DataLink Layer M.C. Juan Carlos Olivares Rojas Department of Computer and System Instituto Tecnol贸gico de Morelia jcolivar@itmorelia.edu.mx 19.72388 lat, -101.1848 long
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Outline Concepts. 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
host schematic application transport network link
cpu
memory
controller link physical
host bus (e.g., PCI)
physical transmission
network adapter card
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
LAN Addresses Each adapter on LAN has unique LAN address
Broadcast address = FF-FF-FF-FF-FF-FF
1A-2F-BB-76-09-AD
LAN (wired or wireless) 71-65-F7-2B-08-53
= adapter 58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
5: DataLink Layer
5-8
Framming datagram
datagram controller
controller
receiving host
sending host datagram
frame
• 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
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
ALOHA The frames are transmitting in arbitrary moment
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 5: DataLink Layer
5-12
Persistent and Not PersistenteCSMA
CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA – collisions detected within short time – colliding transmissions aborted, channel wastage
reducing
• collision detection: – easy in wired LANs: measure signal strengths, compare transmitted, received signals – difficult in wireless LANs: received signal strength overwhelmed by local transmission 5: DataLink Layer 5-14 strength
CSMA Collision Detection
CSMA/CD can be in 3 states: contention, transmission, or idle
Token Passing • control token passed
T
from one node to next sequentially. • token message • concerns: • token overhead • Latency • single point of failure (token)
(nothing to send) T
data 5-16
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,
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 Ethernet sketch
Ethernet: Unreliable, connectionless • connectionless: No handshaking sending and receiving NICs
between
• 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
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! 5: DataLink Layer
5-25
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, Professor Cubicle.
December
5,
2008
at
• 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
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.
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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