Layer 2 Protocols

The essential point is that PPP is the protocol of choice for point-to-point connections. The address component of the frame is generally superfluous, as addresses are unnecessary in a PPP connection. The address part persists in PPP frames because they are derived from the HDLC frame structure. PPP naturally broadcasts to the destination, and with only one other device on the network, that device will always receive the frame intended for it. Although SLIP is an alternative protocol, PPP is favored for its advanced features. Many ISPs utilize PPP to deliver internet services.

Frame Relay is a Layer 2 protocol mainly utilized to connect multiple LANs into a WAN through virtual circuits. It has certain limitations, including the absence of error control and flow management mechanisms. The types of virtual circuits are:

1) Permanent Virtual Circuits (PVCs)

2) Switched Virtual Circuits (SVCs)

Its lack of error checking means it is not deemed reliable, yet it permits quicker data transmission.

The Nokia IP Exam concentrates more on Ethernet and ATM as Layer 2 protocols, which we will explore next.

Asynchronous Transfer Mode (ATM) was designed to merge high-throughput data traffic with real-time, low-latency traffic. The goal was to establish a protocol that could adapt to varying demands for throughput and latency. As a result, ATM provides different classes of service, detailed as follows:

1) AAL1 - Constant bit rate

2) AAL2 - Variable bit rate

3) AAL3/4 - Connection-oriented class

4) AAL5 - Connectionless service

AAL stands for ATM Adaptation Layer. As the name suggests, the AAL enables ATM to interface with non-ATM networks, hence the 'Adaptation' part of its name.

ATM cells are fixed at 53 bytes, unlike some other protocols where the size is variable. This fixed size minimizes queuing delay, offering an advantage in real-time transmissions. Similar to frame relays, ATM also utilizes virtual circuits. It can be configured with Permanent Virtual Circuits (PVC) and Switched Virtual Circuits (SVC).

Among all the classes, AAL5 is the one that employs IP. Remembering the descriptions of these classes will aid you in the exam.

Ethernet serves as a quintessential example of a shared medium, originally designed for broadcast communication. This design allows any transmitted frame to be received by all devices connected to the medium. MAC addressing is utilized to ensure frames reach their intended destination. However, as a frame traverses the medium, signal degradation occurs, leading to inherent distance limitations within the Ethernet protocol. To counteract this, repeaters or amplifiers are commonly employed to boost the signal, with switches being particularly effective. Over time, Ethernet has evolved to encompass two main standards:

1) The 802.3 standard

2) The Ethernet II standard.

These standards share similar frame structures, differing primarily in the maximum frame size and the terminology used for the frame's fifth field.

In practice, if a frame exceeds 1500 bytes, it is classified as Ethernet II; if it is less than 1500 bytes, it falls under the 802.3 standard. In the industry, the terms Ethernet II, 802.3, and Ethernet are often used interchangeably.

The Ethernet Frame

I've noticed several questions arising from this session, so I'll highlight the key points you should know. Please refer to the attached image.

1) Preamble: This is 7 bytes and serves as a precursor to the actual frame. Its function is to signal the receiver of incoming frames.

2) Start of Frame Delimiter (SFD): This is 1 byte and indicates to the receiver that the following bits mark the beginning of the frame.

3) Destination Address: This contains the MAC address of the receiver, which is 6 bytes.

4) Source Address: This contains the MAC address of the sender, also 6 bytes.

5) Length (802.3) / Type (Ethernet II): This specifies the frame's length so the receiver knows what to expect.

6) Payload: This is where the actual data resides.

7) Frame Check Sequence (FCS): This contains checks to ensure everything was received correctly and in the proper order.