• Application Layer
• APIs, Socket libraries, etc
• Transport Layer
• TCP(Connection-oriented), UDP (Connectionless)
• Network Layer
• IP
• Data link layer
• Ethernet 802.3, wireless 802.11n...
• Physical Layer
• Cat 5 twisted pair copper...

• Media types
• Encoding Methods
• Transmission capacity theories

It is the communication media used to connect devices to form a computer network.

Some examples include:

• Coaxial cable
• Twisted pair
• Power line
• Fibre optic 
• ....

• Manchester
• 8b/10b
• 64b/66b

• Clock driven
• Uses double bandwidth due to need for clock (50% efficient)
• Multiplexes data by XORing the clock with the data, and then transmitting the data

• Uses block encoding
• Many uses, e.g. fibre channel, gigabit ethernet
• Avoids long runs of 0's or 1's; helps clock recovery
• Keeps 0 and 1 counting similar using 'running disparity' measure
• 80% efficient

• 10 Gigabit Ethernet
• 97% efficient

(Takes into account signal to noise ratio)

The maximum date rate (or capacity) of a noisy channel whose bandwidth is H Hz and whose signal-to-noise ratio is S/N, is given by:

Maximum number of bits/sec = H log2( 1 + S/N )

ADSL:

• Analogue bandwidth = 1MHz
• S/N around 40dB for 1-2 km (S/N ratio = 10,000)
• dB=10log10(S/N), so S/N ratio of 1000 is 30dB
• Data rate up to 1M x log2(10001) = 13.2 Mbps
• ADSL1 specified up to 12 Mbps

• Receiving datagrams from IP layer
• Transmission of frames over physical media
• Encapsulating data form IP layer into Layer 2 frames
• Synchronization
• Detection of transmission errors

• Personal area network (PAN)
• Local area network (LAN)
• Metropolitan Area Network (MAN)
• Wide area network (WAN)

• IEEE 802.3 Ethernet, 802.11 wireless (WiFi)
• Switched networks, Virtual LANs (VLANs)

• IEEE 802.16 (WiMAX)
• Metro Ethernet, up to 100 Gbit/s

• Transforms raw transmission facility into a line 'free of errors' to the network layer
• IPv4 header has checksum too, IPv6 header does not
• Lets sender break transmission into frames
• Originally important on shared media
• Many senders trying to transmit simultaneously
• Uses checksum for integrity
• Frames may be acknowledged by receiver
• May regulate flow of data
• So that slow receiver is not swamped by fast sender

• Connectionless, no acknowledgements
• For low error rate networks, eg. Ethernet
• Acknowledged, connectionless service
• e.g. wireless 802.11
• 802.11n supports block acknowledgements
• Acknowledged, connection-oriented services
• For long, unreliable links, e.g. satellite

• Variants of Automatic Repeat reQuest (ARQ)
• Stop-and-Wait ARQ
• Go-Back-N ARQ
• Selective-Reject ARQ

1. Sender sends a frame, sets timer and waits for ACK
2. If timer finishes, it re-sends the frame
3. If receives ACK, sends next frame
4. Receiver sends ACK whenever it receives frame

http://www.youtube.com/watch?v=XLEBBBYnE8U

1. If ACK is lost, then the sender will resend its frame and the receiver cannot tell if it is a duplicate or if it is a new farme with the same contents
2. If timer times out before the sender gets the ACK, it will resend the frame. The receive will therefore send a total of two ACKs, but the receiver was only expecting one.

These issues can be solved by using a one bit sequence (0, then 1, then 0, etc) in the header of each frame - this way, the receiver and sender will know when they receive replicate packets or ACKs

http://www.youtube.com/watch?v=XLEBBBYnE8U

1. N packets are sent simultaneously and a timer is started - a 'base' states the current location of the window of current N packets
2. ACKs are sent simultaneously by receiver when packets are received, however, ACKs are only sent for packets which precede the first missing packet
3. If any packets, which are after the missing packet, are received, no ACK is sent, and receiver waits until timer goes off so that sender sends packets base to base+N

• N packets are sent simultaneously and a timer is started - a 'base' states the current location of the window of current N packets
• All ACKs are sent simultaneously by receiver when packets are received
• Base increases only up to the first packet with no ACK
• When timer goes off, it re-sends any packets with no ACKs

• Simplest method is parity bit
• Not efficient, will not reveal all errors
• Ethernet requires more robust method
• Uses CRC: Cyclic redundancy check
• Result held in checksum field

Carrier sense multiple access with collission detection is the protocol for carrier transmission access in Ethernet networks.

On Ethernet, any device can try to send a frame at any time. Each device senses whether the line is idle, and therefore available to be used. If it is, the device begins to transmit its first frame. If another device has tried to send at the same frame, a collision is said to occur and the frames are discarded. Each device then waits a random amound of tiem and retries until successful in getting its transmission sent.

• 48-bit source and destination addresses
• Preamble: 7 bytes plus 1 byte SFD to sync Tx and Rx
• 802.1Q tag for VLAN ID and 802.1p priority
• Minimum data size 46 bytes

• Need to be able to detect collisions
• Node 'A' transmits; after time t the frame has almost reached 'B'
• If 'B' transmits, it detects a collision
• 'A' must then wait for the collission to reach it
• If frame is too small, 'A', may finish transmitting before detecting this

• Consider a 10Mbps LAN
• Max length 2500m, with up to 4 repeaters (as per 802.3)
• An observed 50 microsecod RTT
• At 10Mbps, a bit takes 100nsec to send
• So, smallest frame is 500 bits (50,000 nsec RTT/100nsec)
• Round that up to 512 bits, i.e. to 64 bytes
• Less source and destination addresses (6+6 bytes)
• Less either type and CRC (2+4 bytes)
• Implies 64-12-6 = 46 bytes data

• Media types
• Raw encoding methods
• Shannon theory

• Functions
• Handling ACKs
• Framing
• Ethernet, CSMA/CD