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Mobile Backhaul by Esa Metsala, Juha Salmelin

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4.4 PPP and ML-PPP

4.4.1 PPP over E1/T1/JT1

PPP (Point-to-Point Protocol) defines an encapsulation method assuming a full-duplex communication. The frame format is HDLC-like (High level data link control). From 3GPP standards viewpoint, PPP is one option for Layer-2, below the IP as a network layer protocol. PPP in HDLC-like framing is defined in RFC1662 [25].

The L2 frame format is shown in Figure 4.8.

Figure 4.8 PPP frame format [24].

img

A flag marks the start of the frame with a bit sequence ‘01111110’. Address field is typically a broadcast field of all ‘1’s, meaning all stations. Control fields consists of ‘00000011’, other values may be used if so agreed. Protocol indicates the protocol carried in the information field. Information consists of data to be transmitted, and padding may be included. Together the information and padding may include up to 1500 octets, which is the default value for the Maximum Receive Unit (MRU). Other values may be negotiated.

Frame Check Sequence (FCS), is calculated over the entire frame. 2 byte FCS is the default, 4 byte FCS may be negotiated. In case of mobile backhaul, IP is encapsulated into PPP by marking the protocol field accordingly, and then including data and optional padding.

In IP addressing, the PPP link can be treated as an unnumbered serial interface (no IP address assigned) or then as a network. In the latter case, the network consumes ...

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