Chapter 6: The Link Layer and LANs Flashcards
Give the definition:
Any device that runs a link-layer protocol, e.g. routers and hosts
Node
Give the definition:
Communication channels that connect adjacent nodes along communication paths
Links
What are the three types of links?
Wireless, wired and LANs
Give the definition:
A link layer packet that encapsulates a datagram
Frame
Give the definition:
A layer that has the responsibility of transferring datagrams from one node to physically adjacent nodes over a link
Link layer
Give the definition:
Encapsulating a datagram into a frame by adding a header and a trailer
Framing
What do MAC addresses in frame headers identify?
Source and destination
Give the definition:
Channel access is shared medium
Link access
The link layer offers reliable delivery between
adjacent nodes
Give the definition:
Pacing between adjacent sending & receiving nodes
Flow control
Give the definition:
Receiver detects errors caused by signal attenuation/noise and signals retransmission, or drops frame
Error detection
Give the definition:
Receiver identifies and corrects bit error(s) without retransmission
Error correction
With half duplex nodes, both ends of a link can transmit, but not
at the same time
Where is the link layer implemented?
In every host
How is the link layer implemented?
In an Network Interface Card (NIC) or on a chip
Ethernet, WiFi card or chip implement which layers?
Link & physical
Does the description below describe the sending or the receiving side of a link?
1. Encapsulates datagram in frame
2. Adds error checking bits, reliable data transfer, flow control, etc.
Sending side
Does the description below describe the sending or the receiving side of a link?
1. Looks for errors, reliable data transfer, flow control, etc.
2. Extracts dataframe & passes to upper layer at receiver
Receiving side
The link layer transmits frames over a
physical medium
Give the definition:
Two insulated copper wires
Twisted pair
Give the definition:
Two concentric copper conductors
Coaxial cable
Give the definition:
Glass fiber carrying light pulses, each pulse a bit. With high speed operation & low error rate
Fiber optic cable
Give the definition:
Wireless LAN, wide-area, satellite, terrestrial microwave
Radio link
Give the definition:
The ability of the receiver to both detect and correct errors
Forward Error Correction (FEC)
Give the definition:
Detect single bit errors
Single bit parity
Give the definition:
Set parity bit so there’s an even number of 1’s
Even parity
Give the definition:
Detection and correction of single bit errors
2D bit parity
What sort of errors are cyclic redundancy checks (CRC) particularly well suited for?
Burst errors
Give the definition:
Contiguous sequences of erroneous data symbols in messages
Burst errors
Give the definition:
A link that can have multiple sending and receiving nodes all connected to the same, single, shared broadcast channel
Broadcast link
Give the definition:
A link where there’s a single sender & receiver at each end of the link.
Point-to-Point link
Give the definition:
2 or more simultaneous transmissions by nodes
Interference
Give the definition:
When a node receives 2 or more signals at the same time
Collision
Give the definition:
Distributed algorithm that determines how nodes share channels, i.e. determines when a node can transmit
Multiple access protocol
Which problem does this description fit:
How to coordinate the access of multiple sending & receiving nodes to a shared broadcast channel
Multiple access problem
What are the three classes of MAC protocols?
Channel partitioning
Random access
Taking-turns
Which class of MAC protocols fits this description:
- Division of channels into smaller “pieces” - by time, frequency or code - that are allocated to nodes for exclusive use
- Shares channels efficiently & fairly at high load
- Inefficient at low load due to delay in channel access & 1/N bandwidth allocated even if only 1 active node
Channel partitioning
Which class of MAC protocols fits this description:
- Channel isn’t divided & collisions are allowed. Allows for recovery from collisions.
- Efficient at low load due to a single node being able to fully utilize the channel
- Inefficient at high load due to collision overhead
Random access
Which class of MAC protocols fits this description:
Nodes take turns, and nodes with more to send can take longer turns
Taking-turns
Which sort of channel partitioning MAC protocol is this?
Assigns a different code to each node
Code Division Multiple Access (CDMA)
Which sort of channel partitioning MAC protocol is this?
- Access to channel in rounds
- Each station gets fixed amount of packet transmission time in each round
- Unused slots go idle
Time Division Multiple Access (TDMA)
Which sort of channel partitioning MAC protocol is this?
- Channel spectrum divided into frequency bands
- Each station assigned fixed frequency band
- Unused transmission time in frequency bands go idle
Frequency Division Multiple Access (FDMA)
What do random access MAC protocols specify?
- How to detect and recover from collisions
Which sort of random access MAC protocol is this?
When a node obtains a fresh frame, waits until the beginning of the next slot, then transmits the entire frame in the next slot
- If no collision: node can send new frame in the next slot
- If collision: node retransmits frame in each subsequent slot with probability p until success
Slotted ALOHA
Which sort of random access MAC protocol is this?
Transmits the first frame immediately upon arrival
- Simpler then slotted ALOHA and requires no synchronization
Pure/unslotted ALOHA
Which sort of random access MAC protocol is this?
Listens before transmission
- If channel sensed idle: transmits entire frame
- If channel sensed busy: defer transmission
Has carrier sensing
Simple CSMA (Carrier Sense Multiple Access)
Which sort of random access MAC protocol is this?
CSMA with collision detection
- Collisions detected within short time
- Colliding transmissions aborted
CSMA/CD
Give the definition:
A node listens to the channel before transmitting
Carrier sensing
Give the definition:
The time it takes for a signal to propagate from one node to another
Channel propagation delay
Collision probability depends on what two factors?
Distance & propagation delay
Give the definition:
After mth collision, NIC chooses K at random from {0, 1, 2, …, 2^(m-1)}. NIC waits K*512 times then senses channel
Binary (exponential) backoff
Which taking-turn MAC protocol is this?
A master node invites other nodes to transmit in turn by polling them in a RR-fashion.
Polling
Which taking-turn MAC protocol is this?
A control token, which is a small, special purpose frame, is exchanged among nodes sequentially (in some fixed order)
Token passing
Which MAC protocols do cable access networks use?
FDM, TDM & random access
What is the name of the link layer protocol for cable internet access, that specifies cable data network architecture and its protocols?
Data Over Cable Service Interface Specifications (DOCSIS)
What sort of address is this?
32-bit address used for network layer / layer 3 forwarding
IP address
What sort of address is this?
A 48-bit address burned in NIC ROM or in software that is used “locally” to get a frame from one interface to another physically connected interface
MAC address
What sort of address is this?
An address that can move an interface from one LAN to another. Recalls the IP address if not portable - depends on the subnet to which node is attached
MAC flat address
What protocol is this?
A protocol that makes it possible to determine an interfaces MAC address based on its IP address
Address Resolution Protocol (ARP)
What sort of table is this?
A table that each IP node on LAN has. It contains IP/MAC address mappings for some LAN nodes and Time To Live (TTL) for each address mapping.
ARP table
What do we call the physical topology for the ethernet that has an active link layer / layer 2 switch in the center of several hosts/”wheel” where each “spoke” runs a (separate) Ethernet protocol
Switched
What does Ethernet with switching ensure that nodes don’t do?
Collide
What sort of frame is this?
The frame that a sending interface encapsulates an IP datagram or other network layer protocol packet in
Ethernet frame
What part of the Ethernet frame is this?
Synchronizes receiver and sender clock rates. Has 7 bytes of 10101010 + 1 byte 10101011.
Preamble
What part of the Ethernet frame is this?
6 byte source & destination address
Source and destination addresses
What part of the Ethernet frame is this?
Indicated higher layer protocol (mostly IP) & is used to demux up at receiver. 2 bytes.
Type
What part of the Ethernet frame is this?
Carries IP datagram.
Data (payload)
What part of the Ethernet frame is this?
Cyclic Redundancy Check at receiver that drops a frame if an error is detected. 4 bytes.
CRC
Which attribute of the Ethernet is described below?
No handshaking between sending and receiving NICs
Connectionless
Which attribute of the Ethernet is described below?
Receiving NIC doesn’t send ACKs or NAKs to sending NIC - Data dropped in frames recovered only if initial sender uses higher layer rdt, otherwise the dropped packet is lost
Unreliable
What’s the Ethernet MAC protocol for half-duplex connections / connections to old non-switched equipment?
Unslotted CSMA/CD with binary backoff
What is the difference between the Ethernet standards?
Speeds & physical layer media
What do the Ethernet standards have in common?
MAC protocol & frame format
What attribute of the switch is described below?
Hosts are unaware of the presence of switches
Transparent
What attribute of the switch is described below?
Different links can operate at different speeds and run over different media
Heterogeneous links
What attribute of the switch is described below?
Switch learns which hosts can be reached through which interface
Self-learning
What attribute of some switches is described below?
A device that requires no intervention from a network administrator or user
Plug-and-play
What function of a switch is described below?
The function that determines whether a frame should be forwarded to some interface or dropped
Filtering
What function of a switch is described below?
The function that determines the interface to which a frame should be directed, and then moves the frame to those interfaces
Forwarding
Switching makes it possible for two hosts to transmit …, without collisions
simultaneously
What sort of table has the following information in each entry?
MAC address of host
Interface to reach host
Time stamp
Switch table
What sort of switches are described below?
Self-learning switched that are connected together
Interconnected switched
What two attributes do switches and routers have in common?
Store-and-forward & forwarding tables
Are routers or switches network-layer devices that examine network-layer headers?
Routers
Are routers or switches link-layer devices that examine link-layer headers?
Switches
Do routers or switches compute their forwarding tables using routing algorithms & IP addresses?
Routers
Do routers or switches learn their forwarding tables using flooding, learning & MAC addresses?
Switches
Which layer has the following principles behind their services?
- Error detection & correction
- Sharing a broadcast channel: multiple access & link layer addressing
Link layer
What two link layer technologies have we learnt how to instantiate and implement?
Ethernet & switched LANs
