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Chapter 7Data Link Layer
CIS 81 Networking Fundamentals
Rick Graziani
Cabrillo College
Last Updated: 4/27/2008
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This Presentation
For a copy of this presentation and access to my web site for other CCNA, CCNP, and Wireless resources please email me for a username and password. Email: [email protected] Web Site: www.cabrillo.edu/~rgraziani
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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
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Node to node, not host to host
The Data Link layer provides a means for exchanging data over a common local media.
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Focus on Data Link LayerIP
IP
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DataHTTP Header
TCP Header
IP Header
Data Link Header
Data Link Trailer
IP PacketData Link Header
Data Link Trailer
IP PacketData Link Header
Data Link Trailer
IP PacketData Link Header
Data Link Trailer
IP PacketData Link Header
Data Link Trailer
IP PacketData Link Header
Data Link Trailer
IP PacketData Link Header
Data Link Trailer
DataHTTP Header
TCP Header
IP Header
Data Link Header
Data Link Trailer
Reminder of encapsulation/decapsulation
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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. “Data from only one node can be placed on the medium at any
one time.” (This is only true when using CSMA/CD (hubs), NOT true
with switches or wireless) 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 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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Framing- The Trailer
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
More next week!
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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)
Chapter 7Data Link Layer
CIS 81 Networking Fundamentals
Rick Graziani
Cabrillo College
Last Updated: 4/27/2008
