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EPS Major Protocols . . . .3.1 IETF Dependence .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .3.2 Transport L ayer O ptions. . . .3.3 Disadvantages of TCP . . . .3.4
SCTP ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... 3.5 SCTP F unctions.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .3.6 SCTP Concepts .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .3.7 DiffServ.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 3.8 DSCP ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... 3.9
3GPP-specific Protocols. . . .3.10 Diameter in the EPC. . . .3.11 Diameter Applications. . . .3.12 Diameter Operation . . . .3.13 Legacy GTP .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .3.14 GTP in the EPS .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 3.15
GTPv2-C Packet Header . . . .3.16 GTPv2-C Message Types . . . .3.17 GTPv1-U Packet Header . . . .3.18 GTPv1-U Message Types . . . .3.19 S1AP ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... 3.20
S1AP Functions and Procedures. . . .3.21 NAS Functions and Procedures. . . .3.22 NAS Message Types . .. .. .. .. .. .. .. .. . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 3.23
LTE Evolved Packet Core Network
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At the end of this section you will be able to:
OBJECTIVES OBJECTIVES
Major Protocols
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■ discuss the derivation of the major protocols employed by the EPC and highlight the organizations responsible for specifying them
■ list the set of major protocols defined by the IETF
■ outline the support the EPC provides for deployment of different versions of IP
■ discuss the transport layer protocols that are available for use in the EPC
■ describe the specific features of SCTP that make it suitable for transporting signalling flows over the EPC
■ outline the basic concepts employed by SCTP
■ describe the functions performed by DiffServ within the EPC
■ outline the role of the Diameter protocol and discuss its use within the EPC
■ list the set of 3GPP-specific protocols developed or adapted for use in the EPC
■ describe the role performed by GTP in legacy 3GPP networks and highlight the differences between those versions and GTPv1-U and GTPv2-C
■ outline the functions performed by the S1 Application Protocol and the L3 NAS protocol
■ describe the message types and formats employed by the S1 and NAS protocols
LTE Evolved Packet Core Network
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Major Protocols
EPS Major Protocols EPS Major Protocols
The Evolved Packet System is designed to be an ‘all-IP’ environment. This means that all protocols, whatever their function, will travel between network nodes via an IP transport network.
IP is an open standards Network Layer/Layer 3 packet delivery system specified by the loose community of IT academics and professionals that comprise the IETF.
IETF specifications exist in the form of RFCs (Requests For Comment), which are freely available for download and comment from their website – www.ietf.org.
Most major protocols employed within the EPS were developed by the IETF, which means that the EPS can be regarded as a (mostly) open-standards networking environment.
Where relevant IETF-based specifications do not exist or where a legacy protocol can be employed, 3GPP has developed protocols or reused protocols of its own. These protocols are mainly related to inter-node signalling functions and are either evolutions or combinations of existing 3GPP systems.
Further Reading: 3GPP TS 23.401, 36.300; www.ietf.org
EPS
IETF 3GPP
3.2
LTE Evolved Packet Core Network
IETF Dependence IETF Dependence
Many IETF protocols are employed within the EPS and the IMS.
These include:
■ IP – support for IPv4 and IPv6
■ UDP (User Datagram Protocol) – employed at the transport layer in many protocol stacks
■ TCP (Transmission Control Protocol) – generally only employed on the SGi interface
■ SCTP (Stream Control Transmission Protocol) – employed on signalling interfaces
■ Mobile IP – MIPv4, PMIPv6 and DSMIPv6 are variously employed
■ SIP – employed by the IMS to establish user sessions
■ SDP (Session Description Protocol) – employed within SIP to describe session parameters
■ RTP (Real-time Transport Protocol) – transports user session media flows
■ RTCP (Real-time Transport Control Protocol) – provides control and feedback for RTP sessions
■ DiffServ – provides IP QoS services
■ Diameter – provides secure connections between database nodes
Further Reading: 3GPP TS 23.401; 36.300; www.ietf.org
IETF
SDP SIP
RTCP RTP
Diameter
UDP SCTP TCP
Mobile IP DiffServ
Underlying Transport IPv4/IPv6
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Major Protocols
Transpo
Transport rt Layer OptionsLayer Options
In the layered architecture employed by IP-based systems, IP itself operates at the network layer (layer 3) and performs packet creation, addressing and routing functions. IP is classed as an unreliable protocol as it has no mechanism for ensuring that packets are delivered and no means of requesting a retransmission if a packet is lost. Packet sequencing and delivery guarantees are the responsibility of the transport layer (layer 4).
Traditionally, the IP protocol suite offered two options for use at the Transport Layer: UDP and TCP. UDP was designed to operate in a ‘connectionless’ manner, which sees no correspondence between successive packets transmitted by a host. Instead of an ongoing stream of data travelling over a connection UDP only recognizes individual packets and has no mechanism for tracking their delivery.
UDP is sometimes referred to as a ‘fire and forget’ protocol.
Conversely, TCP was designed to operate in a complex, ‘connection-oriented’ manner. It employs a sophisticated feedback mechanism between transmitting and receiving nodes which ensures that out-of-sequence data can be reordered and that missing packets can be retransmitted.
UDP is employed on connections where there is either no opportunity to retransmit missing data (such as a connection carrying a real-time voice service) or where retransmission is handled by an upper layer protocol. In both cases UDP provides a fast and simple transport layer service. TCP is employed on connections where speed of delivery is less important than guaranteed delivery and is usually employed to carry non-real-time traffic for web browsing and e-mail applications.
UDP is employed on most user plane interfaces in the EPS.
TCP is an option for some signalling and control interfaces in the EPS, but the preferred layer 4 protocol for these is SCTP.
Further Reading: IETF RFC768 (UDP), RFC793 (TCP); 3GPP TS 23.401