Service Specific Connection Oriented Protocol

Description

The Service Specific Connection Oriented Protocol (SSCOP) is a data link layer protocol defined by ITU-T Q.2110 and adopted by 3GPP for use over ATM transport networks, primarily within the Iu interface connecting the Radio Network Controller (RNC) and the Core Network (CN). SSCOP operates as part of the Service Specific Convergence Sublayer (SSCS) above the ATM Adaptation Layer 5 (AAL5). Its primary function is to provide a reliable, connection-oriented data transfer service for signaling messages, ensuring they are delivered without errors, in sequence, and without loss or duplication. This is achieved through mechanisms like sequence numbering, selective retransmission of corrupted or lost Protocol Data Units (PDUs), and flow control to prevent receiver buffer overflow.

SSCOP works by establishing a confirmed connection between peer entities before data transfer begins. It uses a sliding window protocol for transmission control, where each PDU is assigned a sequence number. The receiver acknowledges correctly received PDUs, and the sender retransmits any unacknowledged PDUs after a timeout. SSCOP also includes a keep-alive function to monitor connection status and a protocol error recovery procedure. It offers both assured and unassured transfer modes; assured mode guarantees delivery, while unassured mode provides a simpler, non-guaranteed service suitable for certain management functions.

Within the 3GPP UMTS architecture, SSCOP is a key component of the transport network layer for control plane signaling across the Iu-CS (Circuit Switched), Iu-PS (Packet Switched), and Iur (inter-RNC) interfaces. It sits below the Signaling Connection Control Part (SCCP) or other network layer protocols, insulating them from the underlying ATM network's potential cell loss and misordering. By providing a robust, connection-oriented link, SSCOP ensures the integrity and reliability of critical signaling messages for mobility management, call control, and session management, forming a foundational layer for network stability and performance.

Purpose & Motivation

SSCOP was created to address the need for reliable signaling transport over Asynchronous Transfer Mode (ATM) networks, which were the primary transport technology for early 3G UMTS releases. ATM itself is a connection-oriented protocol but operates at the cell level (53-byte cells) and does not guarantee reliable, in-order delivery of larger data units at the AAL5 layer. Signaling messages for call control and mobility management are critical and require guaranteed delivery. Without SSCOP, higher-layer signaling protocols would need to implement complex error recovery and sequencing themselves, increasing complexity and reducing efficiency.

The protocol solves the problem of providing a standardized, robust data link service for signaling across the Iu interface. It abstracts the imperfections of the underlying ATM transport, such as cell loss or misordering due to network congestion or errors, presenting a reliable byte stream to the upper layers. This separation of concerns allows signaling protocol designers to focus on application logic rather than transport reliability. Its introduction in Release 99 was essential for the UMTS network's operational integrity, ensuring that control plane communications between the RAN and CN were dependable, which is fundamental for services like voice calls and packet sessions.

Key Features

• Provides reliable, connection-oriented data transfer with guaranteed in-sequence delivery
• Implements selective retransmission based on sequence numbers and receiver status reports
• Includes flow control mechanisms to prevent receiver buffer overflow
• Supports both assured (guaranteed) and unassured transfer modes
• Offers connection establishment and release with confirmation procedures
• Incorporates keep-alive and protocol error recovery functions to maintain connection health

Evolution Across Releases

Defining Specifications