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OSI Data Link Layer

Network Fundamentals – Chapter 7

Objectives � Explain the role of Data Link layer protocols in data transmission.

� Describe how the Data Link layer prepares data for transmission on network media.

� Describe the different types of media access control methods.

� Identify several common logical network topologies and describe how the logical topology determines the media access control method for that network.

� Explain the purpose of encapsulating packets into frames to facilitate media access.

� Describe the Layer 2 frame structure and identify generic fields.

� Explain the role of key frame header and trailer fields including addressing, QoS, type of protocol and Frame Check Sequence.

OSI Data Link Layer – DLL

� OSI model layer 2

Application

Presentation

Session

Transport

Network

Data link

Physical

Application

Transport

Internet

Network Access

TCP, UDP

Ethernet,

WAN

technologies

HTTP, FTP, TFTP, SMTP

etc

Segment

Packet

Frame

Bits

Data

stream

� TCP/IP model part of Network Access layer

� Controls access to the transmission medium.

� Encapsulates packets by adding a frame header and trailer including appropriate addressing.D

L L F

u n c ti o n s

Data Link Layer – Accessing the Media

� Provides a means for exchanging data over a common local media.

� Provides services to support communication for each medium over which data is to be transmitted.

� Exchanges frames between nodes over the media of a physical network.

� Performs two basic services:

– Framing technique to allow upper layers to access the media

– Media access control & error detection techniques to control how data is placed/received onto/from the media

Data Link Layer – Accessing the Media

� DLL protocols require control info to enable the protocols to function

� DLL prepares packet for transport across the local media by encapsulating it with a header and a trailer to create a frame.

Header - Contains control info, such as addressing, & is located at the beginning of the PDU

Data - The packet from the Network layer

Trailer - Contains control info added to the end of the PDU

Data Link Layer – Accessing the Media

� DLL protocols are required to control media access

Hops

� There may be a different layer 2 protocol in use on each hop of a journey.

� Different media, different types of link, different bandwidths, LAN/WAN affect the choice of protocol.

� Different protocols have different frames.

� The router removes the old frame and adds a new header and trailer for the next hop.

Data Link Layer – Accessing the Media

� DLL frames the packets for transmission on a given media

Data Link Layer – Accessing the Media

� DLL links the software and hardware layers

Data Link Layer – Accessing the Media

� DLL uses several sources for its protocols and standards

International Organization for Standardization

Institute of Electrical and Electronics Engineers

American National Standards Institute

International Telecommunication Union

DLL Sublayers

Point to point link

� Only two devices on the network

� Full duplex: both can send at the same time, no problem with media access

� Half duplex: data can only travel one way at a time so one device can send at a time. Simple media access control.

Shared medium

� Needs media access control.

� If there is no control there will be many collisions and the frames will be destroyed.

Physical bus Star with hub

Controlled media access

� Predictable, deterministic.

� Each device is given a time when it may send, and it most not send at any other time.

� High overhead.

� No collisions.

� Token passing – each host in turn gets the token and is allowed to send.

� E.g. token ring, FDDI Fiber Distributed Data Interface

Media Access Control Techniques

Contention based media access

� Non-deterministic, first come first served.

� Each device “listens” and sends when the medium seems to be clear.

� Low overhead.

� Collisions occur.

� Need a way of re-sending lost frames.

� Becomes inefficient on large networks.

� E.g. traditional Ethernet.

Media Access Control Techniques

Variation on contention based

� Carrier Sense Multiple Access

� Traditional Ethernet uses CSMA/CD (collision detection): collisions are allowed and detected, frames sent again.

� Wi-fi uses CSMA/CA (collision avoidance): when the medium is clear, host sends signal to say it is about to use the medium. It then sends.

Different environments

� Fragile environment e.g. satellite link – frames are likely to be lost – need large overhead of control mechanisms to make sure data arrives.

� Protected environment e.g. modern LAN – frames not often lost – do not need such elaborate control mechanisms

� Therefore need different layer 2 protocols

Addressing needs

� Point to point link – only one possible destination. Minimal addressing.

� Multi-access network – need full addressing system.

� Therefore need different layer 2 protocols.

Layer 2 frame format

� All protocols have the same general form but there are variations

� Start and stop indicator fields - The beginning and end limits of the frame

� Naming or addressing fields

� Type field - type of PDU contained in the frame

� Quality control fields

� A data field -The frame payload (Network layer packet)

� Fields at the end of the frame form the trailer. These fields are used for error detection and mark the end of the frame.

PPP frame

� Point to point links. Minimal addressing. Control mechanisms.

Start Minimal address

Packet Check and stop

Ethernet frame

� Multi-access links. Full addressing. No control field.

� Same for all Ethernet types/bandwidths.

Timing and start

Addresses 48 bits each

Layer 3 protocol

Packet Check and stop

802.11 Wi-Fi

� LAN wireless protocol

� Fragile environment – lots of interference, risk of lost frames, contention.

� Every transmission needs to be acknowledged.

� No acknowledgement – re-send frame.

� Lots of control mechanisms in frame.

End to end

� PC sends packet to server

� Packet header has PC IP address and source and server IP address as destination.

� Frame header has PC MAC address as source and router MAC address as destination.

Media Access Control Techniques

T ra

d e o

Media Access Control Techniques � Full Duplex and Half Duplex

Half-duplex communication

Devices can both transmit and receive on the media but cannot do so simultaneously.

In full-duplex communication,

Both devices can transmit and receive on the media at the same time.

Logical Topology � The topology of a network is the

arrangement or relationship of the network devices and the interconnections between them.

Logical topology is how a network transfers frames between nodes.

Consists of virtual connections between the nodes independent of their physical layout.

These logical signal paths are defined by Data Link layer protocols.

Influences the type of network framing and media access control used.

� Logical and physical topologies typically used in networks are:

Point-to-Point

Multi-Access

Ring

Physical Topology

� The topology of a network is the arrangement or relationship of the network devices and the interconnections between them.

Physical topology is an arrangement of the nodes and their physical connections.

The representation of how the media is used to interconnect the devices is the physical topology.

Physical topologies

Logical Point-to-Point Topology � A point-to-point topology connects two nodes directly together.

All frames on the media can only travel to or from the two nodes.

The frames are placed on the media by the node at one end and taken off the media by the node at the other end of the point-to-point circuit.

� In point-to-point networks

Half-duplex link: data can only flow in one direction at a time

Full-duplex link: data can successfully flow across the link from each node simultaneously

Logical Point-to-Point Networks � The end nodes communicating in a point-to-point network can be physically

connected via a number of intermediate devices.

The use of physical devices in the network does not affect the logical topology.

The source & dest. may be indirectly connected over some geographical distance.

The logical connection between nodes forms a virtual circuit (VC).

The two nodes on either end of the virtual circuit exchange the frames with each other.

This occurs even if the frames are directed through intermediary devices.

Logical 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.

Every node sees all the frames that are on the medium, but only the addressed node processes the frame.

� Having many nodes share access to the medium requires a Data Link media access control method to regulate the transmission of data and thereby reduce collisions.

The media access control methods used by logical multi-access topologies are typically CSMA/CD or CSMA/CA.

Logical Ring Topology � Each node in turn receives a frame. If the frame is not addressed to the node,

the node passes the frame to the next node.

A ring uses a controlled media access control

technique called token passing.

Nodes in a logical ring topology remove the frame from the ring, examine the

address, and send it on if it is not addressed for that node.

All nodes between the source and dest. node examine the frame.

A node can only place a data frame on the media when it has the token.

Media Access Control Addressing and Framing Data � Encapsulating packets into frames facilitates the entry & exit of data on media

Media access control addressing and framing data

Summary