Cisco ASR 5000 Series, Обзор продукта

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Serving Gateway Overview
▀ Features and Functionality - Base Software
▄ Cisco ASR 5000 Series Product Overview
OL-22938-02
Network Operation Management Functions
This section describes the following features:
Support Interfaces (Reference Points)
Multiple PDN Support
Congestion Control
IP Access Control Lists
Support Interfaces (Reference Points)
S1-U (E-UTRAN EPC)
In an E-UTRAN network S1-U is the per-bearer user plane tunneling reference interface between the S-GW and
eNodeB. The S-GW provides the local mobility anchor point for inter-eNodeB hand-overs. It provides inter-eNodeB
path switching during hand-overs when the X2 handover interface between base stations cannot be used. The S1-U
interface uses GPRS tunneling protocol for user plane (GTP-Uv1). GTP encapsulates all end user IP packets and it relies
on UDP/IP transport.
In order to support S1-U hand-overs the source eNodeB initiates the hand-over by sending the hand-over required
message over the S1-MME interface to the MME. The MME then determines if the S-GW needs to be relocated. The
eNodeB decides which EPS bearers are subject to forwarding to the target base station. In the S1-U hand-over, the hand-
off occurs indirectly from the source eNodeB to the target via the source and target S-GWs.
S11 (E-UTRAN EPC)
S11 is the reference interface that provides the control plane protocol (GTP-Cv2) between the MME and S-GW. As with
all GTP-based interfaces S11 relies on UDP/IP transport. A GTP tunnel is identified in each node with a Tunnel
Endpoint ID (TEID), IP address and UDP port number. The TEID values are exchanged between the tunnel endpoints
using GTP-C. There is one GTP-C tunnel between the MME and S-GW for each mobile terminal. The GTP protocol
provides the following functions:
Bearer management function : This functionality is responsible for bearer management; setting up, modifying
and releasing EPS bearers, which are triggered by the MME. The release of EPS bearers may be triggered by
the P-GW or HSS as well. The messages include Create Session request, Create Bearer request, Create bearer
response etc. Additionally GTP tunnel management messages may be sent for any of the following reasons:
Initial UE attachment
UE requests connection to an additional PDN
Tracking Area Update with S-GW change
S1/X2 handover with S-GW change
GERAN or UTRAN to E-UTRAN Inter-RAT handover with SGW change.
Path management function : This functionality is responsible for managing the path between the tunnel
endpoints. It includes messages like ECHO request, ECHO response and version not supported indication.
Mobility management functions : This functionality consists of messages that are exchanged between GTP end
points to manage UE mobility. Messages such as Forward Relocation request/response are sent between end
points. These messages are not sent on the S11 interface.