1.3 Routing and Switching

Question 1

SOHO LAN

Answer 1

Small Office Home Office Local Area Network

Internet –> Router –> Switch –> Wireless Access Point –> Computer/Laptop/Cellphone

Question 2

The Ethernet Frame

Answer 2

Is a data link layer protocol data unit and uses the underlying Ethernet physical layer transport mechanisms. In other words, a data unit on an Ethernet link transports an Ethernet frame as its payload

Question 3

Preamble

Answer 3

7 bytes

56 alternating ones and zeros used for synchronization (101010)

Question 4

SFD

Answer 4

Start Frame Delimiter
1 byte
Designates the end of the preamble (10101011)

Question 5

Destination MAC Address

Answer 5

6 bytes

Ethernet Mac address of the destination device

Question 6

Source MAC Address

Answer 6

6 bytes

Ethernet MAC address of the source device

Question 7

EtherType

Answer 7

2 bytes
Describes the data contained the payload
Might say there’s IPv4 or 6 traffic in this frame

Question 8

Payload

Answer 8

46-1500 bytes
Layer 3 and higher data
Might contact IP information, TCP data or browsing infromation

Question 9

FCS

Answer 9

Frame Check Sequence
4 bytes
CRC checksum of the frame that looks at of the data sent and it comes up with a checksum.

Question 10

MAC Address

Answer 10

Ethernet Media Access Control address

• The physical address of a network adapter
• Unique to each device

48 bits / 6 bytes long

• Displayed in hexadecimal
• 8c:2d:aa:4b:98:a7
• First half is the OUI Organizationally Unique Identifier (manufacturer)
• Second half is the Network Interface Controller-Specific (Serial Number)

Question 11

Duplex

Answer 11

Half-Duplex
- Device cannot send and receive simultaneously
- All LAN hubs are half-duplex devices
- Switch interfaces cna be configured as half-duplex
but usually only when connecting to other half-duplex devices.

Full-Duplex

• Data can be sent and receive at the same time
• A properly configured switch interface will be set to full-duplex

Question 12

CSMA/CD

Answer 12

CS - Carrier Sense

• Is there a carrier
• Is there a signal avail that we can use to send data?

MA - Multiple Access
- More than one device on the network

CD - Collision Detect

• Collision - two stations talking at once
• Identify when data gets garbled
• Perform random backoff function and try to comm again

Half-duplex Ethernet
- Not used any longer

Question 13

CSMA/CD Operation

Answer 13

Listen for an operation
- Don't transmit if the network is already busy
-
Send a frame of data
- You send data whenever you can
- There's no queue or prioritization 

If a collision occurs

• Transmit a jam signal to let everyone know there was a collision
• Wait a random amount of time
• Retry to the transmission

Question 14

Full-Duplex Ethernet

Answer 14

Sam’s PC creates an Ethernet frame

• Sam’s MAC address is the source
• SGC Server MAC address is the destination

Frame is sent to Switch A

Switch A examines destination MAC address

• Sends frame out the interface that matches the destination MAC
• Device at the other end is Switch B

Switch B forwards the frame to the interfaces that matches the destination (SGC Server) MAC address

• Destination PC receives the frame, identifies a matching destination MAC
• Destination PC access the frame

Sending and receiving happens simultaneously
No Collision

Question 15

CSMA/CA

Answer 15

CA - Collision Avoidance
- Common on wireless networks

Collision detection isn’t possible
- A sending station can’t hear other stations

USE RTS/CTS

• I’m Ready To Send
• You’re Clear To Send

Solves the “hidden node” problem

• Station A can hear the access point
• Station B can hear the access point
• Station A can’t hear station B