A sequence of 56 bits (alternating 1 and 0 values) used for synchronization; gives components in the network time to detect the presence of a signal.
Start Frame Delimiter
A sequence of 8 bits (10101011) that indicates the start of the packet.
Destination and Source Addresses
The Destination Address field identifies the station or stations that are to receive the packet. The Source Address identifies the station that originated the packet. A Destination Address may specify either an individual address destined for a single station, or a multicast address destined for a group of stations. A Destination Address of all 1 bits refers to all stations on the LAN and is called a broadcast address.
Type
Ether type
Data and Padding
This field contains the data transferred from the source station to the destination station or stations. The maximum size of this field is 1500 bytes. If the size of this field is less than 46 bytes, then padding is used to bring the packet size up to the minimum length. A minimum Ethernet packet size is 64 bytes from the Destination Address field through the Frame Check Sequence.
Frame Check Sequence
This field contains a 4-byte cyclical redundancy check (CRC) value used for error checking. When a source station assembles a packet, it performs a CRC calculation on all the bits in the packet from the Destination Address through the Pad fields (that is, all fields except the Preamble, Start Frame Delimiter, and Frame Check Sequence). The source station stores the value in this field and transmits it as part of the packet. When the destination station receives the packet, it performs an identical check. If the calculated value does not match the value in this field, the destination station assumes an error has occurred during transmission and discards the packet.
IP control flags; must be set to 010 to avoid fragmentation
Fragment Offset
Indicates where in the datagram the fragment belongs; not used for TDM-over-packet
Time to Live
IP Time-to-Live field; datagrams with zero in this field are to be discarded
Protocol
Must be set to 0x11 to signify UDP
IP Header Checksum
Checksum for the IP header
Source IP Address
IP address of the source
Destination IP Address
IP address of the destination
表3. UDP报头结构
Field
Description
Source Port Number, Destination Port Number
Either the Source or the Destination Port Number holds the bundle identifier. The unused field can be set to 0x85E (2142), which is the user port number assigned to TDM-over-packet by the Internet Assigned Numbers Authority (IANA). For UDP/IP-specific OAM packets, the bundle identifier is all ones.
UDP Length
Length in octets of UDP header and data
UDP Checksum
Checksum of UDP/IP header and data; if not computed, it must be set to zero
UDP/IPv6报头
图5. UDP/IPv6报头
表4. UDP报头结构
Field
Description
IPVER
IP version number; for IPv6 IPVER = 6
Traffic Class
An 8-bit field similar to the type-of-service (ToS) field in IPv4
Flow Label
The 20-bit Flow Label field can be used to tag packets of a specific flow to differentiate the packets at the network layer.
Payload Length
Similar to the Total Length field in IPv4, this field indicates the total length of the IP header and data in octets.
Next Header
Similar to the Protocol field in IPv4, this field determines the type of information following the basic IPv6 header. It must be set to 0x11 to signify UDP.
Hop Limit
Similar to the Time-to-Live field in IPv4
Source IP Address
Similar to the Source Address field in IPv4, except that this field contains a 128-bit source address for IPv6 instead of a 32-bit source address for IPv4.
Destination Address
Similar to the Destination Address field in IPv4, except that this field contains a 128-bit destination address for IPv6 instead of a 32-bit destination address for IPv4.
MPLS报头
图6. MPLS报头
表5. MPLS报头结构
Field
Description
Outer Labels
These MPLS labels identify the MPLS LSP used to tunnel the TDMoMPLS packets through the MPLS network. They are also known as tunnel labels or transport labels. The label number can be assigned either manually or via the MPLS control protocol. There can be zero, one, or two outer labels.
EXP
Experimental field
S
Stacking bit: 1 indicates stack bottom; S = 0 for all outer labels
TTL
MPLS time to live
Inner Label
The MPLS Inner Label (also known as the PW label or the interworking label) contains the bundle identifier used to multiplex multiple bundles within the same tunnel. It is always at the bottom of the MPLS label stack, and hence its stacking bit is set.
MEF报头
图7. MEF报头
表6. MEF报头结构
Field
Description
ECID
The Emulated Circuit Identifier (ECID) contains the bundle identifier.
L2TPv3/IPv4报头
图8. L2TPv3/IPv4报头
表7. L2TPv3/IPv4报头结构
Field
Description
IPVER
IP version number; e.g., for IPv4 IPVER = 4
IHL
Length in 32-bit words of the IP header, IHL = 5
IP TOS
IP type of service
Total Length
Length in octets of header and data
Identification
IP fragmentation identification
Flags
IP control flags; must be set to 010 to avoid fragmentation
Fragment Offset
Indicates where in the datagram the fragment belongs; not used for TDM-over-packet
Time to Live
IP Time-to-Live field; datagrams with zero in this field are to be discarded
Protocol
Must be set to 0x73 to signify L2TPv3
IP Header Checksum
Checksum for the IP header
Source IP Address
IP address of the source
Destination IP Address
IP address of the destination
表8. L2TPv3报头结构
Field
Description
Session ID (32 Bits)
Locally significant L2TP session identifier, also contains the bundle identifier; all bundle identifiers are available for use except 0, which is reserved
Cookie (32 or 64 Bits)
Optional field that contains a randomly selected value used to validate association of the packet with the expected bundle identifier
L2TPv3/IPv6报头
图9. L2TPv3/IPv6报头
表9. L2TPv3/IPv6报头结构
Field
Description
IPVER
See Table 4
Traffic Class
Flow Label
Payload Length
Next Header
Must be set to 0x73 to signify L2TPv3
Hop Limit
See Table 4
Source Address
Destination Address
L2TPv3报头结构见表8。
控制字
图10. 控制字
表10. 控制字结构
Field
Description
RES
Reserved bits—must be set to zero
L
Local loss-of-sync (LOS) failure. This bit is set by the CPU. A set L bit indicates that the source has detected, or has been informed of, a TDM physical layer fault that impacts the data to be transmitted. This bit can be used to indicate physical layer LOS that should trigger AIS generation at the far end. Once set, if the TDM fault is rectified, the L bit must be cleared.
R
Remote receive failure. This bit is set by the CPU. A set R bit indicates that the source is not receiving packets at the Ethernet port (i.e., there is a failure in the direction of the bidirectional connection). This indication can be used to signal congestion or other network-related faults. A remote failure indication may trigger fallback mechanisms for congestion avoidance. The R bit must be set after a preconfigured number of consecutive packets are not received, and must be cleared once packets are received again.
M
Defect modifier failure. These bits are set by the CPU. This field is optional. When used, it supplements the L-bit meaning.
FRG
Fragmentation field. This field is used for fragmenting multiframe structures into multiple packets in case of CESoPSN structured with CAS bundles. The field is used as follows:
00 - Indicates that the entire (unfragmented) multiframe structure is carried in a single packet
01 - Indicates the packet carrying the first fragment
10 - Indicates the packet carrying the last fragment
11 - Indicates a packet carrying an intermediate fragment
Length
Includes control word, payload, and RTP header (if it exists), unless it is a UDP/IP packet. It is used when this sum is less than 64 bytes. Otherwise, set to zero.
Sequence Number
TDM-over-packet sequence number. This value is defined separately for each bundle and incremented by one for each TDMoP packet sent for that bundle. The initial value of the sequence number is random (unpredictable) for security purposes, and the value is incremented in wrap-around manner separately for each bundle. It is used by the receiver to detect packet loss and restore packet sequence.
The HDLC payload type machine supports three different modes for this field: always zero, incremented in wrap-around manner, or incremented in wrap-around value, but skips zero value.
For OAM packets (see TDM-over-packet payload), it uniquely identifies the message. Its value is unrelated to the sequence number of the TDMoP data packets for the bundle in question. It is incremented in query messages, and replicated without change in replies.
RTP报头
图11. RTP报头
表11. RTP报头结构
Field
Description
V
RTP version—must be set to 2
P
Padding bit—must be set to 0
X
Extension bit—must be set to 0
CC
CSRC count—must be set to 0
M
Marker bit—must be set to 0
PT
Payload Type. One PT value MUST be allocated from the range of dynamic values for each direction of the bundle. The same PT value MAY be reused for both directions of the bundle, and also reused between different bundles.
SN
The sequence number, identical to the sequence number in the control word
TS
Timestamp. The RTP header can be used in conjunction with the following modes of timestamp generation:
Absolute mode: the chip sets timestamps using the clock recovered from the incoming TDM circuit. As a consequence, the timestamps are closely correlated with the sequence numbers. The timestamp is incremented by one every 125µs.
Differential (common-clock) mode: The two chips at bundle edges have access to the same high-quality clock source, and this clock source is used for timestamp generation.
SSRC
Identifies the synchronization source. This identifier should be chosen randomly, with the intent that no two synchronization sources within the same RTP session will have the same SSRC identifier.
PreConfig Configuration
1. Link Type E1
2. Bundle Number ID Location Port in DST, Bundle in SRC UDP Port
3. UDP Mask 1FFF
4. VCCV OAM Mask [0 - 4] 0
5. VCCV OAM Value 1FFF
6. MEF Ethernet Type 88D8
7. MEF OAM Type 0
8. TDMoIP Port Number 1 85E
9. Oscillator Type OCXO (Stratum 3E)
10. RTP Clock Source ABSOLUTE
11. Common clock Rate 19440000
12. IP Version IPv4
13. Clock Recovery Smart Statistics Enable
14. One or Two Clock Mode One
Maxim软件菜单的第二项用来选取所需的Bundle Number ID Location。上面菜单的第二项提供以下可选项：
Bundle Number ID Location
1: Ignore port, Bundle in SRC UDP PORT,
2: Port in DST, Bundle in SRC UDP PORT
3: Port in SRC, Bundle in DST UDP PORT,
4: Ignore Port, Bundle in DST UDP PORT
在上面的菜单中，Maxim设备的默认Bundle Number ID Location是选项2： “Port in DST, Bundle in SRC UDP PORT”。为了使得Maxim的设备能够与其它厂家的设备互通，用户需要适当地选取选项1、3或者4。比如，某TDMoP厂家的设备在Source (SRS)位置插入Destination端口，在Destination (DST)插入绑定端口号。如果用户在上面的菜单中选取选项3，那么UDP源端口Bundle Number ID Location就被设置为0x85E (十进制的2142)，UDP目的端口就会为2，如图14所示。这样就可以匹配那个厂家的TDMoP报文头，因此，他们可以互操作。
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