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Chapter 9. Hardware Addressing And Frame Type Identification

Jing Wang

Towson University

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9.1. Introduction

• Advantage of shared networks– Universal connectivity: because all computers share

the medium, a transmitted signal reaches all computers

• This chapter– Consider transmission across a shared LAN in more

detail– How a pair of computers communicate without other

computers to receive and process a copy of each message

– Hardware addressing– Network interface hardware– Frame type

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9.2. Specifying A Recipient

• How can two computers communicate directly across a shared medium in which all attached stations receive a copy of all signals?

• Most LAN technologies use an addressing scheme to provide direct communication– Each station on the LAN is assigned a unique

numeric value called a physical address, hardware address or media access address (MAC address)

– Each frame begins with a header that contains destination address field and source address field

– Network interface hardware accepts only those frames in which the destination address matches the station’s address

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9.2. Specifying A Recipient

• Each computer attached to a LAN is assigned a number known as its physical address.

• A frame sent across a LAN contains the address of the sending computer, called a source address, and the address of the intended recipient, called the destination address

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9.3. How LAN Hardware Uses Addresses To Filter Packets

• LAN interface hardware handles all the details of sending and receiving frames on the shared medium– Checks the length of an incoming frame– Checks the CRC– Discards frames that contain errors– Sends and receives frames without using the

computer’s CPU

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9.3. How LAN Hardware Uses Addresses To Filter Packets

Figure 9.1. Organization of the hardware in a computer attached to a LAN. Because it is powerful and independent, the network interface hardware does not use the CPU when transmitting or receiving bits of a frame.

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9.3. How LAN Hardware Uses Addresses To Filter Packets

• LAN interface hardware uses physical addressing to prevent the computer from receiving all packets that travel across the LAN– Compares the destination address in the frame to the

physical address of the station– If match, accepts the frame and passes it to the

operation system– If not, discards the frame and waits for the next frame– Ignore the frame addressed to a nonexistent station

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9.3. How LAN Hardware Uses Addresses To Filter Packets

• A shared network system uses physical addresses to filter incoming frames.

• The network interface hardware, which handles all the details of frame transmission and reception, compares the destination address on each incoming frame to the station’s physical address, and discards frames not destined for the station

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9.3. How LAN Hardware Uses Addresses To Filter Packets

• Because a network interface operates without using a station’s CPU, a frame can be transferred across a shared LAN from one computer to another without interfering with processing on the computers

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9.4. Format Of A Physical Address

• Static– relies on the hardware manufacturer to assign a

unique physical address to each network interface

• Configurable– provides a mechanism that a customer can use to set

a physical address

• Dynamic– provides a mechanism that automatically assigns a

physical address to a station when the station first boots

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9.4. Format Of A Physical Address

• Static– Ease of use– Permanence

• Dynamic– Eliminates the need for hardware manufacturers to coordinate in

assigning addresses– Allows each address to be smaller– Lack of permanence and potential conflict

• Configurable– Permanent– Do not need to be large– Can be replaced without changing the computer’s physical

address

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9.5. Broadcasting

• Broadcasting– It makes a copy of data available to all other

computers on the network– Since LANs employ shared media, they make

broadcasting extremely efficient– Extend the addressing scheme

• A special, reserved broadcast address

– If a frame arrives with the special broadcast address or the station’s physical address in its destination address, accepts it

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9.5. Broadcasting

• The network interface hardware in a computer makes a copy of every frame that passes across the shared network.

• The interface accepts the frame and delivers a copy to the operating system if the destination address in the frame is the reserved broadcast address or matches the computer’s physical address.

• Thus, when a frame is sent to the broadcast address, each computer on the network receives a copy

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9.6. Multicasting

• Multicast– A network interface does not automatically

forward multicast frames to the CPU– it is programmed to make decision to accept

or reject frames

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9.7. Multicast Addressing

• Multicast– Reserve an additional set of addresses– When computer boots, the interface is programmed to

recognize only the computer’s address and the broadcast address

– If an application wishes to receive multicast frames, it inform the network interface which multicast address to use

– The interface adds the address to the set it will recognize, and begins to accepting frames sent to that address

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9.8. Identifying Packet Contents

• Explicit frame type– type information is included in the frame– Frame type field - the bits of a frame used to

identify the contents– Self-identifying frame

• Implicit frame type– does not include a type field

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9.9. Frame Headers And Frame Format

Figure 9.2. The general format of a frame sent across a LAN. The header contains information such as the addresses of the sender and the recipient.

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9.9. Frame Headers And Frame Format

• Each LAN technology defines a frame format. • Most technologies have frames that each consist

of a header followed by a data area.• Because the size and format of the header is

fixed, all frames used with a given technology have the same header format

• The size of the data area is determined by the data being sent in the frame

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9.10. An Example Frame Format

Figure 9.3. Illustration of the frame format used with Ethernet. The number in each field gives the size of the field measured in 8-bit octets.

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9.10. An Example Frame Format

Figure 9.4. Examples of frame types used with Ethernet (type values are given in hexadecimal). The table lists only a few examples; many other types have been assigned.

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9.11. Using Networks That Do Not Have Self-Identifying Frames

• Before any data is sent, the sender and receiver agree to use– A single format for data– The first few octets of the data field to store

type information

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9.11. Using Networks That Do Not Have Self-Identifying Frames

Figure 9.5. Illustration of how type information can be included in a frame's data area if the frame header does not include a type field.

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9.11. Using Networks That Do Not Have Self-Identifying Frames

• IEEE LLC/SNAP– Part of IEEE 802.2– Logical Link Control (LLC) SubNetwork Attachment

Point (SNAP) header• Logical Link Control

– Originally designed to be the same for all LANs for interoperability, but not used often today.

– Interoperability is provided by a common network layer protocol.

• SubNetwork Attachement Point– Organizationally Unique Identifier (OUI)

» Identify a particular standards organization– Type

» Type value defined by that organization

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9.11. Using Networks That Do Not Have Self-Identifying Frames

Figure 9.6. An example of the 8-octet IEEE LLC/SNAP header, which is used to specify the type of data. The SNAP portion specifies an organization and a type defined by that organization.

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9.12. Network Analyzers, Physical Addresses, Frame Types

• Network analyzer (network monitor)– A device used to determine how well a

network system is performing– Also called network sniffer– Consist of a standard portable computer (e.g.

a notebook PC) with a standard LAN interface– Network interface card in promiscuous mode

• Accept all frames

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9.12. Network Analyzers, Physical Addresses, Frame Types

• A network analyzer is a device that can be configured to count or display frames as they pass across a shared network.

• An analyzer obtains a copy of each frame, and then uses header fields such as the physical source address, physical destination address, or type information to determine how to process the frame

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9.13. Summary

• Each computer attached to the shared network is assigned a unique physical address

• Network interface hardware handles transmitting and receiving frames

• Broadcast packet is only transmitted once on a shared LAN

• Multicast use the network interface hardware to examine frames

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9.13. Summary

• Physical address could be static, configurable, dynamic

• Frame header includes frame type

• If not, type information could be placed in an LLC/SNAP header in the first few octets of the frame data area

• A network analyzer is a device that can be used to debug problems on a network

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Supplement

• http://www.mhhe.com/engcs/compsci/forouzan/

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Chapter 14

Local Area Networks:Ethernet

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Figure 14.1 Three generations of Ethernet

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Figure 14.2 802.3 MAC frame

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Figure 14.3 Minimum and maximum length

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Figure 14.4 Ethernet addresses in hexadecimal notation

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Figure 14.5 Unicast and multicast addresses

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Figure 14.6 Physical layer

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Figure 14.7 PLS

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Figure 14.8 AUI

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Figure 14.9 MAU (transceiver)

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Figure 14.10 Categories of traditional Ethernet

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Figure 14.11 Connection of a station to the medium using 10Base5

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Figure 14.12 Connection of stations to the medium using 10Base2

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Figure 14.13 Connection of stations to the medium using 10Base-T

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Figure 14.14 Connection of stations to the medium using 10Base-FL