connected by a bridge
802.5 Token Ring MAC
Token Passing Special packet allowing device to transmitDevice receives packet, transmits a frame.
When frame returns to sender, sender puts a new token out on ring.
Early token release-creates/releases new token immediately after sending data frame
Active monitor performs maintenance on ring
Beaconing
Allows some automatic error recoveryUpon ring break, stations send beacon frames until they receive a beacon frame from an “upstream neighbor”Soon only one station (after break) is beaconingMSAU attempts to reconfigure ring around the break
FDDI Standard
Fiber Distributed Data Interface
Physical Layer and MAC sublayer+SMTAssumes 802.2 (LLC sublayer specification)
Fills need for secure high bandwidthBackbone implementation (connects LANs)Computer room networks (mainframes, minis)High Data Rate LANS (CAD/video needs)
FDDI and 802.5
Both use Token passing MAC
Both physical star logical ring
Both (can) use fiber-optic media
FDDI has higher maximum data rate
FDDI Network Design
2 counter rotating ringsPrimary caries data, secondary managementSecondary becomes primary if P. media fails
1000 workstations max, 200 km total cable500 / 100 incase of media failure
Multi-mode fiber optic cable/62.5 micrometer
Repeater required every 2 km or less
FDDI Network Design (con’t)
Class A WorkstationsConnect to both rings. Higher fault tolerance
Class B WorkstationsConnects to primary ring. Can’t reconfigure
FDDI ALWAYS releases new token at end of transmitted frames, so there may be multiple frames on network at once.
See figure 3-27 on page 3-53
WAN ProtocolsDial up connectionsSLIP (Serial Line Internet Protocol)
Physical layer protocolNot a strict standard, may not work
PPP (Point-to-Point Protocol)Physical and Data Link layer protocolAllows Dynamic IP addressingSupport multiple protocols on same linkPassword loginError control
WAN Protocols (con’t)PPTP (Point-to-Point Tunneling ProtocolPPP extensionEncapsulates other protocols for IP transmissionCorporations use Internet to connect their LANsCan read multi-protocol packets
X.25Attaching computer to a packet-switched networkPhysical, Data Link, Network LayersSprintNet, Tymnet, GTE
WAN Protocols (con’t)Frame RelayDesigned for high speed bursts on digital networkNo error checking while transmitting, so fasterError checking at receiving pointEthernet, X.25, Token Ring common56 kpbs to 1.544 Mbps common (56K, T-1, T-3)Physical and Data Link layersPurchased by CIR (Committed Information Rates),
the minimum guaranteed capacity of virtual circuit
WAN Protocols (con’t)
ISDN B-ISDN Integrated Services Digital Network
Channel A 4 KHz analog channel
Channel B 64 Kbps digital channel
Channel C 8 or 16 Kbps digital
Interconnects X.25, PPP, frame relay
Physical, Data Link, Network Layers
Popular with SOHO
WAN Protocols (con’t)
ATM Asynchronous Transfer Mode B-ISDN and Cell Relay (Cell is 53 byte block)
Considered a LAN and WAN protocol
Primarily Data Link and Network Layer
Designed to be independent of Physical Layer for faster processing speeds 155 Mbps and 622 Mbps specified. 10Gbps expected
FDDI or others specify the Physical layer