[Network] Link Layer

1 Introductionthis part explains some similar points of other layers, such as layer service, data format and link. and link layer is a layer of combination of hardware, software and firmware.But why both Link-level and end-end systems need Rdt?

2 Error Detection and correction:edc2.1 single Bit Parity

2.2 Dimensional Bit Parity

2.3 Checksum:crc-cylic Redundancy Check

D bits Data (D) and R bits extension (R)

R+1 bits (G) a generator

Divide to check if it is the same as former R

3 multiple Access protocols

3.1 multiple Access Links

1. Point-to-Point

2. Broadcast:share Wire and Medium

3.2 multiple Access Protocols Channel partitioning: divide channel into smaller" pieces "

1. TDMA
2. FDMA

Random Access

1. Idea:when nodes have pkts to send, the end would use the whole transmission rate.

2. Problem:two ore more transmitting nodes-collision

3. Goals:how to detect collisions and what to recover from collisions.

Examples

1. Slotted ALOHA

1) Assumption

All

frame same sizetime divided same sizenode starts to transmit only slot Beginning.all nodes can detect Collisio Nsnodes is synchronized

2) Operation

If no collision:sendif collision:retransmit in each subsequent slots with probability p until success

3) Advantages use full rate of channel simple 4) Disadvantages idle Slots waste of time clock synchronization

2. ALOHA1. Idea:no synchronization. Transmit immediately.2. Problem:the collision probability increases.
3. Csma:carrier Sense Mutilple Access1. Idea:listen before Transmit2.  Problem:propogation delay means that the nodes may not be hear each other ' s transmission. Look the image below the B ' s transmission are heard by A, C and D at different times with different distances.
3. Operation

Listen before transmission, busy to Waitlisten and transmitting, collision to wait

taking Turns

Idea like the

name.

4 Link Layer Addressing

4.1 MAC address:48 bits4.2 arp:address Resolution Protocol

1. Why: one node could know IP address of another node, but it could not know MAC address of the node. So we simply need to the build ARP table for mapping from IP address to MAC address. And we learn that IP addresses was distrubute to a subnet with logic, and MAC address was related to an interface Physicall Y. So and IP address are just a logical defination which tell the transmission where to go, and the MAC address indicates a Perticular facility.
2. Example

5 Ethernet

5.1 Star topologyopposed to the bus topology, star topology have a center active switch in center. Nodes does not collide with each other directly.

5.2 Frame Structure

preamble:used to synchronize receiver sender and clock rates. Address:adapter check it if matching. If not, the adapter would discard the frame. (6 bytes-48 bits) Type:network Layer Protocol

5.3 Services

1. Unreliable2. Connectionless

5.4 CSMA/CD

1. Sense before transmission

2. Sense while transmitting

1) If detecting another transmission, abort and send jam signal

After aborting, NIC enters exponential backoff:after mth collision, chooseK from {0, 1, 2, ..., 2^m-1}. NIC waitk*512 bit times, and return to sense before transmission.

Jam signal:48 Bits

Bit Time:

6 Link Layer Switch

6.1 Hub

no frame buffering, hub does not detect collisions, host NICs detect collisions.

6.2 Switch

Store and Forwarding Ethernet Framesforward by the MAC addresses. Plug and Play, switch self-learning
multiple simultaneous transmission Because hosts has dedicate connections to the switch. switch buffers frames, and switch frames to outgoing links without collision. Because the ingoing links is not shared.
self-learning:some thing like routing algorithmlearn which host can be reached through which interface. Just Plug into the switch! If some transmission ' s dest is not known, then flood-forward on all but the interface where the frame arrives.
Network:router and switch

7 Ppp:point-to-point Protocol

Error recovery, flow control, data re-ordering all relegated to higher layers.

8 Link Virtualization

8.1 Why virtualization Because there is different local network technology, cable, satellite, telephone and ATM, et C. So we need some work of the virtualization to get them homogeneous.
8.2 Atmbuild Virtual circuits. and work with IP.
8.3 mpls:multiprotocol label switchingspeed up IP forwarding by using fixed length Label (Instead of IP address)