1. LAN Media Access
LAN Media Access Basics
1.1 Media access technologies deal with how devices communicate with each other over a
physical transmission medium.
1.2 The collision problem exists if a shared physical transmission medium is used for data
transmission. If more than one device tries to send frames at the same time, the frames
will corrupt each other in the shared medium.
1.3 The devices that share the same physical transmission medium are in the same collision
domain. A frame sent by a device can cause collision with a frame sent by another
device in the same collision domain. Moreover, a device can hear the packets destined
to any device in the same domain, i.e. sniffing.
1.4 Contention technology refers to any media access technology that requires devices to
share the same physical transmission medium and compete for the same resource, e.g.
1.5 Repeaters do not break up collision domains, whereas bridges, switches or routers do.
(Details about repeaters, bridges, switches and routers can be found in Section 4).
1.6 A Media Access Control (MAC) address (also known as physical address or hardware
address) is the data link layer address of a computer. It is programmed into the ROM of
the computer’s network interface card (NIC). MAC addresses are used by a bridge or
switch to forward frames in the same network segment (IP addresses are used by a router
to route packets across network segments).
1.7 There are 3 types of MAC address. They are:
Unicast address – an address for a specific computer.
Multicast address – an address for a specific group of computers.
Broadcast address – an address for all the computers in a network.
1.8 Address Resolution Protocol (ARP) maps IP addresses to the associated MAC
addresses for the devices in the same network segment (i.e. locally attached devices).
For example, Computer A wants to send a packet to Computer B. Based on B’s IP
address, it is known that B is on the same network segment as A (details about IP routing
can be found in Chapter 4). Therefore, A needs to find out the MAC address of B to
pass the frame to B. It happens as follows:
1. A checks its ARP cache for the mapping of B’s IP address to the MAC address.
2. If there is no entry in the ARP cache for B, A broadcasts (Layer 2 broadcast) an ARP
request for the IP address of B.
3. B replies to the ARP request (with its MAC address).
4. A sends the frame to the MAC address replied by B and stores the IP-address-to-
MAC-address mapping into its ARP cache.
1.9 Reverse Address Resolution Protocol (RARP) maps a MAC address to the associated
IP address. It is usually used by a diskless client to obtain an IP address during a
system boot-up. It works as follows:
When a client boots up, it broadcasts (Layer 2 broadcast) a RARP request with its
A RARP server looks up its table for the entry corresponding to this MAC address.
If the entry is found, the server responds with the associated IP address.
The client uses this IP address as its own address.
1.10 Boot Protocol (BOOTP) and Dynamic Host Configuration Protocol (DHCP) both
provide similar functions as RARP but with enhanced features, e.g. providing the name
server address and the default gateway address to the client. DHCP is the most popular
one among the three protocols. Details about DHCP can be found in Chapter 5.
LAN Media Access Methods
1.11 There are several major types of LAN media access methods. They are:
Carrier Sense Multiple Access / Collision Detection (CSMA/CD).
Carrier Sense Multiple Access / Collision Avoidance (CSMA/CA).
1.12 In a CSMA/CD network, all the computers monitor the carrier activities (i.e. frame
transmissions) on the shared medium continuously. If a computer wants to send a