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Chapter 7
Data Link Layer
João José
http://w3.ualg.pt/~jjose/cisco/
Based on:
Graziani, R. (2008) CIS 81 Networking Fundamentals
Cisco CCNA 1 Exploration - Networking
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Data Link Layer
Ethernet,
PPP,
ISDN,
Frame
Relay
The Data Link layer provides a means for exchanging data over a common
local media.
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Application
Header + data
Generic Data Link Header / Trailer
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Focus on Data Link Layer
IP
IP
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Data Link Frame
The Data Link layer provides a means for exchanging data over a common
local media.
The Data Link layer performs two basic services:
1. Allows the upper layers to access the media using techniques such as
framing
2. Controls how data is placed onto the media and is received from the
media.
1.
2.
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Data Link Frame
The Data Link layer prepares a packet for transport across the local
media by encapsulating it with a header and a trailer to create a
frame.
The Data Link layer frame includes:
Data – Layer 3 (IP Packet or other Layer 3 information)
Header - Control information, such addressing
Trailer - Control information, such as error detection
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Data Link
Frame
Typical field types may include:
Start and stop indicator fields
Addressing fields
Type field - The type of PDU contained in the frame
Quality - control fields
Data field -The frame payload (Network layer packet)
Not all protocols include all of these fields.
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Data Link Layer
Data Link layer
Connects the Network Layer with the Physical Layer
Network Layer and above is software (IP, TCP, HTTP, etc.)
Physical layer is implemented in hardware (converting bits to a transmission signal)
Data Link layer is implemented in both:
Software
Hardware
Data Link Layer prepares Network Layer packets for transmission across some form of media, be it copper, fiber, or the atmosphere.
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Data Link Sublayers
Data Link layer has two sublayers (sometimes):
Logical Link Control (LLC) – Software processes that provide
services to the Network layer protocols.
Frame information that identifies the Network layer protocol.
Multiple Layer 3 protocols, (IP and IPX) can use the same
network interface and media.
Media Access Control (MAC) - Media access processes
performed by the hardware.
Provides Data Link layer addressing and framing of the data
according to the protocol in use.
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Data Link Standards
TCP/IP standards are defined by Internet Engineering Task Force
(IETF).
Data Link layer protocols are defined by:
ISO - International Organization for Standardization
IEEE - Institute of Electrical and Electronics Engineers
ITU - International Telecommunication Union
ANSI - American National Standards Institute
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Media
Access
Control
Media Access Control - Regulates the placement of data frames
onto the media.
The method of media access control used depends on:
Media sharing
Do more than two nodes share the media?
If so, how? (Switches, hubs, etc.)
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Serial vs multi-access
Point-to-Point networks
Only two nodes
/30 subnets are common
Protocols: PPP, HDLC, Frame Relay
Multi-access networks (LANs)
Multiple nodes
Subnets mask range depends upon the number of hosts (nodes)
Protocols: Ethernet, 802.11 (wireless), Frame Relay Multipoint
Point-to-Point
Multi-access
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Duplex Transmissions
Simplex Transmission: One way and one way only.
One way street
Half-duplex Transmission: Either way, but only one way at a time.
Two way street, but only one way at a time (land slide).
Ethernet hubs use half-duplex
Full-duplex Transmission: Both ways at the same time.
Two way street
Ethernet switches use full-duplex
Most serial links are full-duplex
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Physical
Topology
The physical topology
is an arrangement of
the nodes and the
physical connections
between them.
Hub
Switch
Serial
Connections
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Logical Topology
A logical topology -
The way a network
transfers frames from
one node to the next.
Defined by Data Link
layer protocols.
Media Access
Control used.
Type of network
framing
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Point-to-Point topology
A point-to-point topology connects two nodes directly together.
The media access control protocol can be very simple.
Frames from one devices are for the device at the other end.
Point-to-point topologies, with just two interconnected nodes, do not
require special addressing.
11111111
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Logical Point-to-Point Networks
Point-to-point networks may include intermediate devices.
No affect on logical topology.
The logical connection (in some cases) may be a virtual circuit.
A virtual circuit is a logical connection created within a network
between two network devices.
The two nodes exchange the frames with each other.
Data Link Destination address is the device at the other end of
the virtual circuit.
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Multi-access Topology
A logical multi-access topology - Enables a number of nodes to
communicate by using the same shared media.
Every node “may” see all the frames that are on the medium.
Data Link Destination Address denote which device the frame is for.
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Multi-access Addressing
Multi-access networks require an address to specifically identify the
destination.
2222
3333
4444
5555
6666
22226666
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Media Access Control
The media access control methods used by logical multi-access
topologies are typically:
CSMA/CD - Hubs
CSMA/CA - Wireless
Token passing – Token Ring
Later
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Ring Topology
Token Passing media access control
Each node in turn receives a frame.
If data link destination address is not for that device, passes
frame to next node.
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Data Link Frame
No one frame structure meets the needs of all data transportation
across all types of media.
Depending on the:
Environment
Amount of control information needed
Topology
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Data Link Frame - Header Fields
Data Link frame header fields may include:
Start Frame field - Indicates the beginning of the frame
Source and Destination address fields - Indicates the source
and destination nodes on the media
Priority/Quality of Service field - Indicates a particular type of
communication service for processing
Type field - Indicates the upper layer service contained in the
frame
Logical connection control field - Used to establish a logical
connection between nodes
Physical link control field - Used to establish the media link
Flow control field - Used to start and stop traffic over the media
Congestion control field - Indicates congestion in the media
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Data Link Frame - Trailer Fields
The signals on the media could be subject to:
Interference
Distortion
Loss
This would change the bit values that those signals represent.
The trailer is used to determine if the frame arrived without error.
Error detection.
The Frame Check Sequence (FCS) field is used to determine if
errors occurred in the transmission and reception of the frame.
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Cyclic Redundancy Check
Cyclic redundancy check (CRC) is commonly used.
Sending node includes a logical summary of the bits in the frame.
Receiving node calculates its own logical summary, or CRC.
Compares the two CRC values.
Equal – Accepts the frame
Different – Discards the frame
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Bandwidth
LAN typically uses a high bandwidth technology
Supporting large numbers of hosts
WAN
High bandwidth technology is usually not cost-effective across
large geographic areas (cities or multiple cities, for example).
The cost typically results in lower bandwidth capacity.
Note: This is relative and the need for high bandwidth on WANs is
increasing due to video, voice, and other applications.
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Ethernet Protocol for LANs
Ethernet is a family of networking technologies that are defined in
the IEEE 802.2 and 802.3 standards.
Uses 48 bit addressing (Ethernet MAC addresses) for Source and
Destination
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Point-to-Point Protocol for WANs
Point-to-Point Protocol (PPP) is a protocol used to deliver frames
between two nodes.
PPP can be used on various physical media, including:
Twisted pair
Fiber optic lines
Satellite transmission
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Wireless Protocol for LANs
802.11 is an extension of
the IEEE 802 standards.
It uses the same 48-bit
addressing scheme as
other 802 LANs.
Contention-based system using a Carrier Sense Multiple
Access/Collision Avoidance (CSMA/CA)
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This presentation is available at:
http://w3.ualg.pt/~jjose/cisco/
Original presentations from:
http://www.cabrillo.edu/~rgraziani/
Cisco curriculum available at:
http://cisco.netacad.net (Internet Explorer recommended)
After login, under: “Course Materials”