introduction to networking - Ch.7 MAC

MAC

Multiple Access Control (MAC) protocols

Kinds of MAC protocols

coordinates transmissions from different stations in order to minimize/avoid collisions

1. Channel Partitioning MAC protocols
2. Random Access MAC protocols
3. “Taking turns” MAC protocols

Channel Partitioning Mac protocols

1. TDM (Time Division Multiplexing): channel divided into N time slots, one per user; ineffificient with low duty cycle users and at light load.
2. FDM (Frequency Division Multiplexing): frequency subdivided.
3. Other examples include Code Division Multiple Access (CDMA)

4. GSM, Used for mobile phones

Random Access protocols

1. A node transmits at random at full channel data rate R.
2. If two or more nodes “collide”, they retransmit at random time
3. The random access MAC protocol specifies how to detect collisions and how to recover from them

4. E.g. Slotted Aloha, ALOHA, CSMA ...

CSMA - Carrier Sense Multiple Access

1. Listen before transmit, if the channel is busy, defer transmission.
2. If collision happens, the entire transmission time is wasted.

3. Collision detection is easy in wired LANs (eg, Ethernet): can measure signal strength on the line, or code violations, or compare tx and receive signals
4. Collision detection cannot be done in wireless LANs (the receiver is shut off while transmitting, to avoid damaging it with excess power)

LAN

1. Use IP address - Drive the packet to destination network
2. 48 bit Mac address - burnt in the Ethernet card ROM and is unique
3. Use ARP (Address resolution Protocol) to resolve MAC address to IP address and run on network layer

ARP

1. Each IP node (Host, Router) on the LAN has ARP module and Table (aka ARP cache)
2. ARP Table: IP/MAC address mappings for some LAN nodes E.g.
3. <>
   < ………………………….. >
4. TTL (Time To Live): timer, typically 20 min

Hub

Physical Layer devices: essentially repeaters operating at bit levels: repeat received bits on one interface to all other interfaces

Bridge

Link layer device

1. stores and forwards Ethernet frames
2. examines frame header and selectively forwards frame based on MAC dest address
3. when frame is to be forwarded on segment, uses CSMA/CD to access segment

Bridge vs router

1. Both are store-and-forward devices, but Routers are Network Layer devices (examine network layer headers) and Bridges are Link Layer devices
2. Routers maintain routing tables and implement routing algorithms, bridges maintain filtering tables and implement filtering, learning and spanning tree algorithms

Switches

1. Offer large number of interface
2. Layer 2 device
3. Point-to-point connection

Sample network