CS-09 – Data Communications and Networks

AGENDA (Session 9)

Medium Access Sublayer

ALOHA

Carrier Sense Protocols

CSMA/CD

IEEE 802.3

Manchester Encoding

802.3 Frame

MULTIPLE ACCESS SUBLAYER

· Lower Layer of Data-Link Layer

· Broadcast (not point to point) based network

· Broadcast Channel = Multiaccess Channel = Random Access Channel

· Channel Allocation is the primary issue (in context of speed and reliability aspect)

· Channel Allocation can be static or dynamic

Static Allocation:

· FDM type ( Multiple Simultaneous Frequency)

· TDM type (Time Slot Approach)

Dynamic Allocation:

· Channel Allocation as per need of the sender.

· Priority between senders are self managed by the allocation scheme itself.

· Detection of Channel Usage status.

· Detection of Transmission Success Status.

ALOHA

· Let sender transmit whenever he likes.

· Sender Listen Back the broadcast.

· If message is destroyed due to contention send it back after waiting for random time.

· Pure ALOHA throughput is given by

S=Ge^-2g

S: Throughput, G: Attempts/packet time.

Maximum Channel Utilisation is about 18 Percent

Slotted ALOHA

· Station allowed to transmit in the next available time slot.

· Throughput is now given by

S=Ge^-g

Maximum Channel Utilisation is about 36 Percent

· Maximum utilisation with ALOHA system is 1/e.

Carrier Sense Protocols

· Station Listens for carrier before sending message.

· Better Throughputs are available with this scheme.

1-Persistent CSMA (Carrier Sense Multiple Access)

· Station Listens for an idle channel state and then transmits.

· Probability of Transmission is 1.

· Collision can occur because other station can also start together.

Non-Persistent CSMA

· Station Listens for Channel Busy.

· If Busy it waits and then listens after sometime again.

· Better Channel Utilisation.

p-Persistent CSMA

· Applies to Slotted Channel.

· Transmits with a probability p and differs next slot.

CSMA/CD & CDMA

CSMA/CD (CSMA with Collision detection):

· Sender Initiates transmission when channel is idle but terminates as soon as there is collision.

· Channel States are: State of Contention, State of Transmission or IDLE.

· Retries after random Time.

· Used in 802.3 LAN.

· Signal Coding Allows contention detection.

CDMA (Code Division Multiple Access):

· Multiple simultaneous transmissions, but 1s and 0s are coded differently for different stations.

· Receiver identifies by mathematical operation over this code (Inner Product of code – Product is 0 for distinct code).

IEEE 802.3 Standards

· Evolved from Ethernet Implementation (2.94 Mbps, over 1Km).

· Follow CSMA/CD.

Cabling Systems

· 10 Base 5 – Thick Coax, 500m, 100 Nodes.

· 10 Base 2 – Thin Coax, 200 m, 30 Nodes.

· 10 Base T – Twisted Pair, 100 m, 1024 nodes.

· 10 Base F – Fibre , 2000 m, 1024 Nodes

· Linear, Spine, Tree, Segmented Topology.

Signal Encoding:

· MANCHESTER ENCODING :

o High to Low – 1

o Low to High – 0

· DIFFERENTIAL MANCHESTER:

o Transition at start – 0 .

o Transition not at start – 1 .

802.3 Frame

Preamble (=7)

Start (1)

Dest. Address (2 or 6)

Source Address (2 or 6)

Data Length (2)

Data (0-1500)

End

PAD (0-46)

CS

· Preamble – 7 bytes each 10101010 manchester coding make it 10 MHz pulse for 5.6 uSec.

· Start of Frame 10101011

· Dest/Start Address (2 or 6 Byte address)

o MSB – 0 for ordinary address

o MSB – 1 for group address

o All bits – 1 for Broadcasting.

· Local and Global addressing supported.

· Length field 2 Byte.

· Data Length 0-1500 Byte.

· Valid Frame at least 64 Byte long.