Lecture 5 and 6

   1.Unacknowledged connectionless service
        - Using LLC datagram, no flow control or error checking.
   2. Connection service
        - Require a logical connection
   3. Acknowledge connectionless service
        - No logical connection but sender do receive an acknowledge

Service primitives:
        - UNITDATA
        - CONNECT
        - DATA
        - RESET
        - DATA-ACK
        - REPLY
        - REPLY-UPDATE

Basic technique to deal with error control: ARQ ( Automatic repeat request)
        - Error detection : CRC, check sum, etc...
        - Positive acknowledge
        - Retransmission by negative acknowledge
        - Retransmission after time-out

CSMA/CD (Ethernet)

A. Non-persistent CSMA
1. If the medium is idle, transmit.
2. If the medium is busy, wait a random amount of time and repeat step 1.

B. 1-persistent CSMA
1. If the medium is idle, transmit immediately ( p = 1).
2. If the medium is busy, continue to listen until the medium is idle; then transmit immediately.

C. p-persistent CSMA
1. If the medium is idle, transmit with probability = p ( p < 1 ) and delay 1 time unit with probability 1-p
2. If the medium is busy, continue to listen until the medium is idle; then repeat step 1

D.Collision Detection
1. If a collision is detected during transmission, transmit a brief jamming signal to inform other station to discard the transmitting packet.
2. Wait for a random amount of time and transmit again. ( Each time the mean delay is doubled, 16 back-off time reports as an error and gives up)

Note1: The 1-persistent CSMA/CD is the winner and become the standard for IEEE 802.3 (Ethernet)

Note2: This technique imposes a minimum size of the Ethernet Packet and max length for the cable.

Note3: For the star topology on UTP, collision is based on logic rather than voltage swing

MAC Frame

        1. Preamble: 7 octets to setup synchronization. ( 1010101010……. )
        2. Start frame delimiter(SFD): The sequence of 10101011 to indicate the start of the frame.
        3. Destination Address(DA): 16 (Novel) or 48 bit (Ethernet) but it has to be unique across all the stations on the same LAN
        4. Source Address(SA): Same as above
        5. Length: Length of LLC data field
        6. LLC Data: The data we want to send.
        7. Pad: Octets added to satisfy the minimum packet size
        8. Frame check sequence(FCS): CRC of all fields except Preamble, SFD, FCS.

Note:
        1.Repeater is used to expend the length of the network.
        2.Avoid the loop of repeater
        3.Repeater is working at the MAC (data link) layer , it does not isolate one segment from another. So 2 segments join by a repeater is considered a single   collision domain.
        4. Repeater can use to mix 10base5, 10base2, 10base-T together.
        5. Don’t insert a 10base2 segment between 2 10base5 segments (noise, resistance). Locate the high resistance segment at the dead end.

Token Ring

A small frame called token is circulated when all stations are idle.
A station wishing to transmit must wait for the token passes by, it seizes the token by changing 1 bit in the token which transform it to a start-of-frame sequence of a datagram.
Data is appended with all the necessary fields to construct a data frame.
When the token is taken, there is no more token on the bus , so no one can transmit. This is how medium access is control.
When the data frame circulates back to the transmitting station, it absorbs the frame and insert a new token back to bus when these 2 conditions happen
1. The station has completed the transmission of the whole frame
2. And the leading edge of the transmitted frame has returned to the station.

Under lightly loaded conditions, there is some inefficiency with the token ring (wait for the token)
Under heavy loaded, the round-robin like make token ring becomes very efficient.
Priority can be assigned to the token, there is no priority on Ethernet.
A monitor station is needed in token ring to prevent the token is loss or duplicated.