DataLink Layer

DataLink Layer

M.C. Juan Carlos Olivares Rojas

Department of Computer and SystemInstituto Tecnológico de Morelia

jcolivar@itmorelia.edu.mx19.72388 lat, -101.1848 long

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OutlineConcepts.

MAC Addressing.

Framming

Medium Access Control

IEEE 802.x Technologies

Basic Principles.

Token Ring.

Ethernet and its variants.

FDDI

Objectives of the Session• The students will know the basis of

intenrnational computer networks standards.

• The students will know and apply the LAN concepts.

Concepts

DataLink Services

• Provide services to the Network Layer

• Send and receive data in a frame format

• Processing and error correction

• DataFlow Control

• Medium Access Control ***

Where is the link layer implemented?

• in each and every host• link layer implemented in

“adaptor” (aka network interface card NIC)– Ethernet card, PCMCI card,

802.11 card– implements link, physical

layer

• attaches into host’s system buses

• combination of hardware, software, firmware

controller

physicaltransmission

cpu memory

host bus (e.g., PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

MAC Addressing• MAC (or LAN or physical or Ethernet) address:

– function: get frame from one interface to another physically-connected interface (same network)

– 48 bit MAC address (for most LANs)• burned in NIC ROM, also sometimes software settable

5: DataLink Layer 5-8

LAN Addresses

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Framming

• sending side:– encapsulates datagram in

frame– adds error checking bits,

rdt, flow control, etc.

• receiving side– looks for errors, rdt, flow

control, etc– extracts datagram, passes to

upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

Medium Access Control• There are a lot of technices for sharing the

transmision medium. The more used in computer networks are:

• ALOHA

• CSMA

• Protocols without colision

• Wireless Protocol

• Other Multiplexation

ALOHAThe frames are transmitting in arbitrary moment

5: DataLink Layer 5-12

CSMA (Carrier Sense Multiple Access)

CSMA: listen before transmit:

If channel sensed idle: transmit entire frame• If channel sensed busy, defer transmission

• human analogy: don’t interrupt others!

• collisions can still occur:• propagation delay means • two nodes may not hear• each other’s transmission

• role of distance & propagation delay in determining collision probability

Persistent and Not PersistenteCSMA

5: DataLink Layer 5-14

CSMA/CD (Collision Detection)

CSMA/CD: carrier sensing, deferral as in CSMA– collisions detected within short time– colliding transmissions aborted, reducing channel

wastage

• collision detection: – easy in wired LANs: measure signal strengths,

compare transmitted, received signals– difficult in wireless LANs: received signal strength

overwhelmed by local transmission strength

• human analogy: the polite conversationalist

CSMA Collision Detection

CSMA/CD can be in 3 states: contention, transmission, or idle

5-16

Token Passing

• control token passed from one node to next sequentially.

• token message

• concerns:• token overhead • Latency• single point of failure

(token)

T

data

(nothingto send)

T

IEEE 802.x Technologies• The group of standards 802.x is concern about the

implementation and use of Local Area Network (e.g. TokenRing, Ethernet) and Wide Area Network (e.g. FDDI, WiMax).

• These standars are focused in DataLink Layer. The transmission medium can be wired o wireless.

• Some standards are focused in define services in DataLink Layer such quality of service, security, among others.

Basic Principles• The standards only indicate how computer

networks must be works guarantee interoperability between another Equipments.

• The main functions in the 802.x standards are the framming and Medium Access Control.

Token Ring• Token ring is a local area network protocol

which resides at the data link layer (DLL) of the OSI model. It uses a special three-byte frame called a token that travels around the ring. Token ring frames travel completely around the loop.

• Cabling is generally IBM "Type-1" shielded twisted pair, with unique hermaphroditic connectors.

Token Ring• Initially (in 1985) token ring ran at 4 Mbit/s, but

in 1989 IBM introduced the first 16 Mbit/s token ring products and the 802.5 standard was extended to support this.

• Token ring LANs normally use differential Manchester encoding of bits on the LAN media.

Token Ring

Token Ring

Ethernet and its variants

“dominant” wired LAN technology: • cheap $20 for NIC• first widely used LAN technology• simpler, cheaper than token LANs and ATM• kept up with speed race: 10 Mbps – 10 Gbps

Metcalfe’s Ethernetsketch

Ethernet: Unreliable, connectionless• connectionless: No handshaking between

sending and receiving NICs

• unreliable: receiving NIC doesn’t send acks or nacks to sending NIC– stream of datagrams passed to network layer can

have gaps (missing datagrams)– gaps will be filled if app is using TCP– otherwise, app will see gaps

• Ethernet’s MAC protocol: unslotted CSMA/CD

5: DataLink Layer 5-25

Manchester encoding

• used in 10BaseT• each bit has a transition• allows clocks in sending and receiving nodes to synchronize to

each other– no need for a centralized, global clock among nodes!

• Hey, this is physical-layer stuff!

Ethernet Evolution

802.3 MAC Frame

Categories of Standard Ethernet

Encoding in a Standard Ethernet

10Base5 implementation

10Base2 implementation

10Base-T implementation

10Base-F implementation

Summary of Standard Ethernet implementations

A network with and without a bridge

Switched Ethernet

Fast Ethernet implementations

Encoding for Fast Ethernet

Summary of Fast Ethernet

Gigabit Ethernet

Encoding in Gigabit Ethernet

Summary of Gigabit Ethernet

Summary of Ten-Gigabit Ethernet

Exam for Unit 6• Deadline: Friday, December 5, 2008 at

Professor Cubicle.

• Equipments: 3 persons maximum

• Research Structured Cabling (All the contents of Unit 6).

• Documentation 70%

• Oral Exam 30%

Unidad VI Fundamentos de Construcción de una LAN

6.1 Fundamentos

6.2 Cableado estructurado.

6.2.1 Estándares vigentes.

6.2.2 Diseño y documentación básicos de redes.

6.2.3 Seguridad física.

6.2.4 Planificación del cableado estructurado.

6.2.4.1 Backbone

6.2.4.2 Cableado horizontal.

Unidad VI Fundamentos de Construcción de una LAN

6.2.5 Especificación del centro de cableado (SITE).

6.3 Análisis de necesidades.

6.4 Diseño de una LAN.

6.5 Instalación y configuración.

Wireless Ethernet

Basic Service Sets

IEEE 802.11

Extended Service Sets

MAC Layers in WiFi

CSMA/CA with NAV

802.11 Frame Format

802.11 Control Frames

FDDI • Fiber distributed data interface (FDDI) provides a

standard for data transmission in a local area network that can extend in range up to 200 kilometers.

• These protocol is derived from the IEEE 802.4 token bus timed token protocol.

• It uses optical fiber (though it can use copper cable, in which case one can refer to CDDI). FDDI uses a dual-attached, counter-rotating token ring topology.

FDDI• FDDI-II, a version of FDDI, adds the capability to add circuit-switched service to the network so that it can also handle voice

and video signals. Work has started to connect FDDI networks to the developing Synchronous Optical Network SONET.• A FDDI network contains two token rings, one for possible backup in case the primary ring fails. The primary ring offers up to

100 Mbit/s capacity. When a network has no requirement for the secondary ring to do backup, it can also carry data, extending capacity to 200 Mbit/s. The single ring can extend the maximum distance; a dual ring can extend 100 km (62 miles). FDDI has a larger maximum-frame size than standard 100 Mbit/s Ethernet, allowing better throughput.

• Designers normally construct FDDI rings in the form of a "dual ring of trees" (see network topology). A small number of devices (typically infrastructure devices such as routers and concentrators rather than host computers) connect to both rings - hence the term "dual-attached". Host computers then connect as single-attached devices to the routers or concentrators. The dual ring in its most degenerate form simply collapses into a single device. Typically, a computer-room contains the whole dual ring, although some implementations have deployed FDDI as a Metropolitan area network.

• FDDI requires this network topology because the dual ring actually passes through each connected device and requires each such device to remain continuously operational (the standard actually allows for optical bypasses, but network engineers consider these unreliable and error-prone). Devices such as workstations and minicomputers that might not come under the control of the network managers are not suitable for connection to the dual ring.

• Dual-attach FDDI Board• As an alternative to using a dual-attached connection, a workstation can obtain the same degree of resilience through a dual-

homed connection made simultaneously to two separate devices in the same FDDI ring. One of the connections becomes active while the other one is automatically blocked. If the first connection fails, the backup link takes over with no perceptible delay.

• Due to their speed, cost and ubiquity, fast Ethernet and (since 1998) Gigabit Ethernet have largely made FDDI redundant.• FDDI standards include:• ANSI X3.166-1989, Physical Medium Dependent (PMD) -- also ISO 9314-3 • ANSI X3.148-1988, Physical Layer Protocol (PHY) -- also ISO 9314-1 • ANSI X3.139-1987, Media Access Control (MAC) -- also ISO 9314-2 • ANSI X3.229-1994, Station Management (SMT) -- also ISO 9314-6 • ANSI X3.184-1993, Single Mode Fiber Physical Medium Dependent (SMF-PMD) -- also ISO 9314-4

FDDI

References• Forouzan, B. (2008), Data Comunications and

Networking, 4th. Edition, McGraw-Hill.

• Tanenbaum, A (2004). Computer Networks. 4th Edition. Prentice Hall.

• Kurose, J. and Ross, K. (2007) Computer Networking: A Top Down Approach 4th edition. Addison-Wesley, July 2007.

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