Section 3 The OSI Data Link Layer CSIS 479R Fall 1999 “Network +” George D. Hickman, CNI, CNE.

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