Term
| What does the Physical Layer handle? |
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Definition
| The physical layer defines the means of transmitting raw bits over a physical data link connecting network nodes. |
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Term
| In terms of communication define Simplex, Half, or Full Duplex |
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Definition
Simplex - communication only ever occurs in one direction
Half duplex - communication only ever occurs in one direction at an particular instant, but direction can reverse
Full duplex - communication can occur in both directions at the same time |
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Term
| In multiplexing what techniques are used to allow multiple signals to be sent over a single medium without interference? |
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Definition
Frequency division - Divides spectrum into frequency bands
Time division - Users take turns in round-robin fashion
Code division - Extracts desired signal, rejecting everything else |
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Term
| What does the Data Link Layer handle? |
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Definition
| The reliable, efficient communication of packets including error detection/recovery, data rate and propagation delay. |
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Term
| In networking terms what are packets? |
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Definition
| Small blocks of data sent/received individually. Large messages are usually broken up into many packets. This allows for efficient error detection/recovery and fair sharing of communication links. |
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Term
| How are packets sent to the physical layer? |
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Definition
| Packets are encapsulated into frames for transmission. Each frame contains a frame header, payload and frame trailer. |
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Term
| What service types can the Data link layer offer to the Network layer? |
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Definition
- Unacknowledged connectionless service, e.g. Ethernet
- Acknowledged connectionless service, e.g. WiFi
- Acknowledged connection-oriented service, e.g. ATM |
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Term
| The senders data link layer typically... |
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Definition
- Breaks bit stream into discrete frames
- Computes a checksum over the frame
- Sends the checksum as part of the frame |
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Term
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Definition
| A byte is stuffed in the message to differentiate the from the delimiter. This special byte is added to the beginning and end of each frame. |
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Term
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Definition
| Similar to Byte stuffing, but there is no requirement for 8 bits. This ensures minimum density of transitions to help physical layer maintain serialisation. |
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Term
| Two types of transmission errors are? |
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Definition
| Interference that causes a signal to be received incorrectly and interference that causes data to be received when none was sent. |
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Term
| Error detection is cheap and useful on highly-reliable channels to know an error has occurred. What methods can be used in error detection? |
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Definition
- Parity, where the sender and receiver decide on an even or odd parity.
- Checksums, sender treats data bits as a sequence of binary integers and adds them as they are transmitted. Receiver sums entire received data (including checksum) and if result is not 0 an error has occurred.
- Cyclic redundancy checks, sender and receiver agree on a generator polynomial G(x) in advance, append CRC in such a way that the checksummed frame is divisible by G(x). CRC is stronger than checksum or parity. |
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Term
| What does error correction do and where is it useful? |
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Definition
| Error correction adds redundancy to the sent information so it can be corrected without transmission. This is most useful on noisy lines, but the problem is redundant bits are just as likely to be received incorrectly. |
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Term
| What error is managed by flow control? |
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Definition
| This is when the sender can send more data than a receiver can handle. The best case to manage this is for the receiver to provide feedback to the sender. |
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Term
| In CSMA/CD how are multiple transmissions handled across a channel? |
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Definition
If a channel is busy:
- Could just wait until an idle channel is detected
- Could wait a random amount of time and try again
- Could use slots and send with probability p if idle
With Collision Detection can listen to a channel while transmitting, if the detected signal is different from what is being sent, abruptly stop the transmission. |
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Term
| What is the Network layer responsible for? |
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Definition
| The Network layer is responsible for routing and connecting the different networks. It forwards packets to network routers, which rely on algorithms to determine the best paths for the data to travel. |
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Term
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Definition
| In a connection-oriented service this is the path between the sender and receiver established before any packets are sent. |
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Term
When a router receives a packet, where should
it send it? |
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Definition
In datagram networks, this decision is made for every packet
– In virtual circuit networks, only when a new virtual circuit is set up |
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Term
| Routing algorithms - Forwarding |
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Definition
| Is when a router looks up its table to see where to send a packet |
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Term
| Routing algorithms - Routing |
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Definition
| Is filling in and updating the routing tables |
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Term
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Definition
| Every incoming packet is sent to all outgoing links except the one it came in on. |
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Term
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Definition
| Each router maintains a table containing best known distance to each destination and the link used to achieve that. Routers share tables with their neighbors and eventually every router knows the best links. |
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