CCNA Exploration Chapter 9 Ethernet. Study questions.

 

At which OSI and TCP/IP layers do the Ethernet standards operate?

 

OSI layer 1 physical and layer 2 data link layers.

TCP/IP network access layer.

 

Protocols and services for the Internet, Transport and Application layers of the TCP/IP protocol suite are maintained by the Internet Engineering Task Force (IETF). Which bodies describe protocols and services for the Network Access layer, which is equivalent to the Data Link layer and Physical layer of the OSI model?

 

Engineering organizations (IEEE, ANSI, ITU) or private companies.

 

There are many varieties of Ethernet with different media and different bandwidths. Which two things are the same for all of them?

 

Basic frame format and addressing scheme.

 

Which standard defines Ethernet operation at the physical layer and lower half of the data link layer?

 

IEEE 802.3

 

What is the lower half of the data link layer called?

 

Media Access Control (MAC) sublayer.

 

What is the upper half of the data link layer called?

 

Logical Link Control (LLC) sublayer.

 

Which standard defines operations at the upper half of the data link layer?

 

IEEE 802.2

 

Which OSI layer treats data as streams of bits and is concerned with encoding bits into signals to go on the transmission medium?

 

Physical layer.

 

Of the two data link sublayers, which is concerned with the physical devices to be used for communication?

 

MAC sublayer.

 

Which software carries out the functions of the Logical Link Control sublayer?

 

The driver software for the network interface card (NIC).

Which layer/sublayer  carries out the CSMA/CD process which Ethernet uses to determine when signals can be put on the medium?

 

MAC sublayer.

 

The framing process provides delimiters. What is their purpose?

 

To show which bits belong to a frame – where the frame begins and ends.

 

What type of address is included in an Ethernet frame header?

 

MAC address or physical address.

 

The Ethernet frame trailer contains a CRC field. What is this for?

 

The cyclic redundancy check is used to detect whether a frame has been corrupted during transmission. A calculation is carried out before sending the frame and the result is stored in the CRC field. The calculation is carried out again when the frame is received. If the result is different then the frame is corrupt.

 

What is a bus topology?

 

All devices are connected to a single shared cable.

 

What is a star topology?

 

Devices are connected to a central hub or switch.

 

What were the two types of early Ethernet that used a physical bus topology, what cable did they use, and how long could the cables be?

 

10Base5 or thicknet used thick coaxial cable up to 500m long.

10base 2 or thinnet used thin coaxial cable up to 185 m long.

 

What does a hub do when a frame arrives at one of its ports?

 

Copies the frame and sends it out through all its other ports.

 

In what way was a hub-based star network more reliable than a bus network?

 

A fault in the cable will bring down the whole of a bus network. A fault in one cable of a star network will cut off one device but the rest of the network can operate.

 

A hub receives a frame. Where does it forward the frame?

 

Through all ports except the receiving port.

 

A switch receives a frame. Where does it forward the frame?

 

Only through the port leading to the destination address, once it has learned which ports lead to which addresses.

 

Which other development of Ethernet happened at about the same time as the introduction of switches?

 

100BASE-TX Ethernet (with 100 Mbps bandwidth).

 

In order to eliminate collisions you need a fully switched network. What else do you need?

 

Full duplex operation.

 

What is the name given to Ethernet working at 1000 Mbps?

 

Gigabit Ethernet. (1 Gbps = 1000 Mbps.)

 

What has driven the move from 100 Mbps to 1000 Mbps Ethernet?

 

Extensive use of multimedia services, video, voice over IP (VoIP) so that high bandwidth is required.

 

A network is to be converted from 100 Mbps to 1000 Mbps. Must all the cabling be replaced?

 

Not if the existing cabling is of a high standard and well installed. The requirements of 1000 Mbps operation are higher than for 100 Mbps.

 

Ethernet Frames

What are the smallest and largest Ethernet frames allowed by the IEEE 802.3 standard?

 

64 bytes smallest, 1518 bytes largest.

 

What is the size in bytes of the destination and source address fields in the Ethernet frame?

 

6 bytes each.

 

Are the preamble and start of frame delimiter fields included as part of the frame size?

 

No.

 

The length/type field contains a number equal to or greater than 0x0600. What information can be found from this field?

 

The protocol being used at layer 3, e.g. IP.

 

What does the frame trailer contain?

 

The Frame Check Sequence. (Cyclic Redundancy Check.)

 

What does a device do if it receives an Ethernet frame that is smaller than the minimum or larger than the maximum size?

 

Drops it. (Ignores it.) It is not a valid frame.

 

What happens if the packet to be encapsulated is less than 46 bytes?

 

A “pad” is included to make the frame up to the minimum size.

 

What information is used in the cyclic redundancy check calculations?

 

The contents of the frame from the start of the destination address field to the end of the data and pad field.

 

Ethernet MAC address

Give two other names for a MAC address.

 

Physical address, burned in address. (There are others.)

 

How many hex digits are there in a MAC address?

 

12

 

How many bytes are needed to store it?

 

6

 

How many bits is this?

 

48

 

What is the first half of the MAC address?

 

The Organisational Unique Identifier, assigned by the IEEE to the manufacturer of the device.

 

Who assigns the second half of the MAC address for any device?

 

The manufacturer of the device. Each device must have a unique address.

 

Which of these is the correct way of writing a MAC address?

00-60-2F-3A-07-BC

00:60:2F:3A:07:BC

0060.2F3A.07BC

 

They are all used at different times and by different manufacturers. All are correct.

 

Do switches and routers need MAC addresses?

 

Yes, if they handle Ethernet traffic.

 

What does it mean if you see 0x before a number?

 

The number is in hexadecimal.

 

Denary (decimal) is base 10. What base is hexadecimal?

 

Base 16.

 

Convert the following denary (decimal, base 10) numbers to binary and hex.

 

Denary

Binary

Hexadecimal

10

 

 

20

 

 

128

 

 

224

 

 

252

 

 

 

Denary

Binary

Hexadecimal

10

1010

A

20

10100

14

128

10000000

80

224

11100000

E0

252

11111100

FC

 

What command would you give at the command prompt of your PC in order to see the MAC address?

 

ipconfig/all

 

A PC is sending data across the Internet to another PC. It encapsulates the data into a packet with IP destination and source addresses and then into a frame with MAC destination and source addresses. How long does the data keep the IP addresses?

 

All the way to the final destination.

 

A PC is sending data across the Internet to another PC. It encapsulates the data into a packet with IP destination and source addresses and then into a frame with MAC destination and source addresses. How long does the data keep the MAC addresses?

 

Only as far as the first router. The router strips off the frame header and trailer and encapsulates the packet in a new frame with new addresses.

 

What is a unicast? A broadcast? A multicast?

 

A unicast is a message for one specific device.

A broadcast is a message for all hosts on a network.

A multicast is a message for a group of devices.

 

What is the broadcast MAC address?

 

FF:FF:FF:FF:FF:FF

 

9.4

 

What does a hub do when it receives a signal?

 

Retransmits the signal to all connected devices except the one that sent the signal.

 

How will adding a hub and a new group of hosts affect the number of collisions on the network?

 

The collision domain will be bigger and there will be more collisions.

 

What is latency on an Ethernet network?

 

The time it takes for a signal to reach the far end of the cable. Some of the latency comes from travelling along the cable and some from intermediate devices.

 

What is bit time?

 

The time it takes a device to put one bit on the medium. Devices operating at 100 Mbps do it in one tenth of the time taken by devices operating at 10 Mbps.

 

What is interframe spacing?

 

The minimum time between the end of one frame and the start of the next.

 

What happens when a host is transmitting a frame and detects a collision?

 

It stops transmitting the frame and transmits a 32 bit jam signal. It then stops transmitting and waits for a random length of time. It then listens to see if the cable is clear and retransmits the frame when the cable is clear.

 

If hosts have transmitted and their frames have collided, what priority do they have for retransmission?

 

None at all. They have to take their chance along with all the other hosts that want to transmit.

 

What is the purpose of backoff timing after a collision?

 

It is a way of reducing the chance of a second collision. The hosts whose frames collided wait for the normal interframe spacing time when they detect that the cable is clear. They then wait for a random length of time, the backoff time, before trying to send again. If there were no backoff timing then the hosts would be very likely to retransmit at the same time and there would be another collision.

 

How can a valid frame be distinguished from a collision fragment (runt)?

 

The collision fragment is less than 64 bytes long and fails the checksum test.

 

Why can collisions not be managed on Ethernet networks operating at high bandwidths?

 

On a reasonable length of cable, collisions would not be recognised in time for CSMA/CD to work. A host needs to detect a collision before transmitting byte 64 of a frame. If it detects a collision too late then it will not retransmit. Networks operating with high bandwidth must run without collisions.

 

Which wires in a UTP cable does 100 Base-TX use to transmit and receive?

 

It uses wires 1 and 2 to transmit. (2 wires needed to make the circuit). It uses wires 3 and 6 to receive.

 

Which wires in a UTP cable does 1000 Base-T use to transmit and receive?

 

It uses all 8 of them, both to transmit and receive.

 

9.6

What are the advantages of using switches rather than hubs in an Ethernet network?

 

Dedicated bandwidth to each port

Collision-free environment

Full-duplex operation is possible

 

What is store and forward switching?

 

With store and forward switching, the switch receives the entire frame, puts it in a buffer, checks the FSC for errors, and forwards the frame to the appropriate port for the destination device.

 

What information does a switch keep in its switching (bridging) table?

 

A list of its port numbers and the MAC address of the device connected to each port.

 

How does a switch use its switching table?

 

When a frame arrives, the switch reads the destination MAC address. It looks up the MAC address in its switching table, finds the right port and forwards th frame through that port.

 

Which frames are flooded?

 

Broadcast frames and frames to destinations that are not listed in the switching table.

 

How does a switch learn new address-port matches to put in its table?

 

It looks at the source MAC address of incoming frames and matches them with the incoming port.

 

9.7

 

What information does a host keep in its ARP table?

 

A mapping between IP addresses and corresponding MAC addresses.

 

What command would you give to see the ARP table of a hosts?

 

Arp -a

 

How does a host get information to put in its ARP table?

 

From packets it receives and by sending broadcast ARP requests.