Transcript Link Layer - Instituto Tecnológico de Morelia

DataLink Layer
M.C. Juan Carlos Olivares Rojas
Department of Computer and System
Instituto Tecnológico de Morelia
[email protected]
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,
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 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
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.
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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