Chapter 6 - Link Layer Flashcards
What is framing in link layer?
framing in link layer is multiplexing the network layer into a frame.
If all the links in the Internet were to provide reliable delivery service, would the TCP reliable delivery service be redundant? Why or why not?
Although each link guarantees that an IP datagram sent over the link will be received at the other end of the link without errors, it is not guaranteed that IP datagrams will arrive at the ultimate destination in the proper order. With IP, datagrams in the same TCP connection can take different routes in the network, and therefore arrive out of order. TCP is still needed to provide the receiving end of the application the byte stream in the correct order. Also, IP can lose packets due to routing loops or equipment failures
Name three error-detection strategies employed by link layer.
Internet Checksum, Parity Check, Cyclic Redundancy Check
Suppose two nodes start to transmit at the same time a packet of length L over a broadcast channel of rate R. Denote the propagation delay between the two nodes as d_prop. Will there be a collision if d_prop < L /R? Why or why not?
Yes collision will occur for any d_prop when two nodes start transmitting at the same time.
In Section 6.3, we listed four desirable characteristics of a broadcast channel. Which of these characteristics does slotted ALOHA have? Which of these characteristics does token passing have?
- when one node broadcasting rate should be R
- when N nodes broadcasting each node should average R/N.
- No master controller.
- Simple
slotted ALOHA: (1) yes, (2) - due to collisions max efficiency is 0.37, (3) yes, (4) yes
token passing: (1) - token passing time, (2) - token passing time, (4) yes,
In CSMA/CD, after the fifth collision, what is the probability that a node chooses K = 4? The result K = 4 corresponds to a delay of how many seconds on a 10 Mbps Ethernet?
pool = 0 .. 2^n - 1, prob(K=4) = 1/2^n = (1/32) time = (5 \* 512b) / 10Mb
While TDM and FDM assign time slots and frequencies, CDMA assigns a different code to each node. Explain the basic principle in which CDMA works.
code division multiple access CDMA assigns a different code to each node. Each node then uses its unique code to encode the data bits it sends. Allows for nonblocking system.
Why does collision occur in CSMA, if all nodes perform carrier sensing before transmission?
Due to propagation time a node may have started transmitting at t - e but the signal has yet to reach the sensing node.
How big is the MAC address space? The IPv4 address space? The IPv6 address space?
MAC 2^48, IPv4 2^32, IPv6 2^128
Suppose nodes A, B, and C each attach to the same broadcast LAN (through their adapters). If A sends thousands of IP datagrams to B with each encapsulating frame addressed to the MAC address of B, will C’s adapter process these frames? If so, will C’s adapter pass the IP datagrams in these frames to the network layer C? How would your answers change if A sends frames with the MAC broadcast address?
C’s adapter will process the frames, but the adapter will not pass the datagrams up the protocol stack. If the LAN broadcast address is used, then C’s adapter will both process the frames and pass the datagrams up the protocol stack.
Why is an ARP query sent within a broadcast frame? Why is an ARP response sent within a frame with a specific destination MAC address?
An ARP query is sent in a broadcast frame because the querying host does not which adapter address corresponds to the IP address in question. For the response, the sending node knows the adapter address to which the response should be sent, so there is no need to send a broadcast frame (which would have to be processed by all the other nodes on the LAN).
For the network in Figure 6.19, the router has two ARP modules, each with its own ARP table. Is it possible that the same MAC address appears in both tables?
No, the ARP table for each interface will only contain addresses on that NIC’s subnet.
Each host and router has an ARP table in its memory. What are the contents of this table?
The ARP table will contain mappings of MAC, IP Addresses for the local subnet.
What is a hub?
A hub is a physical-layer device that acts on individual bits rather than frames. When a bit, representing a zero or a one, arrives from one interface, the hub simply re-creates the bit, boosts its energy strength, and transmits the bit onto all the other interfaces.
What is the difference between a router and a switch?
Router: has MAC, IP address, forwards packets based primarily on IP address, but can use values in other layer header fields. (link-layer 3 switch)
Switch: no MAC or IP, forwards packets based only on MAC address. (link-layer 2 switch).
