Radio Link Protocol
Radio Link Protocol (RLP) is an automatic repeat request (ARQ) fragmentation protocol used over a wireless (typically cellular) air interface.[1] RLP occurs on the data link layer (Layer 2) of the OSI networking model.[2]
Most wireless air interfaces are tuned to provide 1% packet loss, and most vocoders are mutually tuned to sacrifice very little voice quality at 1% packet loss. However, 1% packet loss is intolerable to all variants of TCP, and so something must be done to improve reliability for voice networks carrying TCP/IP data. A RLP detects packet losses and performs retransmissions to bring packet loss down to approximately .01%, or even .0001%, which is suitable for TCP/IP applications. When the receiver detects a missed packet, it sends a NAK (not acknowledged) frame to the sender, which triggers a retransmission of the missing packet. At the same time, the receiver sets a timer for receipt of the missing packet. Each time the timer expires without receipt of the packet, the receiver resets the timer and sends NAK a number of times equal to 1 + the number of times the timer has already expired, up to a limit.[3]
RLP also implements stream fragmentation and reassembly, and sometimes, in-order delivery. Some forms of RLP rely upon a higher-layer PPP protocol to provide these functions,[4] while newer forms of RLP also provide framing and compression. Because a CDMA IS-95 network's smallest voice packet size (and thus an RLP packet length) can be as little as 88 bits (11 bytes),[5] RLP headers must be very small, to minimize overhead. This is typically achieved by allowing both ends to negotiate a variable 'sequence number space', which is used to number each byte in the transmission stream.[4] In some variants of RLP, this sequence counter can be as small as 6 bits
A RLP protocol can be ACK-based or NAK-based. Most RLPs are NAK-based, meaning that forward-link sender assumes that each transmission got through, and the receiver only NAKs when an out-of-order segment is received. This greatly reduces reverse-link transmissions, which are spectrally inefficient and have a longer latency on most cellular networks. When the transmit pipeline goes idle, a NAK-based RLP must eventually retransmit the last segment a second time, in case the last fragment was lost, to reach a .01% packet loss rate. This duplicate transmission is typically controlled by a "flush timer" set to expire 200-500 milliseconds after the channel goes idle.{{Citation needed|date=June 2025}
The concept of a RLP protocol was invented by Phil Karn with Qualcomm in 1993[4] for CDMA (IS-95) networks.
Cellular networks such as GSM and CDMA use different variations of RLP. In UMTS and in LTE, the protocol is called RLC (Radio Link Control).[6]
References
- ↑ Garg, Vijay (2010-07-28) (in en). Wireless Communications & Networking. Elsevier. pp. 549. ISBN 978-0-08-054907-1. https://books.google.com/books?id=UE2wEc9NfB8C&dq=Radio+Link+Protocol+(RLP)&pg=PA549.
- ↑ Yang, Samuel C. (2004) (in en). 3G CDMA2000: Wireless System Engineering. Artech House. pp. 60–63. ISBN 978-1-58053-758-2. https://books.google.com/books?id=cFD47vNgPN8C&dq=Radio+Link+Protocol+(RLP)&pg=PA60.
- ↑ Bao, Gang (1996-08-01). "Performance evaluation of TCP/RLP protocol stack over CDMA wireless link". Wirel. Netw. 2 (3): 229–237. doi:10.1007/BF01201056. ISSN 1022-0038. https://dl.acm.org/doi/10.1007/BF01201056.
- ↑ 4.0 4.1 4.2 Kim, Euree Y. (May 1998). Packet Delay and Sequence Number Space in the Radio Link Protocol Layer (PDF) (MSc thesis). MIT. Retrieved 2025-06-16.
- ↑ Tipper, David. "IS-95 (cdmaone)". University of Pittsburgh. https://sites.pitt.edu/~dtipper/2720/2720_Slides9.pdf.
- ↑ Voicu, Andreea; Jarnikov, Dmitri (Jan 2011). "Extended architecture for home node base stations with multimedia services digest of technical papers". 2011 IEEE International Conference on Consumer Electronics (ICCE). pp. 767–768. doi:10.1109/ICCE.2011.5722853. ISBN 978-1-4244-8711-0. https://ieeexplore.ieee.org/document/5722853.
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