Page 146 - Computer_Science_F5

Chapter Three: Advanced Computer Networks

of two sublayers: the Logical Link Control (LLC) and the Media Access Control
(MAC) sublayers as shown in Figure 3.8.

FOR ONLINE READING ONLY

Figure 3.8: Sub layers of data link layer

The data link layer consists of two also offer flow control and sequencing
sublayers, which are as follows: of control bits.
(i) Media Access Control (MAC): This Switched LANs key concepts
sublayer determines how packets are
placed on the media. In contention Switched LANs refer to local area networks
media access, all devices share the (LANs) that use switches to connect
same bandwidth, and access is based devices within the network. These switches
on a “first come, first served” basis. operate at the data link layer (Layer 2) of
The MAC sublayer defines physical the OSI model, and they significantly
addressing and logical topologies. enhance network performance, efficiency,
Logical topology refers to the signal and manageability compared to traditional
path through a physical topology. LANs that use hubs.
Additionally, the MAC sublayer can
handle line discipline, error notification (a) Link-Layer Addressing and ARP
(not correction), ordered delivery of Link-Layer Addressing:
frames, and optional flow control. Each device on a local area network
(LAN) has a unique MAC (Media Access
(ii) Logical Link Control (LLC): Control) address, which is used for
The LLC sublayer is responsible for communication within the same network.
identifying Network layer protocols and This MAC address is a 48-bit identifier
encapsulating them. An LLC header usually represented in hexadecimal format
provides instructions to the data link (such as, 00:1A:2B:3C:4D:5E).
layer regarding the handling of a packet
once a frame is received. For example, ARP (Address Resolution Protocol):
the LLC header may indicate that the ARP is used to map a known IP address
packet is destined for the IP protocol at to a MAC address. When a device wants
the Network layer. The LLC sublayer can to communicate with another device

137
Student’s Book Form Five

Computer Science Form 5.indd 137 23/07/2024 12:33