Chapter 7

Data Link Layer

João José

jjose@ualg.pt

http://w3.ualg.pt/~jjose/cisco/

Based on:

Graziani, R. (2008) CIS 81 Networking Fundamentals

Cisco CCNA 1 Exploration - Networking

Data Link Layer

Ethernet,

PPP,

ISDN,

Frame

Relay

The Data Link layer provides a means for exchanging data over a common

local media.

Application

Header + data

Generic Data Link Header / Trailer

Focus on Data Link Layer

IP

IP

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.

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

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.

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.

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.

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

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.)

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

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

Physical

Topology

The physical topology

is an arrangement of

the nodes and the

physical connections

between them.

Hub

Switch

Serial

Connections

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

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

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.

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.

Multi-access Addressing

Multi-access networks require an address to specifically identify the

destination.

2222

3333

4444

5555

6666

22226666

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

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.

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

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

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.

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

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.

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

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

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)

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”